Microstructure Evolution And Properties Of Al-5Cu Alloy With High Temperature And Long Time Thermal Exposure

Al alloys play an important role in the development of aerospace,rail transit as well as defense military industries for weight reduction because of their low density,high specific strength,good formability and excellent corrosion resistance.The Al-Cu and Al-Si-Cu-Ni-Mg system alloys are the main aluminum metallic materials for vehicle pistons fabrication.The?'phase,a semi-coherent lattice orientation with the?-Al matrix,is an important strengthening phase precipitated during the age of piston alloys.As the increment of engine power density,temperature of the top side of the piston could reach to 400?,or even higher.It was reported that the?'phase would lose its strengthening effect when the temperature was higher than250°C since the?'phase gradually transformed into a stable?-Al₂Cu phase which demonstrated an incoherent orientation with the matrix.However,the microstructural evolution of Al-Cu system alloys at the service temperature or higher temperatures for high power density diesel engine pistons has not yet been reported in the literature.The precipitation route and precipitation kinetics of the aging strengthening precipitates are very crucial to determine whether the properties of the material meets the service requirements.Therefore,it is urgent to investigate the microstructure evolution of Al-Cu system alloys above 400°C and its influence on mechanical properties to provide guarantee for the service safety and reliability of the hot bearing components.Thermal exposure can provide an experimental and theoretical basis for optimizing heat treatment techniques by simulating the influence of phase precipitation behavior and microstructure evolution on the mechanical properties of alloys under specific conditions.In this paper,Al-5Cu binary alloy was taken as the research object and the precipitation behavior of Al-Cu phase and the effect of microstructure evolution on the properties of the alloy during solid solution process and thermal exposure at 425°C were systematically studied.It was expected to provide assistance for the alloy design and development of heat-resistant piston aluminum alloy.The main conclusions are as follows:(1)The effect of solution time at 525?on the microstructure and properties of Al-5Cu alloy were studied.The optimal solid solution regime(525?/6 h)was determined by calculating the content of Cu atoms in the matrix statically,the content of Al₂Cu phase precipitated during solidification and the variation of?-Al₍₁₁₁₎interplanar spacing.After solution treatment,the eutectic structure was completely dissolved,and the content of Cu atoms in the alloy matrix reached a maximum value of 4.74 wt.%.There was still a small amount of granular?-Al₂Cu phase remained in the alloy because solid solubility of Cu atoms has already reached to its maximum value in the matrix at 525?.Solution treatment improved the room temperature tensile strength and elongation of the alloy.(2)The precipitation behavior and evolution process of Al-5Cu alloy during thermal exposure were studied through quantitatively analyzing the morphology,volume fraction,number density and phase spacing of precipitates during the thermal exposure process.The precipitation under the thermal exposure at 425?was still Al₂Cu phase rather than other Al-Cu phases with different Al/Cu ratios.But the increase of temperature significantly accelerated the precipitation,growth,coarsening and transition rates of Al₂Cu phase.The samples were mainly comprised of semicoherent??-Al₂Cu precipitate before heat exposure at 425°C for 3 min.The??phase began to transform into?-Al₂Cu when the heat exposure time was within 3-5 min.With the increase of heat exposure time,the??phase was completely transformed into stable incoherent?-Al₂Cu phase after 2 h of thermal exposure treatment.When the thermal exposure time was within 2-36 h,the obvious growth of?-Al₂Cu was observed in width direction but little variation in length.The width of the precipitated?-Al₂Cu grew from 210 nm to 304 nm.When the thermal exposure time was within 36-84 h,the obvious stabilization of?-Al₂Cu was observed in length direction but little variation in width.When the thermal exposure time was within 84-180 h,the regrowth process of the?-Al₂Cu phase was observed.Because of the inhomogeneous size of?-Al₂Cu,it was a potential energy difference between the particles leading to coarsening.According to the statistical results,the length of the?-Al₂Cu phase increased from 562 nm to 1528 nm.It was also found that the?-Al₂Cu phase morphology changed with time,from long strips to nearly spherical and then to lath again.(3)The effect of thermal exposure at 425?on the properties of the alloy was studied.It could be seen that the changing trend of properties during the thermal exposure process could be divided into three stages.Firstly,the stage of rapid decrement stage(3 min-2 h).The quick sdecrease of strength was account for two reasons.One is the transition from semicoherent?'-Al₂Cu phase to the incoherent?-Al₂Cu phase,accompanied by decline of the number density and increasement of the interspace of precipitated particles.On the other side,the rapid outward diffusion of Cu atoms in the matrix weakened the solid solution strengthening effect.Secondly,the stage of mechanical properties stabilization(2-84 h).Although the morphology of?-Al₂Cu transferred from plate to spherical shape,the strengthening effect of?-Al₂Cu was almost the same because there was little change in the interspace of the?-Al₂Cu precipitates during this period.Thirdly,the stage of regrowth(84-180 h)when the dissolution and precipitation of Cu atoms in the alloy matrix gradually reached a dynamic equilibrium,but the coarsening of?-Al₂Cu phase was serious and leaded to a sharp increase in the precipitation interphase spacing again making the volume fraction of the precipitated phase decreased and finally weakened the precipitation strengthening effect of the Al₂Cu phase.Based on the metal strengthening theory,a prediction model for the mechanical properties of the alloy during thermal exposure was constructed.It was concluded that the solid solution strengthening decreased rapidly within 2 h of thermal exposure.There were fluctuations during the subsequent thermal exposure but the overall change was not significant.The overall trend of precipitation strengthening during heat exposure was more in line with the actual strength change of the alloy,so precipitation strengthening was the main factor affecting the mechanical properties of the alloy,and the principal determiner affecting precipitation strengthening were the type of precipitation phase,morphological dimension,volume fraction,number density and interphase spacing.