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Selecting a Material: Getting it Right the First Time

Michael Pfeifer, President, Industrial Metallurgists, LLC

There are several situations during the life cycle of a product when a design team selects a new material to use for a component or a joint between components. In addition to selecting materials during new product development companies also change materials to reduce costs, improve product performance and reliability, and improve manufacturing yields. Regardless of the situation, the goals are the same – find the lowest cost solution that poses no risk to the performance and reliability of the product. There are several steps to the material selection process. This article discusses the first step – defining the requirements for the material.
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“It’s just a simple request”

At least once a week someone calls asking for a recommendation for a material to make a component or form a solder, braze, or adhesive joint. They may be looking for a lower cost material, a material that’s easier to work with, or a material for a new design. In any case, at the end of many of these calls the caller has some homework to do.

During the course of the call I ask about the various requirements that the materials must satisfy. I want to make sure that any options I recommend have a good chance of working, and if there are obvious risks I can mention them. What often happens is that the caller has not identified all of the requirements, and my lawyer has forbidden me from making any recommendations without knowing all the requirements.

By the way, people also call up suppliers asking for recommendations of materials. However, most suppliers don’t ask about all the relevant requirements, and they’re usually all too happy to make a recommendation and leave it to the caller to determine whether the material will really work.

So, to save yourself time when you call me or to improve the chances of getting a low-risk recommendation from a supplier, you should put together a list of all the requirements for the component or joint you want to make.

The 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:

Performance requirements
Reliability requirements
Size, shape, and mass requirements
Cost requirements
Manufacturing requirements
Industry standards
Government regulations
Intellectual property requirements
Sustainability requirements

Requirements Descriptions

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.

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 15^o 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.
. 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

The size and shape of components used for specific applications.
The materials that can and cannot be used for components used for specific applications.
The tests required to verify the properties of the materials used to make a component.

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.