On the applicability of selective laser melting on pistons for the oil & gas industry

Abstract

Pistons for reciprocating compressors for industrial applications are often made of specialised materials. These prove to have problems with manufacture due to the high quality and short production times needed in combination with a low production volume per design. Additive manufacturing, specifically selective laser melting, could solve the production problems, provided that the material retains the needed mechanical properties. Ti6Al4V is the most appropriate material for this application. The most important mechanical properties for the application are the fatigue limit and the stress intensity factor, which are not well established properties for printed materials. For this reason fatigue limit and stress intensity factor tests were performed for both stress relieved and hot-isostatic pressed test pieces on longitudinal and transverse directions. Strength, toughness and fatigue limit is higher in hot-isostatic pressed test pieces of Ti6Al4V, and these are proven to be appropriate for application in compressor pistons. However the fatigue limit of stress relieved Ti6Al4V is lower, anisotropic, and has more scatter, and as such is insufficient for the application, which is due to deleteriously oriented microstructure and the presence of porosities. This can be solved by changing the printing parameters – laser power, cooling rate or heat treatment – although the exact combination of parameters for optimised values is not known and will be part-specific.