Formation Mechanism Of The "Twin Peaks" Phenomenon By Aging Strengthening In Al-Si-Cu-Mg Alloy

The casting Al-Si alloy has the advantages of low density, high strength, good abrasionresistance and heat resistance. Because it has the characteristics of thermal expansion coefficientand high thermal stability, it is the most studied and applied material, Among the commercialcast aluminum alloys, Al-Si-Cu-Mg cast alloy is extensively used mainly because of its goodcastability and excellent mechanical properties in the heat-treated condition.The results show that: the casting Al-Si-Cu-Mg alloy after525℃×10h solution treatmentand ageing treatment in175℃,hardness curves appeared double peaks, after6h and12h agingtreatment, each of them reached the peak hardness.Mg and Cu are the most important alloying element of the casting Al-Si alloy,theydetermine the type and distribution of the the precipitation phase after the alloy heattreatment. the Al-10Si-xCu-0.5Mg alloy brinell hardness value increased with increasingCu content in a certain range. the Al-10Si-1.5Cu-xMg alloy brinell hardness valueincreased with increasing Mg content in a certain range. This thesis is about observing themorphology of the alloy with conventional method which chemical constitution, in thepresence of TEM, SEM, X-ray, etc. and it analysed the strengthening mechanism of the alloyheat treatment strengthening and precipitation characteristics of strengthening phase.Theneedle or strip precipitated phase θ(Al2Cu) could be observed in peak-aged alloy, and thelength of structure phase after aging12hours is clear longer than the aging6hour′s.Three-dimensional atom probe experiments, to flaky aggregation, Cu, Mg and Si elementswithin the same area at175°C artificial aging r2h. the equivalent diameter of about4.58nm,segregation in the average percentage of each element atoms: Al accounted for57.52%, Siaccounted for22.38%, Mg accounted for17.41%, Cu accounted for2.69%. Q phase can beformed early in the aging.The casting of Al-Si-Cu-Mg alloy the aging strengtheningsingular peak phenomenon causes: first aging peak phase caused by Q phase, the second aging peak caused bythe θ phase formation.