Study On Microstructure Properties And Heat Treatment Process Of Free-cutting Al-Cu Alloy Extruded Bar Without Lead

Due to many advantages of low density,high strength,corrosion resistance and easy recycling,aluminum alloy is widely used in various fields such large electronic appliances,transportation,mechanical equipment,aerospace.Massive aluminum alloy parts have to experience the process of turning,milling,drilling,tapping and other machining during manufacturing.To improve the machinability of aluminum alloy,higher speed or larger feed can be used for machining,which is also conducive to obtain aluminum alloy parts with smoother surface and higher dimensional accuracy.Traditional free-cutting aluminum alloy contains Pb,Pb regarded as a heavy metal chemical element,which seriously damaged the health of users and the development of local ecological environment.The modern people's living quality and public awareness of environmental protection are undergoing constant improvement,and thus the production and promotion of traditional free-cutting aluminum alloy are gradually banned by the state.In this paper,Sn and Bi were used as the substitution of Pb to prepare a free lead Al-Cu alloy extruded bar with good cutting performance.The influence of Bi and Sn contents on microstructure,cutting properties and tensile strength of Al-Cu alloy extruded bars without lead and the heat treatment process of free-cutting Al-Cu alloy extruded bars without lead were studied by using optical microscope,scanning electron microscope,transmission electron microscope,tensile testing machine and high-speed lathe.The following results were obtained:(1)When the Bi content is lower than Sn,Bi exists in the form of BiSn eutectic phase in the free-cutting Al-Cu alloy extruded bars without lead.When the Bi content is higher than Sn,Bi exists in the form of Bi phase and BiSn eutectic phase.As the Bi content increases from 0%to 1.0%,the number of Bi phases and BiSn eutectic phases increases,the better the cutting performance of the alloy,the shorter the chip length,but the mechanical properties are slightly reduced.The chip length of the free-cutting Al-Cu alloy extruded bars without lead has been reduced from 13.5mm to 3.0mm,the tensile strength has been reduced from 318.0MPa to 283.5MPa,and the elongation has been reduced from 28.0%to 25.9%.(2)Sn exists in the form of low melting point Sn phase and Sn Bi eutectic phase in free-cutting Al-Cu alloy extruded bar without lead.The cutting performance of the alloy increases with the increase of low melting point phases,but the mechanical properties decrease.As the Sn content increases from 0%to 1.0%,the chip length of the free-cutting Al-Cu alloy extruded bars without lead has been reduced from 12.0mm to 2.0mm,the tensile strength has been reduced from 315.0MPa to 280.5MPa,and the elongation has been reduced from 28.20%to 26.00%.(3)The higher the solution temperature,the more fully the coarse and massive Cu Al₂phase in the free-cutting Al-Cu alloy extruded bar without lead is dissolved,and the better the solution effect.When the aging treatment is performed at the temperature of 180?,190?and 200?respectively,the tensile strength of free-cutting Al Cu alloy extruded bar first rises and then falls,while the elongation first falls and then rises with the extension of the aging time.The best heat treatment process for free-cutting Al-Cu alloy extruded bars without lead is 520?×2h solid solution+180?×8h aging.The tensile strength of the bar is463.8MPa and the elongation is 9.70%.