Weldability, microstructure and mechanical properties of laser-welded selective laser melted 304 stainless steel joints

Laser welding is a promising process for joining small components produced by selective laser melting (SLM) to fabricate the large-scale and complex-shaped parts. In the work, the morphology, microstructure, microhardness, tensile properties and corrosion resistance of the laser welded stress-relieved SLMed 304 stainless steel joints are investigated, as the different sections of stress-relieved SLMed 304 stainless steel are joined. Results show that the SLMed 304 stainless steel plates have a good laser weldability. The microstructure of laser-welded joints consists of the cellular dendrites in austenite matrix within columnar grains, exhibiting a coarser dendrite structure, lower microhardness (~220HV) and tensile properties (tensile strength of ~750 MPa, and area reduction of ~27.6%), but superior corrosion resistance to those of SLMed plates. The dendrite arm spacing of the joints varies from ~3.7μm in center zone, to ~5.0 (xm in fusion zone, to ~2.5 in epitaxial zone. The SLMed anisotropy shows a negligible effect on the microstructure and performance of the laser-welded joints. The laser welding along the building directions of the SLMed base plates can induce a slightly finer dendritic structure and higher tensile properties.

Cracking behavior and control of β-solidifying Ti-40Al-9V-0.5Y alloy produced by selective laser melting

Aβ-solidifying Ti-40 Al-9 V-0.5 Y(at.%)alloy with a high cracking sensitivity has been successfully fabricated by selective laser melting(SLM)in this study.The influence factors for cracking sensitivity,cracking behavior and crack inhibition mechanism were investigated.The results show that the effects of process parameters on cracking sensitivity strongly depend on the cooling rate in molten pool with different heat transfer modes.The conduction mode with higher cooling rates exhibits a higher cracking sensitivity in comparison to the keyhole mode.Microstructure characteristics and phase transformations controlled by cooling rate determine the inherent ductility ofβ-solidifyingγ-Ti Al alloys during SLM.On this basis,the formation and inhibition mechanism of solidification and cold cracking are proposed.Finally,the crackfree Ti-40 Al-9 V-0.5 Y sample with fine equiaxed microstructures and favorable mechanical properties(microhardness of 542±19 HV,yield strength of 1871±12 MPa,ultimate strength of 2106±13 MPa and ultimate compressive strain of 10.89±0.57%)can be produced by SLM.The strengthening mechanism can be attributed to grain refinement and precipitation strengthening.

Microstructural evolution and mechanical properties of 300M steel produced by low and high power selective laser melting

300 M ultra-high strength steel has been widely used in critical structural components for aviation and aerospace vehicles,owing to its high strength,excellent transverse plasticity,fracture toughness and fatigue resistance.Herein,low and high power selective laser melting(SLM)of 300 M steel and their microstructural evolution and mechanical properties have been reported.The results show that the optimal energy density range with the highest relative density for SLMed 300 M steel is between 60 and160 J/mm^3.Furthermore,molten pools for deposition exhibit a conduction mode with semi-elliptical shape at a lower laser power of 300~600 W but a keyhole mode with"U"shape at a higher laser power of 800~1900 W.The heterogeneous microstructure of as-built samples is cha racterized by a skin-core structure which is that tempered troostite with the coarse non-equiaxed grains in the molten pool is wrapped by tempered sorbite with the fine equiaxed grains in the heat-affected zone.The skin-core structure of SLMed 300 M steel has the characteristics of hard inside and soft outside.The average microhardness of samples varies from 385 to 341 HV when laser power increases from 300 to 1900 W.Interestingly,ultimate tensile strength(1156-1193 MPa)and yield tensile strength(1085-1145 MPa)of dense samples fabricated at diffe rent laser powers vary marginally.But,the elongation(6.8-9.1%)of SLMed 300 M steel is greatly affected by the laser power.

Effects of circular beam oscillation technique on formability and solidification behaviour of selective laser melted Inconel 718:From single tracks to cuboid samples

The inherent drawbacks of selective laser melting technique including serious micro-pore and element microsegregation problems destroy the mechanical property of the component.To overcome this problem,a new approach,circular beam oscillation,was successfully applied in the SLMed Inconel 718 samples including single tracks,thin walls and cuboid samples.On one hand,circular beam oscillation reduces the micro-pores in molten pools and cuboid samples,increasing the relative density of the cuboid sample to 99.95%.On the other hand,circular beam oscillation suppresses the element microsegregation,reducing the formation of Laves phases in SLMed Inconel 718 samples.Moreover,circular beam oscillation enhances the<001>texture of thin walls and the<101>texture of cuboid samples.The improvement of formability and microstructure of the SLMed samples with oscillation is closely related to cooling rate,thermal gradient and stirring effect during the solidification process.Therefore,circular beam oscillation shows the possibility to overcome the key bottlenecks of the traditional SLM technology and to realize a further industrial application of SLM technology.

Controllable in-situ aging during selective laser melting: Stepwise precipitation of multiple strengthening phases in Inconel 718 alloy

controllable in-situ aging treatment of Inconel 718 alloy was successfully developed by governing the types and times of the thermal cycles during selective laser melting (SLM). The sequential precipitation of γ' composite phase of primary/secondary y ' coated by a γ″shell, and γ″ occurs as the times of vertical thermal cycles increase when laser peak temperature (Ip) is between the precipitation temperature of strengthening phase {TpTe) and liquid temperature (7l ), which causes an increase in microhardness by 4 0 HV. Also, short-term aging mechanisms on three types of strengthening phases are proposed based on a great number of vacancies in the matrix.