Microstructure and Strength-Toughness of FSP-Assisted Laser Deposited AlSi10Mg Alloy
Objective In recent years,laser additive manufacturing based on direct laser deposition has attracted widespread attention because of its flexibility and efficiency.This technology has a wide range of applications and high additive manufacturing efficiency.It is widely used in the aerospace,rail transit,and ship component maintenance equipment fields.However,high-strength aluminum alloys such as those in the Al-Zn-Mg-Cu series have a high content of alloying elements.During the solidification process,the semi-solid range of the alloy may exceed 100 ℃,which can easily leave gaps between aluminum grains.The α-Al layer of"liquid film"generates cracks under the action of thermal stress,making laser deposition repair difficult.AlSi10Mg alloy,as a cast aluminum alloy,has the characteristics of a short semi-solid range and high strength,and is suitable for additive manufacturing and the laser repair of high-strength aluminum alloy components.However,during the laser deposition process,process fluctuations often cause defects such as pores to appear in the components,leading to cracks and ultimately component failure during use.Therefore,exploring a method to eliminate pores in components produced using AlSi10Mg laser deposition is of great significance for improving the mechanical properties and service life of AlSi10Mg components.Methods Atomized AlSi10Mg alloy powder with a particle size ranging from 53 μm to 150 μm is adopted.During the laser deposition process,the laser power is 2700 W,deposition speed is 600 mm/min,powder feed rate is 5.8 g/min,overlap amount is 2.5 mm,argon flow rate is 5 L/min,and protective argon amount is 20 L/min,resulting in a single-layer thickness of 0.5 mm.After depositing eight layers to achieve a thickness of 4 mm,stir friction treatment is performed on the deposited AlSi10Mg alloy.The height of the mixing needle of the mixing head is 4 mm,with a four-prism shape and diameter of 6 mm at the end of the prism.During the stirring friction treatment process,the rotational speed is 800 r/min,stirring speed is 100 mm/min,and variation in the stirring friction treatment passes is 5 mm.Subsequently,the laser deposition of eight-layer AlSi10Mmg alloy is continued on the surface of the AlSi10Mg alloy after the stir friction machining,and then stir friction machining is used.Wire cutting is used to cut the AlSi10Mg alloy into five samples,and stir friction-assisted laser deposition is conducted,followed by room-temperature rolling treatment.Rolling deformation values of 20％,46％,and 68％are achieved on three of the samples.The five tensile specimens of the AlSi10Mg alloy are treated as mentioned above,and their strength and elongation values are measured using a tensile testing machine.After vibration polishing,the five metallographic samples are observed using a scanning electron microscope and backscattered electron diffractometer,and their microhardness values are measured.A thin film sample of the AlSi10Mg alloy is prepared and its microstructure is observed using a transmission electron microscope after electrolytic double spraying.The fracture of the tensile specimen is observed using the scanning electron microscope.Results and Discussions The hardness values of the AlSi10Mg alloy in the five different states are listed in Table 1.It can be observed that the hardness of the deposited AlSi10Mg alloy is approximately 109 HV.Because at high temperatures,the solid solubility of the Si element in the α-Al matrix is relatively high,and when the temperature rapidly drops,it is difficult for the Si element to recover from α-Al matrix,and a large number of Si atoms on α-Al matrix play a role in solid solution strengthening.The Al matrix plays a role in solid solution strengthening.After stir friction processing,the solid solution strengthening effect is significantly weakened,and the hardness of the AlSi10Mg alloy decreases to 75 HV.Based on the hardness values of the rolled AlSi10Mg specimens listed in Table 2,it can be observed that the rolling process improves the effects of dislocation strengthening and fine grain strengthening in the AlSi10Mg alloy,ultimately increasing the hardness of the laser-deposited AlSi10Mg alloy after stir friction processing to 116 HV.As shown in Fig.9,after stir friction processing,the strength of the AlSi10Mg alloy is close to 200 MPa,and the elongation distribution is 33％-40％.It can be seen that stir friction processing can simultaneously improve the strength and plasticity of the laser-deposited AlSi10Mg alloy.Figure 10 shows that there are a large number of dimples in the tensile fracture surface of the AlSi10Mg alloy in the laser deposition state and stir friction processing state,indicating that the fracture mode of both AlSi10Mg alloy specimens is the plastic fracture mode.The research on hardness shows that the strength and elongation of the laser deposited AlSi10Mg alloy cannot reach high levels.However,after stir friction processing,the larger shoulder pressure and stirring effect eliminate the porosity defects in the alloy,reduce the stress concentration,and thus significantly increase its elongation.Conclusions After friction stir processing,the columnar α-Al and eutectic phases in the laser deposited AlSi10Mg become equiaxed α-Al grains and Si particles,while the Mg2Si precipitate phase is significantly refined.After rolling,when the deformation of the AlSi10Mg alloy increases to 68％,dislocation strengthening further refines the grains.After rolling,the dislocation strengthening effect in the alloy is significantly enhanced.In addition,increasing the rolling amount also brings about a fine grain strengthening effect.Therefore,the hardness of the AlSi10Mg alloy can be increased to 116 HV at most,exceeding the microhardness of the laser deposited AlSi10Mg alloy.Although the solid solution strengthening effect in the laser deposited AlSi10Mg alloy is remarkable,the solidification defects in the alloy lead to the formation of early cracks during the tensile process,which results in an alloy strength of less than 200 MPa and an elongation of less than 20％.After friction stir processing,the strength and toughness of the AlSi10Mg alloy are simultaneously improved,with a strength close to 200 MPa and an elongation of 33％-40％.After rolling,the dislocation strengthening effect of the AlSi10Mg alloy gradually increases,and its strength continues to rise,reaching a maximum of approximately 400 MPa.The localized hardening area in the alloy leads to a decrease in its plastic deformation ability,and the elongation gradually decreases to 25％.
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