Study On The Forming Technology,Structure And Properties Of TiAl Based Alloy By Laser Melting Deposition

TiAl based alloy has good high temperature oxidation resistance,good creep resistance,high specific strength,good thermal conductivity and low density.Based on the above advantages,TiAl based alloy is expected to partly replace the most widely used nickel-base superalloy in aerospace,and becomes the most promising new generation of lightweight high-temperature structural materials,thereby achieving the purpose of reducing the weight of aero engines and improving the efficiency of aerospace engines.However,due to its low plasticity at room temperature and difficulties in forming,there are many disadvantages in the traditional forming process,such as large machining allowance,low utilization rate of materials and many pre-treatment and post-treatment processes,which increase the processing time and production cost.The development of advanced near-net forming technology is the main technology to solve the forming problem of TiAl based alloy material.In this paper,the thin-walled and bulk TiAl based alloy were prepared under different processing conditions.The deposition quality,microstructure and mechanical properties of TiAl based Alloy under different processing conditions by laser melting deposition were studied.This paper analyzed the crack problems in the process of thin-wall and block deposition,as well as the crack mitigation mechanism under the optimized process.The main conclusions are as follows:(1)During the deposition of thin-walled TiAl based alloy,TC4 titanium alloy was used as the transition layer by optimizing the process conditions,and deposited under the protection of argon.By using its good strength and toughness and the thermal accumulation during deposition,the temperature gradient and thermal stress level of subsequent deposition of TiAl based alloy were reduced.And the tendency of thin-walled cracks of TiAl based alloy was alleviated.(2)Under different processing conditions,the microstructure of the deposited thin wall is composed of equiaxed grains and columnar grains of different sizes.All of the substructure inside the grains is lamellar with the lamellar spacing of about 0.29 ?m.(3)Compared with local argon protection and direct deposition of TiAl based alloy,the mechanical properties of TiAl based alloy samples were greatly improved by introducing TC4 transition layer in argon environment.The tensile strength was increased from 402 MPa to 502 MPa.And the elongation was increased from 0.3% to 0.5%.The fracture mode of tensile fracture was brittle cleavage fracture.(4)During the deposition of bulk TiAl based alloy,the pores between the bulk and the substrate,as well as the poor joint,are easy to become the source of crack initiation.And then the interface will be separated under the effect of thermal stress.The thermal expansion coefficient between the TC4 transition layer and the deposited bulk TiAl based alloy is closer,so the interface crack is restrained.With less deposition defects and better bonding quality,the crack tendency of the bulk TiAl based alloy has been greatly alleviated.And the formability has been improved.The results of present study provide a process method for crack control during laser melted deposition TiAl based alloy,which is helpful to control crack and improve formability.