Control metal properties

For any component or joint we want to select materials that have specific metal properties and we want to use manufacturing processes that are capable of transforming a material into the desired shape with the desired properties.

This lesson teaches the relationship between the properties of a metal and its composition, microstructure and the manufacturing processes used to form components and joints. Obtaining the desired properties in a metal requires understanding these factors and their interactions.

The concepts and information presented in this lesson applies to all metals.

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Stainless steel is known for its corrosion resistance in many environments in which carbon and low alloy tool steels would corrode. The corrosion resistance is a result of a very thin (about 5 nanometers) oxide layer on the steel's surface. This oxide layer is referred to as a passive layer since it renders the surface electrochemically passive in the presence of corrosive environments.

The passive layer forms because of the chromium added to stainless steel. Stainless steel must have at least 10.5% chromium in order for the passive layer to form. The more chromium that is added, the more stable the passive layer becomes, and the better the corrosion resistance. Other elements such as nickel, manganese, and molybdenum can be added to enhance stainless steel corrosion resistance.

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Another requirement for the formation and maintenance of the passive layer is that the steel surface must be exposed to oxygen. Corrosion resistance is greatest when the steel is boldly exposed and the surface is maintained free of deposits. If passivity is destroyed under conditions that do not permit restoration of the passive film, then stainless steel will corrode much like a carbon or low-alloy steel. For example, covering a portion of the surface - for example, by biofouling, painting, or installing a gasket - produces an oxygen-depleted region under the covered region. The oxygen-depleted region is anodic relative to the well-aerated boldly exposed surface, possibly resulting in the corrosion of the covered region.

Pitting in 304 stainless steel

Pitting in 304 stainless steel

Under certain circumstances, the passive layer can break down at localized spots on a well exposed stainless steel surface. When this happens, the metal can corrode in the localized spots. This is called pitting corrosion. One common cause of pitting corrosion is exposure to aqueous environments that contain chloride. Examples are coastal atmospheres, road salt combined with rain water, and even tap water containing high levels of chloride.

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Intergranular corrosion of 304 stainless steel

Intergranular corrosion

During the fabrication of stainless steel components or structures it is possible to degrade the corrosion resistance. This occurs when austenitic stainless steels (e.g. 304) are exposed to temperatures between about 425 °C (797 °F) and 870 °C (1598 °F). If the exposure time is too long, then the areas near the metal's grain boundaries lose their corrosion resistance and can be preferentially attacked when exposed to a corrosive environment. The grains fall out and the metal loses strength. The increased susceptibility to corrosion by this change in microstructure is called sensitization.

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For more information about stainless steel corrosion and corrosion in general take our online, on-demand Corrosion of Metals course or read Corrosion: Understanding the Basics by J.R. Davis or Corrosion and Corrosion Control by R.W. Revie and H.H. Uhlig.

In metallurgy, the term phase is used to refer to a physically homogeneous state of matter, where the phase has a certain chemical composition, and a distinct type of atomic bonding and arrangement of elements. Within an alloy, two or more different phases can be present at the same time. The images below show the phases in aluminum-copper and iron-carbon alloys.

Al2Cu precipitates in an aluminum matrix. © DoITPoMS Micrograph Library, Univ. of Cambridge
Ferrite (white) and cementite (dark) in steel.

Each phase within an alloy has its own distinct physical, mechanical, electrical, and electrochemical properties. For example, in carbon steel, ferrite is a relatively soft phase and cementite is a hard, brittle phase. When they are present together, the strength of the alloy is much greater than for ferrite and the ductility is much better compared to cementite. Thus, an alloy with more than one phase can be considered to be a composite material.

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The phases present in an alloy depend on the alloy composition and the thermal treatment to which the alloy has been exposed. Phase diagrams are graphical representations of the phases present in a particular alloy being held at a particular temperature. Phase diagrams can be used to predict the phase changes that have occurred in an alloy that has been exposed to a particular heat treatment process. This is important because the properties of a metal component depend on the phases present in the metal.

Phase diagrams are useful to metallurgists for selection of alloys with a specific composition and design and control of heat treatment procedures that will produce specific properties. They are also used to troubleshoot quality problems.

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Iron-Carbon Phase Diagram
An example of a commonly used phase diagram is the iron-carbon phase diagram, which is used to understand the phases present in steel. The amount of carbon present in an iron-carbon alloy, in weight percent, is plotted on the x-axis and temperature is plotted on the y-axis. Each region, or phase field, within a phase diagram indicates the phase or phases present for a particular alloy composition and temperature. For the iron-carbon phase diagram, the phase fields of interest are the ferrite, cementite, austenite, ferrite + cementite, ferrite + austenite, and austenite + cementite phase fields.

The phase diagram indicates that an iron-carbon alloy with 0.5% carbon held at 900 °C will consist of austenite, and that the same alloy held at 650 °C will consist of ferrite and cementite. Furthermore, the diagram indicates that as an alloy with 0.78% carbon is slow cooled from 900 °C, it will transform to ferrite and cementite at about 727 °C.

Fe-C_PhaseDiagram

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Aluminum-Copper Phase Diagram
Another commonly used phase diagram is the aluminum-copper phase diagram, which is useful for understanding precipitation strengthening in Al-Cu alloys. The amount of copper present in an alloy is plotted on the x-axis. The phase fields of interest are the Al, θ, and Al+θ phase fields on the left hand side. For precipitation strengthening an Al-Cu alloy, this phase diagram indicates the minimum temperature to which an alloy must be heated to put all the copper in solution. This is indicated by the solvus line on the phase diagram. The maximum amount of copper that can contribute to precipitation strengthening is indicated by the maximum amount of copper (5.45 %) that can go into solid solution in the aluminum.

Al-CuPhaseDiagram

Equilibrium Conditions
Phase diagrams indicate the relationship between the phases present, alloy composition, and temperature under conditions of slow heating or cooling. Slow heating or cooling allows the atoms within a metal to move around so that the alloy is at equilibrium. However, with many heat treatment processes, a metal is exposed to fast heating and cooling. Under these conditions it is possible to have phases missing or present compared to what is indicated by the phase diagram. Therefore, it is also important to understand the kinetics of phase transformations, i.e. the effects of temperature, time, cooling rate, and heating rate on phase changes within an alloy. This will be a topic of another article.

You can learn more about how to read and use phase diagrams in a few of our courses. Metallurgy of Steel and Metallurgy of Steel Heat Treating teach about the iron-carbon phase diagram. Metallurgy of Precipitation Strengthening teaches about the aluminum-copper phase diagram.

Corrosion of metals is an electrochemical reaction that involves changes in both the metal and the environment in contact with the metal. While the mechanisms of corrosion are the same on a microscopic level, various microstructure, composition, and mechanical design issues will lead to different manifestations of corrosion.

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There are seven common types of metal corrosion

Uniform corrosion occurs over the entire exposed surface of a metal. Rust on a steel structure or the green patina on a copper roof are examples of uniform corrosion. The driving force for this type of corrosion is the electrochemical activity of the metal in the environment to which the metal is exposed.

Galvanic corrosion occurs near the junction between two dissimilar metals.The driving force for the corrosion reaction is the difference in electrode potentials between the two metals.

Crevice corrosion occurs in crevices between components and also under polymer coatings and adhesives. The driving force for the corrosion is the difference between the oxygen concentration inside the crevice and outside the crevice.

Pitted304Stainless_cropped


Pitting occurs in metals that are normally passive, when the passive layer breaks down. Examples of passive metals are aluminum and stainless steel. Pitting is a problem if it leads to weakening or perforation of the metal. In applications where appearance is important pitting is a problem.

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Interganular corrosion involves corrosion along the grain boundaries of the affected metal. The result is that the metal grains fall away and the metal is weakened. Austenitic stainless steels and precipitation strengthened aluminum alloys such as 2xxx alloys are examples of metals that can suffer from intergranular corrosion if the alloys are not properly processed and if they are exposed to corrosive environments.

Stress corrosion cracking involves the combined action of stress and exposure to a corrosive environment. In most cases, the stress or environment by themselves are insufficient to cause degradation of the metal. That is, the stress is below the metal’s yield strength and the metal would not corrode in the specific environment if the stress was absent.

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Dealloying is the selective leaching of one element from an alloy. This results in the formation of a porous structure that is not strong enough to support the applied mechanical loads. One common example is dezincification of brass alloys used for plumbing, where the zinc is leached out of the alloy.

Factors that influence corrosion mechanism

The specific type of corrosion that occurs depends on the several factors including metal composition, metal microstructure, environment, component geometry, stress on the component, contact between metals, and the manner in which components are joined together.

In some cases, the root cause of metal corrosion failures is selection of materials that are inherently incompatible with the environment. In other cases, corrosion is a result of mechanical design, where incompatible metals are joined together or components meet in a manner than results in narrow spaces between the components. Corrosion can also be the result of faulty manufacturing processes that result in microstructures that render an alloy susceptible to corrosion.

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Failure AnalysisHave you ever run into the following situation? A component within your product broke or your manufacturing line was producing bad components, and you wanted to determine the root cause of the failure. This required determining the failure mode and failure mechanism and whether there were any metallurgical deficiencies in the metal. So, you sent a sample to a metallurgical lab and got a report, but the report didn't have the information you needed or you didn't know what to do with the information in the report.

Failure analysis results

There are things you can do to prevent these problems from occurring, and improve your chances of determining the root cause of the failure. This article discusses how to work with a metallurgical lab to ensure the likelihood of getting the information needed to determine the root cause of a failure and to ensure that working with the lab is a positive experience.

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1) Provide samples and background information before asking for a quote.
It's difficult for a metallurgist to accurately quote the costs and time required to perform a failure analysis without getting a chance to visually examine the samples and without getting some information about the failure circumstances. The type of failure and the information needed by the client will are factors in determining which analyses will be required. Also, the size and shape of the samples and the materials that comprise the samples will influence the preparation required for the analysis and whether all the required analyses can be performed.

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2) Selecting the analyses to perform.
To many people, metallurgical failure analysis techniques are a mystery. Is scanning electron microscopy or a metallographic exam necessary? Many people don't know what to ask for when submitting a sample for failure analysis. The best thing to do is to provide the metallurgist with detailed background information about the failure and ask him to determine the failure mode and failure mechanism. Also, ask him to determine whether there were any metallurgical deficiencies that might have contributed to the failure. Metallurgical deficiencies include alloy composition, microstructure, tensile properties, and hardness that did not meet specification or were not appropriate for the application.

Let the person performing the failure analysis select the analyses needed to obtain the desired information. It's best not to try to steer the metallurgist in any direction or to select the analyses to perform without getting the metallurgist's input on the analyses required to get the information needed to determine the root cause of the failure. I've seen reports from metallurgical labs that provided the information the client requested, but did not lead to a complete understanding of the failure or its cause. Many labs will do what a client requests. It's best not to constrain the metallurgist by giving too much direction.

Also, ask the metallurgist to determine the root cause of the failure, if it is possible. Many times, it is possible for a metallurgist to determine the root cause of a failure. However, the ability to do this often depends on the background information you can provide.

3) Samples for analysis for manufacturing or assembly problems
For manufacturing or assembly problems, send samples of components or sub-assemblies that meet specifications, along with the samples that do not meet specifications. If needed, analysis results of the “good” samples can be used for comparison. Also, if a metallurgical exam has never been performed on “good” samples, the results will be helpful to verify whether the “good” samples are in fact metallurgically “good.”

4) I don't understand the report
What's transformed austenite? What's dimple rupture or cleavage? What's a grain boundary precipitate? Let's face it, metallurgists have their own language. It makes plenty of sense to us. Unfortunately, many reports require a translator. After reading the report, call the metallurgist and ask him to go through the report with you. Have him explain the results and what they mean. By the way, do this soon after receiving the report, when the analysis and results are still fresh in the metallurgist's mind.

5) Don’t expect the metallurgist to be able to determine the root cause of the failure
Assuming that it was possible to determine the failure mode and mechanism, you still need to figure out the root cause of the failure. It may be possible for the metallurgist to determine the root cause, if you provided enough background information about the failure. However, in many cases, especially for manufacturing and assembly failures, you will probably need to get more information about the circumstances leading to the failure. However, in many cases the information from the failure analysis will point you in the direction of where to look for the additional information.

6) Treat the metallurgist like a member of your engineering team
Find a metallurgist you're comfortable working with and treat her like a member of your team. Invite her to meetings about the failure. The information she gains from participating can be huge for helping her figure out the analyses required, the samples to analyze, and possibly the root cause of the failure. Too often, people keep the metallurgist in the dark, which can slow down the failure analysis and root cause analysis process. Remember, this person is supposed to be an expert

Successful failure analysis

A successful failure analysis results in getting information that leads you to the root cause of the failure. Following the advice in this article will increase the likelihood of getting the information you need, and make the process less frustrating, or maybe even enjoyable, if a failure analysis can be enjoyable.

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Interested in learning more about failure analysis and root cause analysis? Check out these courses, webinars, and videos that we offer Failure Analysis of Metal Fractures (video), Root Cause Analysis of Metal Problems (video), Metal failure analysis (course), Failure Analysis of Metal Fractures (webinar), Failure Analysis of Metal Problems (webinar),  Root Cause Analysis of Metal Problems (webinar)

Orange peel was present on bowls that were deep-drawn from low-carbon steel sheet. It was present on bowls produced from some batches of steel and not present on bowls produced from other batches of steel. Failure analysis of the bowls was performed as part of the effort to determine the root cause of the problem.

Orange Peel

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Failure analysis results

Orange peel is associated with the grain size of a metal. Therefore, cross-section metallography was used to examine the microstructure of bowls with and without the defect. The images below show the results of the analysis. The bowl without defects had small grains throughout the cross-section. The bowl with orange peel had large grains at the surface.

OrangePeel_grains

For a given amount of deformation, there is a limit to the maximum grain size before orange peel appears. In this instance, the grains at the surface were too large for certain batches of steel sheet.

Root cause

The grain size of metal is controlled through a combination of cold rolling and annealing. Proper control of annealing temperature, annealing time, and cooling methods after annealing are critical to obtain the desired grain size. The top surface of the sample with orange' peel was not properly cooled, which allowed the grains at the surface to grow too large.

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For more information

The roughened surface of large grained metals after forming is called orange peel because the surface has the appearance of the surface of an orange. See the article Orange Peel for more information about the subject, including how to prevent it from occurring.

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Interested in learning more about failure analysis and root cause analysis? Check out these courses, webinars, and videos that we offer Failure Analysis of Metal Fractures (video), Root Cause Analysis of Metal Problems (video), Metal failure analysis (course), Failure Analysis of Metal Fractures (webinar), Failure Analysis of Metal Problems (webinar),  Root Cause Analysis of Metal Problems (webinar)

rivet failure analysisProblem
Brass rivets were cracking during assembly, when the rivets were being set. Failure analysis of the rivets was performed as part of the process to determine the root cause of the cracking.

The rivets were manufactured by machining them from wire stock. The main requirements for the rivet material were:

  1. Easy to machine rivet from wire.
  2. Easy to form rivet during assembly.

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Meeting these two requirements put mutually exclusive constraints on the brass composition. Lead is added to improve machining, however lead reduces brass cold forming properties. The alloy selected had 0.9 to 1.5 % Pb.

Rivet Failure Analysis
Stereo zoom microscope (up to 70x) examination revealed that rivets were cracking at the portion deformed during the rivet setting process, when two components were being joined.

Composition analysis of cracked and uncracked rivets was performed using atomic absorption spectrocopy. The results indicated that uncracked rivets had less than 1.1% Pb and the cracked rivets had more than 1.3% Pb.

Conclusions
The root cause of the cracking was poor design. A new alloy was selected that had lower Pb. This required slowing down the machining process, but eliminated the cracking.

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The problem could have been prevented by conducting a failure modes and effects analysis (FMEA) when the product was being designed. An FMEA involves reviewing components, assemblies, and subsystems to identify failure modes, and their causes and effects. Had the design team performed a design FMEA, they may have identified the mutually exclusive behavior of brass with Pb. Information about how to perform an FMEAs is available in the publication "Potential Failure Mode & Effects Analysis", which is available from AIAG at www.aiag.org.

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Interested in learning more about failure analysis and root cause analysis? Check out these courses, webinars, and videos that we offer Failure Analysis of Metal Fractures (video), Root Cause Analysis of Metal Problems (video), Metal failure analysis (course), Failure Analysis of Metal Fractures (webinar), Failure Analysis of Metal Problems (webinar),  Root Cause Analysis of Metal Problems (webinar)

Product design and manufacturing often involves trade-offs to optimize product performance, reliability, and cost. One aspect of cost includes the time and effort required to perform the manufacturing steps. In the case of fabricating carbon steel components, free-machining steels are often used to improve ease of machining. However, if components made of free-machining steel must be welded together, then issues arise, and costs rise, because of the poor weldability of these steels.

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The machinability of carbon steel containing up to about 0.5% carbon is improved by the addition of sulfur. Most resulfurized steels contain 0.08 to 0.13% sulfur, though some alloys allow sulfur content as high as 0.35%. The sulfur combines with manganese to form manganese sulfide (MnS) inclusions within the steel. These inclusions enhance the formation of microvoids during machining, which leads to the formation of broken chips rather than continuous chips. However, steels are typically specified to have a maximum of 0.050% sulfur because steel weldability decreases with increasing sulfur content. So, the number of MnS inclusions is small in standard grades of steel.

aMnS_inclusions

Manganese sulfide inclusions in steel

Phosphorous and lead may also be added to resulfurized low carbon steel. Phosphorus increases the strength and hardness of the ferrite phase in the steel. This promotes chip breaking rather than the formation of long, stringy chips. Lead is present as soft particles that enhance the formation of microvoids during machining.

Welding Free-Machining Steel
Free-machining steel with sulfur and phosphorous has poor weldability because the MnS inclusions and phosphorous compounds have a lower melting point than the steel. As the weld metal cools and solidifies stresses start to build across the weld due to shrinkage. Due to the presence of the still molten low melting point compounds within the weld metal the metal grains tear apart under the shrinkage stresses, resulting in solidification cracks.

The major concern with lead is its toxicity because lead can melt during welding and volatilize into the weld fumes. Occasionally, lead may cause weld porosity and embrittlement.

Welding free-machining steel is usually inadvisable because of these welding problems. If one of these steels must be welded, special electrodes must be used to try to reduce the sulfur and phosphorus content of the weld metal. Also, low welding currents should be used to minimize base plate dilution. Still, there is no guarantee that these precautions will eliminate the problem. Sometimes the best that can be achieved is a reduced level of cracking.

The amount of cracking that is tolerable will depend on the required service conditions and reliability of the weld. For a weld joint that will be part of a structural member, any amount of cracking is unacceptable. For a weld joint that is part of a structure that will bear light or no loads, some amount of cracking may be acceptable.

Design solutions to prevent weld cracking
Another solution to this problem is to select a standard carbon steel alloy with a composition, microstructure, and hardness that enables optimization between ease of machining and ease of welding, while meeting the product performance and reliability requirements.

Low and medium carbon steel for machining consists of ferrite and spheroidized cementite or ferrite and pearlite. Ferrite is a soft material and cementite is a hard material. Pearlite is a composite consisting of plates of ferrite and cementite. The amount of cementite or pearlite in a steel increases as the steel carbon content increases.


Ferrite and pearlite. The dark phase is cementite. The light phase is ferrite.

aFerriteAndSperoidizedCementiteSpheroidized and partially spheroidized cementite in a ferrite matrix.

Ferrite can be readily cut and causes little tool wear. However, it contributes to the formation of a built-up edge on the tool because of its low hardness. The presence of large quantities of massive cementite particles can cause significant wear on a tool since cementite is very hard. Pearlite is harder than ferrite and generally causes greater tool wear, with wear increasing as pearlite plate spacing decreases. However, a built-up edge is less common when machining pearlite than when machining ferrite.

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A few different heat treatments are used to obtain the different microstructures. Normalizing and full annealing result in ferrite and pearlite. Among normalized and annealed steels, those with lower hardness and smaller amounts of pearlite can be machined at higher speeds for equal tool lives. Spheroidization annealing is used to obtain ferrite and spheroidized cementite.

The machinability of as-rolled or annealed low-carbon steel improves with increasing pearlite content and with smaller ferrite grain size because microvoids form at the interface between pearlite and ferrite. Maximum machinability of low-carbon steels is achieved at 0.15 to 0.25% carbon in the as-rolled or annealed condition.

With medium carbon steels, normalizing or annealing, combined with cold drawing give a slight increase in machinability compared to as-rolled cold-drawn steel.

Cold working increases the hardness of ferrite, which results in shorter chip lengths and less built-up edge on the tool. However, using cold-worked steel may require a stress relief heat treatment prior to machining to minimize distortion during machining.

Well-designed components are essential to every product’s success. Whether a component meets its performance and reliability requirements and is low-cost to manufacture depends on two things of equal importance:

Both mechanical form and metals can be engineered. Unfortunately, engineering a component’s metals is overlooked at many companies, leading to problems such as component costs over-budget, product launch delays, and supplier quality problems. Additionally, opportunities for innovation are missed.

Engineering component form and metal enables trade-offs for design optimization – meeting component performance and reliability at low total cost. Sometimes, selecting a more expensive alloy enables using less expensive fabrication processes, resulting in lower total cost.

Optimizing designs also includes thinking about ease of component fabrication. For any fabrication process, some alloys are easier to fabricate into components than others. Designing for ease of fabrication reduces costs and improves the supply base. Components that are difficult to fabricate result in high quotes from suppliers who want to be compensated for expected problems and the extra care required.

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Problems with the current approach

Focusing on engineering a component’s form and overlooking metals engineering leads to decision making without all the pertinent information and missing opportunities to improve designs. The result is sub-optimum results - components that fail to meet performance and reliability requirements, are more expensive than necessary, and are difficult to fabricate.

Also, overlooking metals engineering leads to inefficient problem solving for product failures and quality problems. The result is delayed and panicked product launches and lingering quality problems.

Metals engineering presents a transformative opportunity

Incorporating metals engineering is a transformative opportunity to make better, lower-cost products and increase innovation. It adds an important perspective to enable better-informed, faster decisions for product design, supplier evaluation, and solving quality problems. Also, it opens up new avenues of product design for better functionality or capability.

This may sound too good to be true, but it‘s not, and it isn’t novel. Some companies have been actively applying metals engineering for years. The only mystery is why so many manufacturers overlook the benefits it can provide, something I will discuss next month.

We help companies with metals engineering to design components and perform failure analysis for quality problems and component failures. Call or email if you would like to discuss a project. 847.528.3467 mike@imetllc.com

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There are many different types of wrought aluminum alloys used in a wide variety of applications such as wire for power distribution, automobile radiators, airplane fuselage, fasteners, and soda cans. The main criteria for selecting a particular alloy are strength, electrical conductivity, corrosion resistance, ease of manufacturing and assembly, and cost. This article discusses the effects of alloy composition on strength.

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There are several families of wrought aluminum alloys. Each family is based on specific major alloying elements added to the aluminum. These alloying elements have a large influence on the properties. The different families of alloys and the major alloying elements are

The first number in the alloy designation indicates the particular alloy family. Within each family there are different alloys based on the amounts of the major alloying elements present and the types and amounts of minor alloying elements that have been added.

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For the 1xxx group, 10xx is used to designate unalloyed compositions. The last two digits in the designation indicate the impurity content. Designations having a second digit other than zero indicate special control of one or more impurity elements.

For the 2xxx through 7xxx alloy groups, the second digit indicates alloy modification. A second digit of zero indicates the original alloy. Integers 1 through 9 indicate modifications of the original alloy. The last two digits have no special significance other than to identify the different aluminum alloys in the group.

The strength of aluminum alloys can be modified through various combinations of cold working, alloying, and heat treating. All the alloys can be strengthened by cold working processes such as cold rolling or wire drawing. Furthermore, except for 1xxx alloys, additional strength can be obtained by solid solution strengthening, dispersion strengthening, and precipitation strengthening. The particular strengthening mechanism possible depends on the alloy.

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Solid solution strengthening occurs in 3xxx and 5xxx alloys through the addition of manganese (3xxx) and magnesium (5xxx). Aluminum can hold more magnesium than manganese in solid solution. Consequently, greater solid solution strengthening is possible in 5xxx alloys than in 3xxx alloys. However, in the 3xxx alloys, the formation of Al-Mn-Si particles results in increased strength. These particles are obstacles to dislocation motion through the metal.

© DoITPoMS Micrograph Library, University of Cambridge.

The alloys can be divided into two groups based on whether the alloys can be precipitation strengthened. The 2xxx, 6xxx, and 7xxx alloys can be precipitation strengthened and the 3xxx, 4xxx, and 5xxx alloys cannot. Precipitation strengthening in 2xxx, 6xxx, and 7xxx alloys occurs through the formation of Al-Cu (2xxx), Al-Mg-Si (6xxx), and Al-Zn-Mg-(Cu) (7xxx) sub-micron sized particles in the alloys. The particles form as a result of a series of heat treating processes and are obstacles to dislocation motion through the metal.

The yield and tensile strengths possible in the different alloy families depends on the strengthening mechanisms available. The table shows the maximum nominal yield and tensile strengths for the different alloy families and the methods by which the strength is increased. There is a wide range of strengths possible with aluminum alloys.

Data from Aluminum Standards and Data 2009, The Aluminum Association

Data from Aluminum Standards and Data 2009, The Aluminum Association

Selecting a particular alloy requires consideration of other requirements such as electrical conductivity, corrosion resistance, ease of manufacturing and assembly, and cost. A future article will discuss the general properties of the alloys from the different alloy families.

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Stress corrosion cracking (SCC) is a fracture process that involves the combined and simultaneous action of a tensile stress and a corrosive environment. SCC occurs when the tensile stress and a specific environment are able to cause failure by their combined action, but are insufficient to cause failure by either one acting alone. In fact, the tensile stresses are usually below the metal’s yield strength. Furthermore, the metal would suffer only minimal corrosion in the absence of the applied stress.
StressCorrosionCracks
There are three requirements for SCC to occur:

  1. A susceptible metal.
  2. Tensile stresses applied to the metal.
  3. A specific environment containing an aggressive species that promotes SCC.

Elimination of any of these three factors will prevent SCC.

SCC occurs in specific combinations of metals and environments. The number of metal environment combinations that promote SCC is fewer than the number of metal environment combinations that will result in corrosion. Other environmental factors, such as pH and temperature, can also influence the severity of SCC.

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The source of the tensile stress may be externally applied stress or residual stresses. Externally applied stresses arise from applied mechanical loads such as tensile or bending loads. Residual stress is an internal stress that exists in a metal without an external load being applied. Residual stresses can result from cold working, heat treating, or welding.

Other than metal composition and the specific corroding species in the environment, other factors that influence SCC include total tensile stress on the metal, metal microstructure, and metal yield strength.

Increasing the yield strength of a metal is one way to improve its resistance to SCC because the threshold stress for SCC increases as the yield strength increases. The yield strength can be increased through alloying, heat treating, cold-working, and combination of these approaches. There is one very important consideration when increasing the yield strength. The increase in strength must not be accompanied by a significant reduction of the metal’s toughness, because decreasing the toughness will have a detrimental effect on a metal’s resistance to SCC and on its fracture toughness.

SCC can be controlled by any of the following three approaches: 1) design, which includes selection of the mechanical and materials aspects of components, 2) controlling the materials, 3) and controlling the environment.

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Also, the books Corrosion and Corrosion Control (4th edition) by R.W. Revie and H.H. Uhlig and Corrosion: Understanding the Basics by ASM International are good resources for information about the SCC and the different corrosion mechanisms.

Abstract: Annealing is a heat treating process used to modify the properties of cold-worked metal. This article discusses the reasons for annealing, the metallurgical changes that take place within a metal during cold working and annealing, the effects of these metallurgical changes on the properties of metals, and the effects of annealing temperature and time on the final microstructure and properties of annealed metals.

Many metal fabrication processes involve cold-working, such as cold rolling sheet and plate, wire drawing, and deep drawing. Due to metallurgical changes that occur to a metal during cold working, the ductility of a metal decreases as the amount of cold-working increases. There comes a point when additional cold working is not possible without causing the metal to crack. At this point, it is necessary to anneal the metal if continued cold-working is required.

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During an anneal, metallurgical changes occur that returns the metal to its pre-cold-worked state. These changes result in a reduction of the metal’s yield and tensile strength and an increase in its ductility, enabling further cold working. In order for these changes to occur, the metal must be heated above its recrystallization temperature. The recrystallization temperature for a particular metal depends on the metal’s composition. This specific annealing process is sometimes called a recrystallization anneal, though other names like process anneal are also used.

Metallurgical effects of cold working
During cold-working there is an increase in the number of dislocations in a metal compared to its pre-cold-worked condition. Dislocations are defects in the arrangement of atoms in a metal (discussed in Principles of Metallurgy).

The increase in the number of dislocations causes a metal’s yield and tensile strength to increase and its ductility to decrease.  After a certain amount of cold work, a metal cannot be cold worked further without cracking.  The amount of cold working that a particular metal can withstand before cracking depends on its composition and microstructure.

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Metallurgical effects of recrystallization anneal
During a recrystallization anneal, new grains form in a cold-worked metal.  These new grains have a greatly reduced number of dislocations compared to the cold-worked metal.  This change returns the metal to its pre-cold-worked state, with lower strength and increased ductility.

With continued time at the annealing temperature, some of the newly formed grains grow at the expense of neighboring grains.  There is some further decrease in strength and increase in ductility as the average grain size increases during the grain growth phase of the annealing process.

The final grain size depends on the annealing temperature and annealing time.  For a particular annealing temperature, as the time at the temperature increases the grain size increases.  For a particular annealing time, as the temperature increases the grain size increases.  A piece of metal with large grains has lower strength and more ductility than a piece of metal of the same alloy with smaller grains.

The figure shows micrographs of a brass alloy that was cold-rolled to 50% of its original thickness and annealed at two different temperatures.  Figure (a) shows the microstructure of the cold rolled sample.  Figure (b) shows the microstructure of a sample that was cold rolled and then annealed at 1022 °F (550 °C) for 1 hour.  Figure (c) shows the microstructure of a sample that was cold rolled and then annealed at 1202 °F (650 °C) for 1 hour.

The cold-rolled sample had a yield strength of 80 ksi (550 MPa).  The sample that was annealed at 1022 °F (550 °C) for 1 hour had yield strength of 11 ksi (75 MPa).  Many small grains are present in this sample.  The sample that was annealed at 1202 °F (650 °C) for 1 hour had yield strength of 9 ksi (60 MPa).  Fewer, large grains were present in this sample compared to the sample in Figure (b).

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Other reason for recrystallization anneal
In addition to enabling additional cold-working, recrystallization annealing is also used as a final processing step to produce metal sheet, plate, wire, or bar with specific mechanical properties.  Control of the annealing temperature and time, heating rate up to the annealing temperature, and amount of cold-working prior to anneal is important for obtaining the desired grain size, and therefore the desired mechanical properties.

Rockwell and Brinell hardness tests are common metal characterization methods used to determine whether metal stock or a metal component has the required properties. The reason for this is that these tests are simple and quick to perform, in addition to being inexpensive. However, while these tests do provide useful information, there is a danger to the common practice of specifying only hardness and alloy composition on component design drawings.

There are circumstances where a metal can meet the composition and hardness requirements, and still be unsuitable for use in the intended application. This can occur when the metal microstructure is deficient in a way that is undetectable by Rockwell or Brinell hardness testing.

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ferriteatgbsConsider the microstructure of a steel component. The microstructure consisted of martensite, with ferrite on what were austenite grain boundaries (arrows) before the austenite transformed to martensite. This was the result of improper quenching during heat treatment. The sample had a hardness of Rockwell C 38. However, the ferrite on the prior austenite grains compromised the mechanical properties of the metal by allowing a fracture path through the relatively soft ferrite. Without specifying that the microstructure should be completely martensitic, the component designer risked using components that met the spec, but were potentially unreliable.

Other examples where it is possible to have a Rockwell or Brinell hardness value that meets specification, but still have a component that is unreliable or unacceptable are:

Want to learn more about metallurgy of strength and hardness? See our metallurgy courses page for training options.

The main thing to take away from this discussion is that specifying composition and hardness is often not enough. In many cases, it is important to specify the microstructure of stock metal or a component to improve the likelihood of consistently getting the desired metal properties. The importance of this increases as the expected component performance and reliability increases. Other analyses, such as tensile testing may also be required.

More information about the effects of microstructure on properties and the effects of processing on microstructure is in our Principles of Metallurgy, Metallurgy of Steel Heating, and Corrosion of Metals courses. There are also many books that discuss the relationship between processing, microstructure, and properties.

Need help selecting an alloy for a component? We can help. See our metallurgy consulting page for information.

This is the second article in a series of posts on the materials selection process. The first article gave an overview of the entire process for selecting a material to use for a component or a joint between components. This article discusses the first step of the process - identify the design requirements for the component or joint. As a reminder, here are the steps for the materials selection process:

  1. Identify the design requirements
  2. Identify the materials selection criteria.
  3. Identify candidate materials.
  4. Evaluate candidate materials.
  5. Select materials.

Need help selecting an alloy for a component? We can help. See our metallurgy consulting page. info@imetllc.com

The design requirements

Here is a list of the categories of the requirements to consider when selecting a material for a component or a joint between components:

  1. Performance requirementsSelection criteria
  2. Reliability requirements
  3. Size, shape, and mass requirements
  4. Cost requirements
  5. Manufacturing requirements
  6. Industry standards
  7. Government regulations
  8. Intellectual property requirements
  9. Sustainability requirements

Below is an explanation of each category of requirements

Performance Requirements
The performance requirements describe the attributes that the component or joint must have to function as required. The attributes can be described in terms of mechanical, electromagnetic, thermal, optical, physical, chemical, electrochemical, and cosmetic properties.

Reliability Requirements
The reliability of a component or joint refers to its ability to function as required over a specific use period when exposed to a specific set of use conditions. A component or joint fails once the material degrades to the point where the component or joint no longer performs as required. The reliability requirements describe the use conditions to which the materials will be exposed and the expected response of the materials to the use conditions. Examples of use conditions are exposure to high temperatures, salt water (corrosion), and vibration.

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Size, shape, and mass requirements
The size, shape, and mass requirements for a component or joint will have a huge influence on the materials that can be used. Consider a component that must carry five amperes of current without heating up by more than 15o C above the ambient temperature. The electrical conductivity for a component with a 1 mm diameter must be about four times greater than the electrical conductivity for a component that can be 2 mm in diameter. A bicycle frame that must weight 10 pounds must have frame tubes made of a lower density material compared to a 20 pound frame. For a component that must support 200 pounds, the yield stress for the material in a component that must be 0.20 inches diameter must be much greater compared to the material in a component that can be 0.50 inches in diameter.

Cost requirements
The cost to form a component or joint or purchase a component depends on 1) the materials that comprise a component or joint, 2) the manufacturing processes used to form a component or joint, 3) whether a component is custom made or purchased “off-the-shelf supplier”, 4) the quantity of materials or components being purchased and 5) quality problems associated with a material or component. If you want to reduce costs, consider what will be required from the materials engineering perspective to make manufacturing process changes that address items 2 and 5.

Manufacturing requirements
Companies may require that specific processes be used for fabricating components and building assemblies or sub-assemblies. Perhaps a company has internal manufacturing capabilities that must be used or a company is familiar and comfortable with component or joints fabricated using a familiar manufacturing process.

Restrictions on the processes that can be used to build a product will restrict the materials that can be used to make components because the materials must be compatible with the processes and other materials used to make the product. For example, components to be joined using a specific welding, brazing, or soldering process must be made of materials that enable good joints to be formed using the specific joining process. This may exclude off-the-shelf components from one or more suppliers because their components are made of materials that are incompatible with the process. For a custom component, the restriction may require the use of certain materials in order to form a good joint.

Want to learn about metals engineering considerations for component design? See our metallurgy courses page for training.

Restricting the manufacturing process to only familiar ones will restrict the options of materials that can be used to form a component or joint since many manufacturing processes are limited to processing certain materials. In some respects manufacturing constraints are acceptable, and may in fact be desirable, since the use of familiar processes and materials reduces the risk associated with a change or new product. However, in cases when a new product is significantly different than older products, the constraints of using specific manufacturing processes may seem to be a burden.

Industry standards
There are industry standards concerned with the performance and reliability of components and joints. In some cases, a specific standard will discuss component and joint requirements. For component specific standards, the standards discuss

Government regulations
Government regulations regarding the materials used in a product are typically related to requirements on the materials from which components and joints can and cannot be made. The requirements address the materials that can or cannot be used in a component or joint and the expected quality and reliability of the materials for specific applications. Every country has its own set of regulations.

Intellectual property requirements
There are many patents regarding the design and manufacture of component or joints. If a patent is found that is applicable to the component or joint being selected or designed, then the design team has to decide whether to license the patent or engineer the component or joint order to avoid conflict with the patent.

Sustainability requirements
These requirements restrict the materials that can be used in components and joints to materials that can be re-used or recycled. The requirements might also restrict the manufacturing processes than can be used to form components and joints to processes that do not harm the environment and do not use chemicals and materials that are manufactured using environmentally unfriendly processes. The sustainability requirements for a product become the sustainability requirements for its components and joints.

Time and money; Focus and discipline

This list of different types of requirements to consider might seem long. It might take a bit of time to come up with a complete description of all the requirements for your next project. However, it will speed up the process of evaluating materials and suppliers that are identified based on the complete set of requirements. Consider the time and money associated with evaluating materials and suppliers that are found to be unsuitable.

It takes a bit of focus and discipline to implement this process, but the rewards of fewer problems and faster implementation or design are well worth it.

There are several situations during the life cycle of a product when a design team selects a material to use for a component or a joint between components - new product development, cost reduction, improve product performance and reliability, and improve manufacturing or assembly yields. Regardless of the situation, the goals are the same – find the lowest cost material that enables the product’s performance and reliability. There are several steps to the material selection process. This article provides an overview of this process.

materials selectionThe performance, reliability, and cost of any product depends on the performance, reliability, and cost of its components and the joints between components. And the performance, reliability, and cost of components and joints depend on two things: 1) their physical construction and 2) the materials of which they are made. Physical construction refers to shape and dimensions. Examples of different physical constructions are shown in the figure for two different size shafts and two different types of weld joints.

Need help selecting an alloy for a component? We can help. See our metallurgy consulting page for information.

Materials selected impact product success

So, the materials used in a product have a huge impact on the product’s success. Select materials with properties that don’t enable meeting the product’s performance or reliability requirements and be prepared to have poor sales or many returns. Select materials that are more expensive than necessary or are difficult to work with during manufacturing and assembly and be prepared to endure lower than expected profits.

Materials selection process

These problems can be avoided by adopting a rigorous approach to the materials selection process, with an eye on selecting materials that optimize product performance reliability and cost. The materials selection process for a component or joint between components involves these steps:

  1. Identify the design requirements
  2. Identify the materials selection criteria.
  3. Identify candidate materials.
  4. Evaluate candidate materials.
  5. Select materials.

While each step might seem obvious, there are many organizations that do not have the structure in place to follow each step. Consequently, they end up selecting sub-optimum materials. The remainder of this article gives a brief overview of each step of the materials selection process. Future articles will provide more details about each step of the process.

Step 1: Identify the design requirements
The design requirements include the following items:

Identifying as many of the requirements as possible is critical for increasing the likelihood of learning whether potential materials exist. For many products, some of these requirements are not applicable, making the information gathering process easier. Regardless, as the number of requirements increases, the chance of finding a set of potential materials decreases.

Need help with a component failure or quality problem? We can help. See our failure analysis page. 847.528.3467 info@imetllc.com

Step 2: Identify materials selection criteria
The materials selection criteria are specific materials properties derived from the requirements identified during Step 1. For example, for a component that must support a specific load, the minimum yield stress that is required for the component’s material can be determined. This will be one of the material selection criteria.

Step 3: Identify candidate materials
Use the materials selection criteria to rule out materials that will not satisfy all the materials selection criteria. When evaluating whether a material might be appropriate for the application, be sure to consider the materials’ range of values for the properties of interest. Do not rely upon nominal properties values.

Step 4: Evaluate candidate materials
There may be candidate materials for which there insufficient data available to indicate whether the materials satisfy certain selection criteria. These materials will have to be analyzed and tested to determine whether they do meet the selection criteria.

Want to learn more about metals engineering and component design? See our metallurgy courses page for training options.

Step 5: Select materials
Select the materials that satisfy all the materials selection criteria at the lowest cost. Remember, cost includes the cost of the material and the cost to fabricate a component or form a joint between components.

Detailed information about the materials selection process is in my textbook Materials Enables Designs: A Materials Engineering Perspective to Product Design and Manufacturing.

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