Study Of Electromagnetic Casting And Strengthening-Toughening Of High Strength Aluminum Alloy

Al-Zn-Mg-Cu-Zr age-harding alloys with high strength and low density are widely used in aero field.However,a series of problems such as difficulties in casting,insufficient solid solubility,low fracture toughness and weak stress-corrosion resistance also occur because of its high alloying.In this paper,a new type of Al-Zn-Mg-Cu-Zr alloy is designed.With the method of using optical microscopy(OM),x-ray diffraction spectrometry(XRD),scanning electron microscopy(SEM),electronic probe microanalysis(EPMA),transmission electron microscopy(TEM) and tensile test,the effects of electromagnetic casting(EMC) and different heat treatments on the microstructure and mechanical properties of alloy are studied.Al-Zn-Mg-Cu-Zr aluminum alloys were prepared by electromagnetic casting(EMC) and direct chill casting(DCC),respectively.The effects of electromagnetic field on as-cast microstructure,alloy elements solid solubility inside crystals and mechanical tensile properties of Al-Zn-Mg-Cu-Zr alloy were investigated.The results show that the Al-Zn-Mg-Cu-Zr alloy billets prepared by EMC have better surface qualities,microstructures and higher solid solubility inside crystals of alloy elements.The degree of microsegregation in the EMC billet decreases.Comparing DCC specimens with the EMC specimens,the relative intracrytalline solubility of elements Zn,Mg and Cu increase from 56%,62%and 25%to 79%,74%and 44%,respectively.After rolling and aging at 120℃for 24h,the strength and toughness of alloy increase significantly than that of as-cast.The tensile strength,yield strength and elongation in the longitudinal direction of DCC alloy are 573MPa,502MP and 12.2%.Compared with the DCC alloy,the value of strength of EMC alloy has no obvious change,but the elongation increases by 30%.This result can be seen the effects of higher solid solubility inside crystals of alloy elements and smaller grain size by EMC.Microstructures and mechanical properties of the forged DCC Al-Zn-Mg-Cu-Zr alloy were investigated under different single and two-step ageing treatment.The tensile strength, yield strength,elongation in the longitudinal direction and electrical conductivity of alloy aged at 120 for 24h are 588MPa,505MP,11.6%and 33%IACS.When the second aging temperature is 160℃,the precipitates within grain grow slowly and obvious precipitate free zones(PFZ) can be found.In this period,the strength of alloy increases in the first 2h and then decreases gradually,and the fracture mechanism changes from intergranular fracture to shearing and transgranular mixed fracture mechanism.The elongation of alloy keeps increasing in this period.When the second aging temperature is 170℃,the precipitates within grain grow significantly and the width of PFZ is large.In this period,the strength of alloy reaches the peak for 1h and then decreases sharply,and the fracture mechanism changes from intergranular fracture to dimple fracture mechanism.The elongation increases at first and decreases later.Suitable two-step aging temper is aging at 160℃for 6h followed by 120℃for 6h.At this condition,the tensile strength,yield strength,elongation in the longitudinal direction and electrical conductivity of alloy is 541MPa,514MP,10.3%and 38%IACS respectively.Meanwhile,the L-T toughness fracture is 43.7 MPa·m^1/2.