Microstructure – texture - mechanical property correlation in additively manufactured stainless Steel 316L and Cu-Ni-Sn

Abstract

Metal additive manufacturing (AM) has gained considerable interest in recent years because of its ability of near net shape building of the components, one-go production, ability to make intricate designs and minimized wastage. Further, the mechanical properties of the manufactured alloys are often superior and governing factors for this comes because of excellent metallurgical benefits that are attributed to high cooling rate and steep thermal gradient. The notable metallurgical benefits offered are highly refined microstructure, increased dislocation density, enhanced solid solubility, presence of crystallographic texture. Even though there are notable achievements reported as mentioned, certain mysteries are yet to get resolved. The evolution of microstructure and texture and their correlation to important mechanical performance is yet to be understood. The role of heat treatment in altering the microstructure – mechanical property correlation also needs to be re-visited. The present thesis primarily focusses on the microstructure - crystallographic texture - mechanical property correlation in additively manufactured materials prepared through selective laser melting and wire arc additive manufacturing. While most of the investigations were carried out on stainless steel 316L, a heat treatable alloy Cu-15Ni-8Sn is also investigated. Chapter 1 deals with the general introduction on metal additive manufacturing, its benefits and various types. A brief overview of available literature is presented for SLM and WAAM process related to the evolution mechanism for microstructural features such as solidification structure, dislocation density, crystallographic phases, texture and their overall consequence on mechanical properties. Chapter 2 details about the materials and method used in carrying out the investigation. Chapter 3 addresses the evolution mechanism behind microstructure, increased dislocation density, formation of residual stresses in additive manufacturing. This investigation has been conducted through a careful experimentation where deconvolution is done for the primary contributors such as thermal gradient and cooling rate in microstructure. It has been observed that repeated heating/cooling leads to accumulation of dislocation density and residual stress. It has been established that difference of melt droplet transfer mode in SLM and WAAM generates textures with extra <112> orientation in WAAM in addition to usual <100> and <110> orientation developed during SLM. Chapter 4 deals about the effect of SLM manufacturing variables hatch style (dividing individual layers into sub-parts and manufacture these in a sequential manner) and interlayer hatch rotation. The role of these variables on controlling the microstructure – texture – mechanical property is discussed. It has been observed that texture control can be enacted by varying interlayer hatch rotation or hatch style. In chapter 5, a comparison of tensile properties as well as fracture toughness is done between SLM and WAAM processed sample in the as built state and the mechanism has been established. A comparison of thermal stability of as-built microstructure is also done between these two routes. The effect of heat treatment on microstructure - mechanical property relationship is also explored. It has been observed that increased propensity of twin formation, presence of melt pool boundaries whereas presence of δ-ferrite enhances the fracture toughness in SLM and WAAM processed stainless steel 316L, respectively. In chapter 6, the benefits of AM in enhancing the mechanical properties is studied for a case when a different hardening mechanism become active, other than what has been observed for stainless steel 316L, that is, slip or twinning. The response of heat treatment in one of the popularly spinodal and age hardened Cu-15Ni-8Sn alloy manufactured through SLM has been explored and compared with solutionised and heat-treated sample. It has been observed that heat treatment in additively manufactured Cu-15Ni-8Sn gives superior mechanical properties and it is associated with highly refined spinodal decomposed structure, fine precipitates in addition to refined as built microstructure. Chapter 7 lists the overall conclusions of the present thesis explored through experimental investigations. The suggestions for future work have also been given.