It should be simple to select a material for metal additive manufacturing (AM), after all, most materials are metals in powder form. But it’s not as simple as it seems, claims Renishaw, a company that has been working with metal alloys in additive manufacturing for some time.
There are limitations in terms of the physical properties of the alloys that are available. Even though there are internationally recognized standards for metal alloy composition, there are proprietary versions of many alloy powders.
Renishaw offers these tips to consider when selecting a metal material:
1.Read metal performance documentation carefully. The tests used to document performance may not give the information you seek. For example, many early adopters of additive technology made do with vague static tensile test values from samples that did not undergo heat treatment or another stress relieving cycles.
Where values are given for tensile strength, know that this is an indication to permit some level of comparison. Therefore, treat this as an introduction.
Where fatigue data has been published for commonly used alloys, chances are these data were acquired from rotating bending fatigue tests, the method favored for achieving quick qualitative results.
2. Post-build heat treatment. The selection of an appropriate post-build heat treatment cycle can be critical to achieving desired final properties in metal materials. Most additive parts, for example, have properties that are anisotropic in at least the directions parallel and perpendicular to the build plane.
This characteristic may not necessarily mean something bad, or that the parts cannot be used but simply that these properties need to be fully understood. Much in the same way as understanding the effects of either hot or cold rolling on wrought alloys or the effects of different grain sizes from sand casting or high pressure die casting.
3. The problem with “wrought.” Parts built with the metal AM process have been described as having material properties better than sand cast parts but not usually as good as wrought materials.
To understand this better, think of the laser bed powder fusion process as delivering a massive number of overlaid welds, which can mean that the weld metal has a different microstructure and material properties to the base metal and that there are more differences in the heat affected zones (HAZ). In a metal AM created part there are thousands upon thousands of micro-weld metal regions, and even more heat-affected zones. Therefore, materials properties will be different, so saying that properties are similar to wrought is simply inaccurate.
A laser rapidly melts the powder, which then solidifies quickly. The resulting metallographic grain size can vary significantly and is dependent on many process parameters. The grain size and microstructure will determine the mechanical performance of the final built object. Most system vendors spend a great deal of time attempting to optimize the laser process parameters to achieve reliable and repeatable materials properties.
4. Post-processing methods, such as heat treatment (including hot isostatic pressing), can affect the mechanical properties of the alloy.
5. Not all alloys are the same. There is a range of allowable compositions with all alloys that are described by the relevant international standard for the alloy. This means there can be small changes in alloy composition from one vendor to the next, which can lead to significant variation in the materials properties. However, the final properties very much depend on the exact heat treatment history of the parts concerned.
For example, a common mistake was to insufficiently cool parts made from 17-4PH to obtain a fully martensitic structure prior to carrying out the age hardening. This led to significantly lower than expected mechanical properties and the false belief that this stainless steel could not be used to produce quality parts with AM.
6. Heat treatment, part 2. Early investigations found that the corrosion resistance of the AM produced parts, after just an age hardening cycle, were vastly inferior to the conventionally wrought equivalents. This points out that the heat treatment is a critical step in the post-processing chain for these steels if the best performance is to be achieved.
This information is excerpted from a Renishaw paper, The status quo of metal alloys for additive manufacturing.