LPW Technology has published the findings of a study exploring the effect on atomising gases on the properties of metal powders in additive manufacturing processing.
Specifically, LPW compared Nitrogen and Argon atomisation of 17-4 PH stainless steel. 17-4 PH is a strong, ‘Martensite’, grade of stainless steel widely used in aerospace, petroleum, and chemical processing industries. ‘Martensite’ refers to a very hard form of steel crystalline structure. Its name comes from Adolf Martens, a German metallurgist.
The effect of atomising gas on the properties of metal powders, in conjunction with the specific additive manufacturing process parameters, plays a key role in the ultimate microstructure and mechanical properties of fabricated parts. Using an inappropriate combination of powder and process can result in the expected final properties of a component not being fully realised.
17-4 PH can be atomised using argon or nitrogen, but the resulting microstructure must be martensitic to deliver the optimum mechanical properties.
Table showing test plan and resultant microstructure of 17-4 PH stainless steel with four different builds.
The additive manufacturing solution provider found when the powder is both atomised and processed in a nitrogen atmosphere it is plausible that the nitrogen content increases to the point at which it starts to inhibit the ‘Martensite’ formation. LPW summarised that, although the two feedstock materials had only small differences in chemistry, the change in powder production processing route and SLM fabrication conditions resulted in significant differences in the final mechanical properties. This was attributed to different microstructures obtained using different processing routes.
“It’s important to understand the effect that powder production methods and AM processing parameters can have on the resulting component microstructure,” said Dr Rob Deffley, LPW’s Research & Development Manager. “LPW’s highly-trained team understands the behaviour of powders processed under different conditions, and can recommend the appropriate powder to achieve the required mechanical properties. This latest case study addresses a question we have been asked by several AM end users, and adds further intelligence to our portfolio of technical information.”
The chemistries of both atomisation gases, nitrogen and argon, were within the 17-4 PH specification. Preliminary powder cross sections of the argon atomised powder revealed a martensitic microstructure, while the nitrogen atomised powder was austenitic. Four total builds were completed to study the effect of powder atomisation process and machine environment on the resultant microstructure. All builds that featured argon as either the atomisation gas or SLM processing gas, or both, had a martensitic structure. However, the build which featured Nitrogen as the atomisation gas and processing gas had an austenite structure.
LPW Tech Gases
Comparison of material hardness 'as-built' and after heat treatment.
Additionally, the hardness of each build was measured in Vickers Pyramid Numbers (HV). Vickers Pyramid Numbers represent the strength of a material by its resistance to indentation. The hardness of all builds that featured argon as either the atomisation gas, processing gas, or both, reached over 400 HV following heat treatment. Meanwhile, even after heat treatment, the build with nitrogen atomisation and processing remained at 235 HV.
LPW’s findings prove that increasing the concentration of nitrogen, a small and interstitial element, even by small amounts, will reduce the amount of Martensite at room temperature and thus affect the heat treatability of the material.