Qualifying Materials for AM 

Additive Manufacturing introduced us to a new way of creating parts, but it also introduced us to a new way of creating materials. This was immediately followed by many mistakes and failed attempts to qualify materials that were produced by AM. These mistakes were all caused by the expectation that AM materials should be the same as any other well-known form of materials. In the case of metals, this was usually the cast or wrought forms of the alloys concerned. AM parts have been continually tested and rejected because their properties were not the same as when produced from either of these methods.

These mistakes can be avoided by realising that AM materials are in fact entirely different from any other conventional form of materials. Their microstructures will be different, and sometimes their chemistry, or composition will be slightly different; even if based on the same alloy as designated by an already existing naming convention. So for example, from the Unified Numbering System (UNS) designation A96061, this alloy can be referred to as 6061 when it is in the wrought form of that particular aluminium alloy. It has well-understood materials properties in each of the conditions that it can be supplied in. Parts formed via any of the AM technologies from feedstock materials with the same chemical composition as A96061 will not have the same microstructure as any of the supplied forms of wrought 6061, so it should be easily understandable that the properties will also be different. Once this is understood, then moving to qualify the AM materials should become a little less troublesome.

The basic steps prior to qualifying materials for AM 

It is essential to get the basics right and to not go off blindly trying to qualify materials for AM. Mostly importantly, an open mind is required to accept that you may be required to do things differently, and that you may have to compromise by following certain existing processes that are not optimised for dealing with AM materials. A certain amount of standard testing will be required to establish a starting point in the understanding of the materials properties. Here it cannot be assumed that ince a particular alloy has already been studied via one form of AM technology that the properties will be the same , or similar via another AM technology.

• Understand the historical context of the material of choice.
• Identify the baseline materials properties in the “as-built” condition.
• Assess the application design requirements.
• Understand the differences between materials design requirements and any as-built properties. 
• Identify opportunities to alter the as-built properties via post-processing, including heat treatment. 
• Build and test to gather statistical relevant data.

The next steps to Materials Qualification (MQ) for AM 

Materials Qualification (MQ) is perhaps one of the most commonly misunderstood requirements in the application of additive manufacturing, even more so when certification is required. MQ is often seen as something that is very time consuming, costly, and involving many processes. Here, the simple truth is that MQ should be what it needs to be for the application, nothing more and nothing less. MQ is a particular subset of the Production Qualification (PQ) process, and can be left out entirely if deemed not to be on the critical path of the part, or parts, concerned. It’s important to assess the application design requirements as part of a well documented process for qualification when choosing the correct route for MQ.

As an example, the chemical composition of a final part may allow for an oxygen concentration of 0.3wt.%,. So, in this case, there is usually very little point in setting a requirement in either MQ or PQ processes of trying to achieve 0.01wt.%. However, this type of false target can sometimes creep into requirements when broad statements like, “the oxygen concentration needs to be as low as possible”, are coupled with advertised system performance data such as, “capable of achieving an atmosphere of 100ppm oxygen”. Referring to the initial design requirements would ensure that MQ is controlled by design and not hearsay.

From the basic steps detailed above it is essential to establish performance data following the selection of the desired materials change steps. By this we mean, in post-processing the AM materials was it necessary to change the surface condition via another process, or was it necessary to change the microstructure via heat treatment? The statistical data gathered during the trials should allow for the selection of the correct post-processing route for any given application. Thus MQ becomes a matter of gaining confidence in that process, and proving that it can be repeated with the degree of reliability required by the application. Note, no comparison is being made here to any other form of materials property data for wrought, cast or forged materials.

• Determine the materials properties that are required for each specific application, and determine the specification minimums
• Establish and document the post-processing route to obtain the required properties.
• Build and test to gather statistical relevant data to prove repeatability reliability. Note this may have to be repeated for each change in geometry since some AM technologies are more sensitive to the part layout than others.
• If necessary this should be repeated for each machine used in a production series since each machine should be certified as being capable of producing the same quality of material.

NOTE: A more simplified version of MQ can be used if tied to the initial Operation Qualification (OQ) of a system. Here, universally accepted materials properties values are set as the acceptable values for qualification. In this case, no application specific requirements are set for the materials properties and post-processing follows a predefined set of steps. This is the situation where an international materials standard may be used for MQ.