Selective Laser Melting Of Aa2195 Al-Li Alloy And Crack Defect Control

Compared with traditional aluminum alloys,Al-Li alloy has enhanced specific strength and rigidity.In addition,Al-Li alloy also has a series of advantages such as good low temperature performance and excellent corrosion resistance.Hence,Al-Li alloy has gradually replaced the use of traditional aluminum alloys in the aerospace industry.Nowadays traditional material processing methods are difficult to meet the needs of complex structural design.Selective laser melting(SLM)technology can not only shorten the structural design and manufacturing cycle,but also effectively reduce costs of the product.However,the current research on Al-Li alloy by SLM is very scarce.In this article,2195 Al-Li alloy powder was used to study the forming quality,defect formation mechanism and grain structure of 2195 Al-Li alloy.Zr was adopted to inhibit the formation of defects.Also,Er-Zr modified Al-Li alloy by SLM process was analysed.In order to explore the forming window of Al-Li alloy,the SLM process of Al-Li alloy was carried out within a larger range of process parameters,the influence of different process parameters on the relative density was studied,and a comprehensive analysis was obtained to find the optimum forming window.Using a small scanning interval(90?m),low laser power(80-160W)and low scanning speed(60-100mm/s)could process the bulk 2195 material with the optimal relative density.The maximum relative density was 99.97%.The analysis on the surface quality of SLM-formed bulk Al-Li alloy specimens revealed that there were Droplet splash,surface-bound powders,and cross crack defects on the surface.The results revealed that there were crack defects within the range of different process parameters.When the power was low(P=80W),it was a continuous holes causing crack perpendicular to the building direction,as the laser power was larger(>120W),it was the hot crack parallel to the building direction.Combined with the simulation results,the formation of solidification cracks were attributed to three factors.At first,the large solidification temperature range of 2195Al-Li alloy resulted in a wide temperature range with hot crack sensitivity(640?-480?),the low-melting eutectic was easy to form at the grain boundary.Second,the coarse columnar crystals with grew along the building direction,the length of which was up to 100?m and the width of which was in a range of 20-30?m.In addition,the columnar grew across the multi-layers.Third,the simulation result proved that the cooling rate in the range of 480 ? and 640? was greater than 0.48×106?/s,and the melt pool suffered continuous tensile transverse stress during the solidification process.And the residual transverse tensile stress increased gradually during the entire SLM process.Therefore,when the liquid film of low-melting eutectic generated at the columnar grain boundary was broken and the shrinkage could not be replenished,the cracks might initiate at the grain boundary due to the tensile stress,and the cracks further expanded with a increased residual tensile stress.Two kinds of schemes were adopted to inhibit the crack defect.The first way was increasing the preheating temperature.Second,Zr modified 2195 powder and(Er-Zr)modified 2195 were used to suppress hot crack defects.And the result showed that the ways mentioned above were effective.The crack defects were successively eliminated in the(Er-Zr)modified 2195 bulk material when the laser power was higher than 120 W.Meanwhile,the crack defects of Zr modified bulk Al-Li alloy were eliminated when the laser power was higher than 160 W.Compared with the Zr modified Al-Li alloy,the way using(Er-Zr)was more effective,while more pore defects were found in the(Er-Zr)modified Al-Li alloy,which resulted in the density reduced by 2% to 4%.The microstructure and properties of Zr alloyed were also analyzed.Compared with the SLM formed Al-Li alloy,the grains of Zr modified Al-Li alloy were significantly refined.Due to the equiaxed grains with a random orientation,the <100> orientation in the SLM-formed Al-Li alloyed was eliminated in the Zr modified 2195 alloy.