Aging Behavior And Composition Control Of SiCp/Al-Mg-Si-(Cu)Composites

To obtain SiCp/Al composite components with precise dimensions,complex shapes and great load-bearing capacities,it is demanded to conduct cold plastic forming during natural aging(NA),and then to improve strength and hardness by subsequent artificial aging(AA).The process means that naturally aged SiCp/Al composites should have weak hardening and great plasticity to ensure adequate formability.Among SiCp/Al composites based on heat-treatable aluminum alloys,SiCp/Al-Mg-Si-(Cu)(SiCp/6xxxAl)composites have the above features,so are ideal composites to achieve cold plastic forming.However,the AA strength and hardness of SiCp/Al-Mg-Si-(Cu)composites are relatively low.Moreoever,the AA hardening ability of these composites can be weakened by precedent NA,leading to impaired load-bearing capacities.This phenomenon is called the negative NA effect.To develop cold-workable high-strength SiCp/Al-Mg-Si-(Cu)composites,it is fundamental to understand and solve the negative NA effects,and then provide a basis for composition design and aging process control.However,these have hardly been reported up to now.In this study,by fine characterization of microstructures and mechanical properties,the features of the negative NA effect in SiCp/Al-Mg-Si-(Cu)composites were investigated,and the solutions for the negative NA effect were discussed.Furthermore,the schemes of composition design and aging process control for cold-workable high-strength SiCp/Al composites were established.Compared to traditional SiCp/Al composites,the newly developed SiCp/Al composites can better meet the requirements of cold plastic forming during NA and service after AA.Firstly,the discrepancy in the negative NA effect between a 17 vol.%SiCp/Al-1.2Mg-0.6Si(in wt.%,the same below)composite and Al-1.2Mg-0.6Si alloy was compared.It is found that in both the composite and Al-Mg-Si alloy,clusters formed during NA significantly inhibit the formation of β" phases and promote the formation of coarse β’ phases,leading to the reduced volume fraction of precipitates and enlarged precipitate sizes,thereby causing the negative NA effect.However,the difference from the Al-1.2Mg-0.6Si alloy is that the quenching dislocations in the composite promote the formation of dislocation-induced precipitates and thus mitigate the reduction of NA on the volume fraction of precipitates.In addition,the composite has a lower proportion of coarse β’ phases due to the depletion of Mg by chemical reactions at or near SiCp-Al interfaces.Consequently,the negative NA effect in the composite is weaker than that in the Al-Mg-Si alloy.Secondly,according to the mechanism that Cu can promote the formation of β"phases and L phases(a Cu-containing precipitate with good stability),and inhibit the formation of coarse β’ phases,this study tries adding Cu to inhibit the negative NA effect of the 17 vol.%SiCp/Al-1.2Mg-0.6Si composite.Results show that Cu plays two opposite roles on inhibiting the negative NA effect.On the one hand,Cu promotes the formation of β" and L phases,and inhibits the formation of β’ phases,which helps mitigate the negative NA effect.On the other hand,Cu promotes the formation of clusters during NA.The clusters that cannot transform into β" and L phases impair AA hardening.Therefore,the inhibition effect of Cu on the negative NA effect in Cu-containing composites fluctuates with the increase of Cu content.In the 17 vol.%SiCp/Al-1.2Mg-0.6Si-1.0Cu composite and Al-1.2Mg-0.6Si-1.0Cu alloy,some NA clusters can transform into β" and L phases during AA.Different from the situation in the Cu-free samples that the transformation of NA clusters into coarse β’phases can aggravate the negative NA effect,the transformation of NA clusters into β"and L phases is beneficial for mitigating the negative NA effect because of the excellent stability of β" and L phases.In the composite,the chemical reaction at the SiCp-Al interface reduces Mg/Si ratio,which makes the transformation easier to happen,so the composite exhibits a weaker negative NA effect than the Al-1.2Mg-0.6Si-1.0Cu alloy.Thirdly,following the pre-aging process in Al-Mg-Si-(Cu)alloys,the 17 vol.%SiCp/Al-1.2Mg-0.6Si-1.0Cu composite was also pre-aged immediately after quenching.It is found that clusters formed during pre-aging have weaker hardening abilities but stronger transformation abilities to β" and L phases than NA clusters,so pre-aging can further mitigate the negative NA effect without aggravating NA hardening.Under the optimal pre-aging process,the negative NA effect of the composite can even be eliminated.The discrepancy in pre-aging effects(means the role of pre-aging to mitigate the negative NA effect)between the composite and Al-1.2Mg-0.6Si-1.0Cu alloy was compared.For the composite,it is easy for supersaturated solute atoms to participate in pre-aging clusters during NA after pre-aging,so β" and L phases are not affected by NA.In contrast,in the Al-1.2Mg-0.6Si-1.0Cu alloy,it is easy for supersaturated solute atoms to form new clusters during NA after pre-aging,leading to the reduced nucleation rate and thus enlarged sizes of β" and L phases with prolonged NA time,thereby impairing pre-aging effects.Finally,based on the above investigations,this study developed two kinds of new SiCp/Al composites through composition design,which can better meet the requirements of cold plastic forming and service.First,for the occasions with high load-bearing demands,this study developed a new SiCp/Al-2.0Cu-1.2Mg-0.6Si composite by controlling the Cu content higher than the SiCp/Al-1.2Mg-0.6Si-1.0Cu composite but lower than the traditional high-strength SiCp/Al-Cu-Mg series composites(such as SiCp/2009Al).The new composite has higher strength than SiCp/2009Al composites after AA due to the Si-modified precipitation,thereby exhibiting greater load-bearing capacities.Moreover,the new composite has a much lower content of alloying elements than SiCp/2009Al composites,so shows lower strength and higher ductility in the NA state,which solves the problem that SiCp/2009Al composites are difficult to cold plastically form caused by the severe NA hardening.Second,for occasions with high demands of cold plastic forming,this study developed a new SiCp/Al-1.2Mg-0.6Si-1.0Zn composite.Zn hardly affects NA hardening,so the formability of the new composite in the NA state is comparable to that of the SiCp/Al-1.2Mg-0.6Si composite.After being conducted a new two-stage AA process including a high-temperature treatment at 170℃ and then a low-temperature treatment at 120℃,Zn can be precipitated,thereby increasing the hardness of the new composite.