Subgrain microstructures and tensile properties of 316L stainless steel manufactured by selective laser melting

316L stainless steel samples were manufactured by selective laser melting(SLM).The microstructure of SLM-made 316L stainless steel and the room temperature tensile properties both perpendicular and along the building direction were studied and characterized.The static temperature field during the molten pool formation was simulated by finite element simulation.It indicates that the nonlinear asymmetrical inclined temperature gradient in SLM process produces a large surface tension gradient.The melt forms a Marangoni flow with different convection modes under the action of surface tension as well as a micro-molten pool morphology with subgrain structures such as strip,hexagonal and elongated cellular structures.In addition,there are also epitaxially grown columnar grains.The growth of columnar crystals is not affected by the boundary of the molten pool.Subgrain structures and low-angle grain boundaries make the tensile strength and the elongation of SLM-made 316L sample higher as compared to those of the cast and wrought samples.The room temperature tensile strength of the sample perpendicular to the building direction is higher than that of the sample along the building direction,while the elongation is lower than that of the sample along the building direction.

Heat transfer performance of porous titanium

Porous titanium fibre materials with different structural parameters were prepared by vacu- um sintering method. The thickness, porosity and wire diameter of prepared materials were investigated to understand the effects of structural parameters on pool heat transmission performance of titanium fibre porous material. As a result, better heat transfer performance is obtained when overheating is less than 10 ℃. In addition, when the wire diameter is smal- ler, the heat transfer is better. However, when superheating is above 10 ℃, heat transfer performance can be improved by increasing the wire diameter. Moreover, thickness influ- ences the superficial area of the prepared material and affects the thermal resistance when bubbles move inside the material; superficial area and thermal resistance are the two key factors that jointly impact the heat transfer in relation to the thickness of the materials. Ex- perimental results also show that the materials of 3 mm in thickness exhibit the best per formance for heat transmission. Furthermore, changes in porosity affect the nucleation site density and the resistance to bubble detachment; however, the nucleation site density and the resistance to bubble detachment conflict with each other. In summary, the titanium fi- bre porous material with a 50% porosity exhibits suitable heat transfer performance.