Investigation On The Formation Mechanism And Control Of Welding Cracks In Electron-beam-welded IN738LC Joint

As the speed of aerospace vehicle increased continually,higher engine efficiency is required.Raising the temperature of engine hot components is beneficial to improve the engine efficiency.Due to higher content of Al and Ti elements,IN738LC alloy presents an increased service temperature and therefore it’s used in the manufacture of engine hot components.However,IN738LC alloy shows an extremely high cracking susceptibility after fusion welding.As a result,the manufacture of engine hot components（IN738LC alloy）by welding technology has become a key problem.Based on the application in the hot-section components of aircraft engine,the microstructure evolution,formation mechanism and control of welding cracks and post-weld heat treatment process optimization of IN738LC joint by electron beam welding（EBW）were investigated.Optical microscopy,scanning electron microscopy and transmission electron microscopy were applied to analyze the phase compositions and cracks feature of IN738LC joint.The crack formation was reproduced by Gleeble thermal-mechanical simulator and heat treatment furnace with rapid heating rate.According to the microstructure analysis,formation mechanism of welding cracking was elaborated and effective cracking control method was put forward.The mechanical properties of IN738LC joint subjected to post-weld heat treatment were evaluated by micro-hardness,tensile strength and stress-rupture life and we obtained the high-quality IN738LC joint by electron beam welding.After EBW of IN738LC alloy,the fusion zone of IN738LC joint consisted ofγ-Ni matrix,γ’-Ni₃（Al,Ti）,MC carbide and trace borides.The solidification process of fusion zone was:L₀→L₁+γ₁→L₂+γ₁+（γ₂+MC）_eut→γ₁+（γ₂+MC）_eut+（γ₃+γ’）_eut→γ₁+（γ₂+MC）_eut+（γ₃+γ’）_eut+γ’.γ’particles in heat affected zone（HAZ）were dissolved significantly.Liquation crackings were observed in HAZ of IN738LC joint.The fusion zone of IN738LC joint after solution heat treatment formed serious strain-age cracking.As a result,IN738LC alloy showed a poor weldability for electron beam welding.The results of microstructure analysis and thermal simulation tests showed that the constitutional liquation ofγ’precipitates and MC carbide and the boride melting resulted in the formation of intergranular liquid film.The constitutional liquation ofγ’precipitates was the main source of liquid film.During welding thermal cycle process,intergranular liquid film resulting from the dissolution and constitutional liquation ofγ’precipitates formed in HAZ.When the thermal stress of IN738LC joint exceeded the surface tension of liquid film in the process of cooling,liquid film would be pulled apart to form the resultant liquation cracking.The temperature range of strain-age cracking formation was 700°C～1000°C.This work designed a series of heat treatment experiments and found that the aging-contraction stress from the precipitation ofγ’particles and residual stress level of IN738LC joint were the main factors to cause the initiation and propagation of strain-age cracking.Moreover,the rapid precipitation ofγ’particles and the weld metal oxidation caused the obvious hardening of fusion zone,reducing the grain-boundary plasticity of fusion zone and facilitating the initiation of crack along grain boundary.Based on the post-weld heat treatment tests,we obtained the critical curve of crack formation（dual C curve shape）,and calculated the critical heating rate v_cri=106°C/min for free of cracks during post-weld heat treatment（the peak temperature T≥1125°C）.Electron beam thermal compensation process and rapid heating treatment process were applied to controll the liquation cracking and strain-age cracking.Electron beam thermal compensation process could reduce the length of liquation cracking by increasing the liquid film thickness and relieve the thermal stress of IN738LC joint.Compared with the direct welding（DW）,the length of strain-age cracking of IN738LC joint by electron beam thermal compensation process increased dramatically and the cracks propagated from fusion zone to HAZ,indicating that the control effect of electron beam thermal compensation process on the two kinds of cracks was poor.Rapid heating treatment process obtained crack-free joint by suppressing the precipitation ofγ’particles in the dendrite core regions of fusion zone and improving the plasticity of IN738LC alloy.In addition,HAZ liquation cracking was filled effectively by primaryγ’particles.As a result,rapid heating treatment process could completely eliminate both of liquation cracking and strain-age cracking.After post-weld solutin treatment,the results showed that the increase of solution temperature promoted the dissolution of primaryγ’precipitates and MC carbides,resulting in the tensile strength of IN738LC joint first increasing and then decreasing,while the rupture life of joint significantly reducing.It’s shown that 1125°C/2h was the optimized solution heat treatment process.The growth kinetics ofγ’precipitat es during aging treatment showed that the stableγ’size and excellent mechanical properties of IN738LC joint could be obtained at 850°C.After aging treatment at850°C,the growth rate of secondaryγ’particles of the joint was K=1.003×10^-27m³/s.With the increase of aging time,M23C6 carbides in grain boundary of fusion zone changed from granular shape to chain distribution,and the content of M23C6 carbide also increased significantly.The tensile strength at room temperature and 900°C decreased gradually,while the rupture life increased first and then decreased as the aging time increased.The IN738LC joint sujected to 1125°C/2h+850°C/24h heat treatment showed the highest elevated elongation and rupture life（900°C/350MPa）,about 16%and 2.5h,respectively.Based on the elevated mechanical properties and dimensional stability of γ’precipitates,1125°C/2h+850°C/24h was the optimized post-weld heat treatment schedule for IN738LC joint.