| Owing to the uniform microstructure,fine grains,and excellent mechanical properties of the spray-formed（SF）7055 aluminum alloy,it has been widely used in the aerospace industry.However,due to the notorious hot-crack susceptibility and severe softening fusion-weld joints welded by the traditional filler rods,the application of the SF 7055 aluminum alloy has been limited.Although the hot-crack susceptibility of fusion-weld joints could be reduced by adjusting the welding parameters,the deterioration of microstructure and properties have not been ideally solved.Therefore,it is necessary to fabricate new filler rods to weld the SF 7055 aluminum alloy.According to the"ceramic particle-metal melt-solidification front"model,Ti C and Ti B2 ceramic particles have been chosen as the reinforced phases.Ti C/7055（TC）and TiC-TiB2/7055（BC）aluminum matrix composites have been fabricated by in-situ mixed-salt reaction.These two aluminum matrix composites have been used as TIG filler rods for the SF 7055 aluminum alloy.The effects of ceramic particles on the microstructure and properties of filler rods and fusion weld joints have been systematically investigated.In addition,the weld joints have been subjected to heat treatments.The effects of solution and aging treatments on the microstructure and properties of weld metal（WM）have been investigated.The research contents are as follows:（1）The investigation and fabrication of filler rods.According to the force balance mechanism and"ceramic particles-metal melt-solidification front"model,the van der Waals force and van der Waals potential between ceramic particles and metal melt have been calculated.Ti C and Ti B2 ceramic particles are chosen as the reinforced phases.The TC and BC filler rods have been fabricated by the in-situ mixed-salt reaction.The microstructure and mechanical properties of ceramic particle reinforced filler rods are superior to those of the as-casted 7055 aluminum alloy in both cast and T76 heat treatment.（2）The investigation of the as-welded WM.The microstructure of WM tansforms from coarse dendrites to fine equiaxed crystals by the addition of ceramic particles.Ceramic particles reduce the segregation of alloying elements and modify the hot-crack susceptibility.The tensile strength,ductility,microhardness,and impact toughness of weld joints have been effectively increased with the addition of ceramic particles.Grain refinement strengthening,Load-bearing strengthening,Zener pinning strengthening,coefficients of thermal expansion mismatch strengthening,and solution strengthening are the dominant strengthening mechanisms of as-welded ceramic particle reinforced weld joints.（3）The effect of solution-isothermal aging treatments on the microstructure and properties of WM.In the process of solution-isothermal aging treatment,ceramic particles restrict the growth of grains,accelerate the solution and aging behavior of alloying elements,and shorten the time to peak-aging.Ceramic particles also promote the growth and precipitation of GP-regions andη’phase.With 475/2.5 h+120/15 h treatment,the tensile strength and ductility of BC-WM and TC-WM are 530±15 MPa,3.9±0.29%and 474±15MPa,3.04±0.35%,respectively,which are higher than 7055-WM（358±20 MPa,2.38±0.42%）.（4）The effect of solution-non-isothermal aging treatments on the microstructure and properties of WM.In the process of the non-isothermal aging treatments,with the decrease of the variable temperature rate,the aging time increases,the mechanical properties,and electrical conductivity increase,but the corrosion resistance decreases.The tensile strength and ductility of BC-20 are 580±25 MPa and 4.16±0.25%,respectively.Compared with the traditional isothermal-aging（T6,T74,and RRA）treatments,BC-40 can achieve good mechanical properties similar to T6 treatment and corrosion resistance close to T74 and RRA treatments simultaneously with much less aging time.The narrow PFZ,fine and dispersed precipitates,and discontinuous grain boundaries precipitates are attributed to the superior mechanical properties and excellent corrosion resistance.Grain refinement strengthening,Load-bearing strengthening,Zener pinning strengthening,coefficients of thermal expansion mismatch strengthening,and precipitation strengthening are the dominant strengthening mechanisms of ceramic particle reinforced weld joints with solution-isothermal aging and solution-non-isothermal aging treatments. |
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