Effect Of Semi-solid + Plastic Deformation Composite Process On Microstructure And Properties Of Al-Cu Alloy

The age-hardening Al-Cu alloy with metastable Al₂Cu precipitates as the main strengthening phase has good mechanical,heat resistance and workability,but it needs a time-consuming heat treatment process.In this paper,a new low-cost and efficient process different from the traditional solution aging is proposed.Non age hardening nano particle reinforced Al-Cu alloy with high strength and low alloy content was prepared by refining the micro lamellar eutectic of gravity cast Al-x Cu（x=2.4,4.5,7.1,10.6 wt.%）alloy to nano size by semi-solid isothermal treatment,breaking the nano eutectic lamellar to nano particles and dispersing them in combination with subsequent plastic deformation.This paper mainly focuses on the refinement and fragmentation of（α-Al+Al₂Cu）eutectic phase in Al-Cu alloy,the specific points are as follows.1)Effects of Cu content and extrusion temperature on microstructure and properties of as-cast and extruded Al-Cu alloys;2)Evolution of liquid layer thickness and distribution,eutectic layer thickness and distribution with semi-solid process parameters in Al-Cu alloy microstructure under different semi-solid process parameters（temperature,time,cooling rate）3)Thermal stability of nano eutectic;4)The effects of Cu content,extrusion temperature and deformation on the particle size and distribution of Al₂Cu;5)The contribution of nanoparticles to strength and the enhancement mechanism in the Al-Cu alloy.The results show that the microstructure of as-cast Al-Cu alloy is composed of dendrite and interdendritic eutectic.With the increase of Cu content,the content of the second phase increases,the strength increases,but the plasticity decreases;After the first extrusion,the micro lamellae of eutectic are broken and distributed in strips along the extrusion direction,and the strength and elongation are improved;After secondary extrusion,the particle dispersion and size of the second phase are improved to a certain extent,and the strength increases with the decrease of extrusion temperature,but the coarse second phase in the initial state cannot be refined.In terms of mechanical properties,the more coarse phase,the smaller the increase of strength.The volume of liquid phase and the regularity of eutectic lamella in semi-solid Al-Cu alloy are affected by Cu content,holding temperature and time at the same time;In the process of heat preservation,the small liquid pool in the solid phase will undergo Ostwald ripening and coarsening with the increase of heat preservation temperature and time,and part of the solid phaseα-Al will be split by the liquid phase.At the same time,shorter heat preservation can obtain finer eutectic lamellae;In the quenching process,nano eutectic lamellae can be obtained when the cooling water temperature is below 40℃,and the cooling rate is inversely proportional to the thickness of eutectic lamellae;Nano lamellar eutectic has good thermal stability below 250℃,with no coarsening of eutectic lamellae and precipitates.During plastic deformation of semi-solid Al-Cu alloy,the size and distribution of broken particles are affected by both deformation and extrusion temperature.In the process of"semi-solid+secondary extrusion",the second phase is further broken with the reduction of extrusion temperature,but the coarse particles still cannot be refined.Al-4.5Cu alloy obtains the best combination of mechanical properties:UTS is 330 MPa,YS is 253 MPa and EL is17.89%;In the"semi-solid+ECAP",the increase of deformation and the decrease of extrusion temperature make the second phase more fine and dispersed,with an average particle size of60-80 nm,which breaks the coarse particles to a great extent.At the same time,there are a large number of subcrystals in the structure,with a size of about 200 nm.The strength of Al-4.5Cu alloy reached the highest at room temperature ECAP:UTS is 460 MPa,YS is 447 MPa,and El decreased significantly to 1.55%.The improvement of the strength is mainly due to the joint action of multiple strengthening mechanisms such as work hardening,grain boundary strengthening and dispersion strengthening:1)The growth rate of dislocations produced by work hardening is higher than that of dislocation annihilation during recovery,and the fine subgrain hinders the slip of dislocations in the crystal;2)Fine particles are dispersed in the matrix,hindering the movement of grain boundaries and dislocations,and playing a significant role by dispersion strengthening.