Laser welding behavior of laser powder bed fusion additive manufactured 304L stainless steel

Abstract

Additive manufacturing (AM) is becoming of great interest due to the ability to create one of a kind, complex components in a relatively rapid manner. However, applications of AM components are limited because little is known about the welding behavior of such components and, therefore, these components are limited to applications where a joint is not required. The laser welding behavior of AM 304L is compared to wrought 304L since the weldability of Type 304L is well known. Spot weld and bead-on-plate laser welds were generated using a 1070 nm Ytterbium-fiber laser with varying parameters to create a range of microstructures. Laser-matter coupling is typically a function of alloy composition, surface roughness, and process parameters. Thus, laser coupling efficiency was measured for 10 ms spot welds made in wrought 304L, AM 304L, and wrought 17-4 martensitic stainless steel. The coupling efficiency was found not to depend strongly on microstructure, composition, or residual stress. The variables that had the largest impact on coupling efficiency was laser power and surface roughness. As laser power increased, the coupling efficiency increased. A higher surface roughness allowed for multiple reflections to take place at lower laser inputs, thereby increasing the coupling efficiency. The welding behavior of AM 304L stainless steel was similar to wrought 304L stainless steels at low laser powers and short welding times. The weld widths and weld depths for spot welds and bead-on-plate welds made in wrought and AM 304L were similar. However, for bead-on-plate welds differences occurred at higher laser powers and low travel speeds. At higher heat inputs, the conduction region of welds made in AM 304L were “V” shaped while welds in wrought 304L were “U” shaped. The keyhole width was wider in welds produced in AM 304L compared to welds made in wrought 304L. The weld solidification behavior of AM 304L is different from wrought 304L. Spot welds produced in AM 304L exhibited dual solidification modes of primary austenite and primary ferrite modes, while spot welds produced in wrought 304L exhibited only one solidification mode, which was primary ferrite mode. The weld microstructures were controlled by composition and solidification rates which resulted in different solidification behavior. The solidification rate increased as the solid/liquid interface approached the center of the weld. Given the compositional differences between wrought and AM 304L, an increase in solidification rate shifted solidification mode from primary ferrite to primary austenite. The composition of wrought 304L was such that primary ferrite solidification was maintained with increasing solidification rate. For bead-on-plate welds, the microstructure of welds made in AM 304L solidified as primary austenite as planar and cellular solidification. Due to solute segregation and an increase in solidification rate, the weld shifted to solidify as primary ferrite dendrites that transformed to austenite with discontinuous vermicular and lathy ferrite. Bead-on-plate welds produced in wrought 304L, solidified as primary ferrite dendrites that transformed to austenite with continuous networks of vermicular and lathy ferrite.