Effect Of TIG Repair Welding And Heat Treatment On Microstructure And Properties Of Al-Cu-Ni Casting Casing

A new type of heat-resistant aluminum alloy casting Al6.5Cu2Ni0.5Zr0.3Ti0.25V because of their excellent high-temperature mechanical properties is applied to the aviation engine box this kind of structure complicated thin-walled components.However,due to the large crystallization temperature range,poor fluidity of alloy castings,and limited by the casting process itself,quality defects such as pores,slag inclusion,shrinkage,and cracks are easily generated in the production process of industrial castings.In the service process,the accident rate and repair rate are high,which will seriously cause life safety and huge economic losses.To guarantee the yield of castings,repair welding has become an essential part of the actual production.To explore the influence of microstructure at the repair site of casting Al-Cu-Ni heat-resistant aluminum alloy casing defects on casing performance,this paper conducted TIG repair for the casting defects that frequently occur in the actual production of casing castings.The optimal welding process parameters of the repaired joint were investigated,and the microstructure and mechanical properties of the repaired joint under the optimal welding process parameters were analyzed.After heat treatment（solution+aging）,the changes in microstructure and mechanical properties of the repaired welded joints before and after heat treatment were compared and analyzed.The relationship between high-temperature mechanical properties and microstructure of the repaired joints in solution+aging state was studied,so that the comprehensive mechanical properties of the repaired joints could meet the actual requirements of casing production.Aiming at the casting defects of the engine casing,a pneumatic milling cutter was used to remove the holes,and TIG wire filling was used to repair the defects.Through the repair welding process test,the optimal process parameters were determined as 220 A welding current,3-6 V welding voltage,45-60°wire feeding Angle,the argon flow rate of 10-15 L/min,and preheating temperature of 180℃.The microstructure,macro morphology,and mechanical properties of the repair welded joint under typical process conditions were tested as follows:From the base metal zone to the weld zone,the grain morphology gradually changed from columnar to equiaxed grain,and the grain size in the base metal zone was 150μm.A large number of the second phase structures were distributed in the base metal zone,either fragmented or fishbone,forming a closed mesh shape.The grain size of the heat-affected zone is 57.54μm,and the microstructure is flocculent eutectic.The grain size of the weld zone is 20.68μm,the microstructure is fine mesh,and there are a few micron pores in the weld zone.The large Angle grain boundary is more in the repaired joint and there is no preferred orientation between grains.The average hardness of the base metal zone,the heat affected zone,and the weld zone is 78.24HV,93.43 HV,and 87.66 HV respectively.The tensile strength,yield strength,and elongation of the rewelded joint at 25℃are 182 MPa,141 MPa,and 2.35%respectively.Compared with the as-cast,it increased by 6.7%,24.8%,and 18.7%respectively.The fracture location of the repaired welded joint is mostly located in the base metal area,and the fracture shows typical cleavage fracture characteristics.The solid solution+aging heat treatment process was used to conduct the whole heat treatment of the repaired welded specimen.The microtexture and mechanical properties of the repaired welded joint in the solid solution+aging state were characterized.The results showed that compared with the repair welded joint without heat treatment,the second phase structure in the base metal area of the repair welded joint in the solid solution+aging state was coarser,part of the mesh structure dissolved and disappeared,and dendrite spacing became larger.The floccule structures in the heat-affected zone disappear,and the second phase is replaced by small and discontinuous island structures intermixed with fragments,and the boundary with the base metal zone becomes blurred.The microstructure of the second phase in the weld zone becomes spot-like and rod-like,and the distribution is more uniform and fine.Generally speaking,after heat treatment,the grain size of the repaired welded joint is slightly larger,the proportion of small Angle grain boundary is more,and the grain orientation is weak.In terms of mechanical properties,the base metal zone has the highest microhardness with an average hardness of 135.7 HV,the heat-affected zone has an average hardness of 125.3 HV,and the weld zone has the lowest microhardness with an average hardness of 113.6 HV.Compared with the repair welded joint without heat treatment,it increased by 73.4%,34.1%,and 29.6%respectively.The tensile strength and yield strength of the repair welded joint in the solution+aging state is 288.5 MPa and 245 MPa respectively at 25℃.Were 87.2%and 90.7%of the base metal,respectively.At 300℃,the tensile strength,yield strength,and elongation of the repair welded joint in the solution+aging state are174 MPa,137 MPa,and 11.8%,which are 92%,87.8%,and 92.9%of the base metal respectively.At 350℃,the tensile strength is 114.2 MPa,the yield strength is 98MPa and the elongation is 12.78%,which is 89.6%,90.7%,and 79.3%of the base metal,respectively.The microstructure and high-temperature mechanical properties of the repair welded joint in the solution+aging state was analyzed.The results showed that the nanocrystallineθ’-Al₂Cu and Al₃M（M=Ti,Zr,V）in the grain,and the distribution ofτ-Al₇Cu₄Ni,δ-Al₃Cu Ni and residualθ-Al₂Cu on the grain boundary,Through the synergistic action of multiple parties,the solute solution+aging state repair welded joint exhibits excellent high-temperature mechanical properties at 300℃and 350℃,which is sufficient to meet the working requirements of aircraft engine casing at high temperature.