| The layered composite material maintains the advantages of a single component material and overcomes the shortcomings of the respective component materials,and has a more excellent comprehensive performance and a wide range of industrial application prospects.In recent years,the lightweight of brake systems in rail transit,aerospace,national defense and military industries has become increasingly urgent.It has important theoretical significance and application value to develop the low-cost and short-process manufacturing technology of structure and function integration,to develop high-strength wear-resistant laminated aluminum-based composite brake parts,and to replace steel with aluminum.This thesis took the brake drum with a practical engineering application background as the research object.PAMC/Al layered composite brake drum with outer wear-resistant layer of SiCp/A357 aluminum matrix composite and inner of 7050 high-strength aluminum alloy was designed.The calculation simulation model of solid-liquid rheological compound casting of PAMC/Al layered composite brake drum was established.A new technology for rheological die forging of high-strength wear-resistant layered aluminum composite had been developed through the combination of numerical simulation and experiment.The influence of process parameters on microstructure and properties was studied,the solid-liquid bonding mechanism of dissimilar materials was revealed and the solid-liquid metallurgical bonding of layered composite was realized,and the high-strength wear-resistant layered aluminum composite castings with integrated structure and function were prepared.The main results in this thesis are as follows:(1)The influence of liquid die forging process parameters on the formability and defects of 7050 high-strength aluminum alloy castings was studied through numerical simulation and experimental verification.The results show that casting hot spots exist at the junction of brake drum spoke and rim.The increase in casting temperature,the decrease of forming specific pressure and the increase of die temperature will increase the existence time of hot spots.The optimized process parameters for liquid die forging of 7050 high-strength aluminum alloy brake drum are casting temperature of 660?,forming specific pressure of 100 MPa,and die temperature of 200?.Under these conditions,7050 aluminum alloy brake drum castings are well formed without shrinkage and porosity defects.(2)The effects of electromagnetic homogenization melt treatment and microalloying on the structure and properties of 7050 high-strength aluminum alloy rheological die forging brake drum castings were studied.The results show that after applying electromagnetic homogenization melt treatment to 7050 aluminum alloy melt and adding 0.15 wt.%Sc for microalloying,the obtained rheological die forging 7050 high-strength aluminum alloy brake drum castings have significantly refined crystal grains,and the mechanical properties are significantly improved.Compared with ordinary liquid die forging parts,the grain size of rheological die forging parts is reduced from 136.9 ?m to 42.7 ?m,the tensile strength is increased from 559 MPa to 597 MPa,the yield strength is increased from 464 MPa to 518 MPa,and the elongation is increased from 6.1%to 13.7%.(3)Through numerical simulation and experimental research,the structural parameters of the wear-resistant ring were optimized,and the influence of the key parameters of the solid-liquid compound casting process on the solid-liquid composite interface bonding was studied,and the law of achieving good interface bonding was revealed.Ensure that the alloy melt and the surface of the wear-resistant ring is wetted,the surface of the wear-resistant ring must be remelted to a certain extent and fused with the melt,and the liquid eutectic zone at the fusion joint should be as narrow as possible.With chemical method used to remove the surface oxide layer,while the preheating temperature of the wear-resistant ring is 200?and the waiting time before pressurization is 10 s,and structure parameters of the wear ring are 5 mm in thickness and 60 mm in height,a good solid-liquid composite interface can be obtained.(4)The morphology of the microstructure,element distribution,phase composition and mechanical properties of the solid-liquid composite interface of the PAMC/Al layered composite brake drum were analyzed.The results of the study found that during the formation of the solid-liquid interface,the surface layer structure of the wear-resistant ring forms fine-grained,spheroidized,and dendritic zones.The SiCp/A357 aluminum composite layer at the solid-liquid composite interface has a transition layer of about 250 ?m thick,al where there are many T phase and Mg2Si phase.After T6 heat treatment,the T phase disappears at the interface and a new W phase is formed.The Vickers hardness at the interface increased from 121.5 HV to 172.0 HV,and the interface shear strength increased from 83.3 MPa to 124.6 MPa,which is about 50%higher than that in as-cast state.(5)On the basis of the above research,large-scale PAMC/A1 layered composite brake drum castings with an outer diameter of 470 mm and a height of 120 mm were manufactured.The solid-liquid interface of the casting is well bonded.The mechanical properties of the casting after T6 heat treatment are as followed:the axial tensile strength of the rim is 582 MPa,the yield strength is 512 MPa,and the elongation is 7.9%;the radial tensile strength of the spoke is 590 MPa,the yield strength is 530 MPa,and the elongation is 6.4%;The radial shear strength of 304 MPa.The friction performance is:friction coefficient 0.5776,wear rate 3.99×10-7 cm3/(N·m).Through the bench test assessment,it is well performed and has a good industrial application prospect. |
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