Study On Manufacturing Process And Mechanical Properties Of Hastelloy X-TiB₂ Composites Fabricated By Selective Laser Melting

Hastelloy X(HX)alloy is a kind of nickle-based high temperature alloy,which has been widely applied in Commercial shipbuilding industries.HX alloy,however,which is susceptible to hot cracking in selective laser melting(SLM)process,has different demand for mechanical properties in different positions in parts.Therefore,this research proposes new kinds of materials fabricated by SLM to solve this problem.This kind of material is HX-TiB2 composites materials.Firstly,the powder spreading process of pure HX alloy and HX-TiB2 composites were simulated using EDEM software.Under the same powder supply,the volume of the powder scraped into the building chamber rose first and then decreased with the increase of the wiper moving forward speed.This is because when the wiper moving forward speed is low or high,there are insufficient areas for powder in the powder bed in the building chamber,and the powder spreading quality of the building chamber is low.The simulating results of EDEM software indicate that the optimum wiper moving forward speed of pure HX powder,HX-1 powder and HX-3 powder is 30 mm/s,25 mm/s and 30 mm/s,respectively.The optimum powder supply of all materials is 250%.For another,results of SLM fabricating experiments indicate that the optimum scanning speed of pure HX,HX-1 and HX-3 is 700 mm/s,900 mm/s and 1100 mm/s.The laser power of all materials is 190 W,the layer thickness is 40 μm,and the hatch spacing is 110 μm.Compared with pure HX fabricated with optimum parameters,microscale crack was not detected in HX1 and HX-3.It is because the high-melting-point TiB2 particles provided nucleation site for HX-TiB2 composites and decrease nucleation energy in SLM process,which refined the grain size inner HX alloy and increased the number of grain boundaries.Therefore,the crack propagation was inhibited.Furthermore,HX-1/HX-3 gradient composites specimens with dimension of 8 ×8 × 15 mm3 was successfully fabricated via SLM and showed good bonding between two graded layers.The ultimate tensile strength(UTS)of HX-1 and HX-3 is 38.1%and 75.8%higher than that of pure HX at ambient temperature test,respectively.Furthermore,the UTS of HX-1 and HX-3 is 77.5%and 106.8%higher than that of pure HX at high temperature test,respectively.After heat treatment,Mo-rich M23C6 and M6C precipitates were detected inner HX alloy.With the increase of heat preservation time,the area of precipitates increases.Cracks perpendicular to building direction were detected in the fracture surface of HX-1/HX-3 gradient composites s and cannot be eliminated by heat treatment.It is because the powder exchanging process increased the residual stresses inner material and the large volume test specimens were susceptible to warping.Finally,the elongation of HX-1/HX-3 gradient composites s improved from 2.89%to 19.6%after heat treatment(1050℃ for 2 hours and then cooled using water).This is because heat treatment eliminates the melt pool of HX-1/HX-3 gradient composites melt pool,which alleviates the stress concentration at the sharp area of the melt pool,and finally leads to the increase of the elongation of the material.