Study On The Sub-Rapid Solidification Behavior,Microstructure Evolution And Mechanical Properties Of Al-Mg-Si Alloys

Aluminum（Al）alloys have great applications in automobile industry,which is of great significance to realize automobile light-weight and improve energy efficiency.The new twin-roll casting technology based on sub-rapid solidification（SRS,cooling rate 10²–10³℃/s）is a promising process,which can further improve the comprehensive properties and production efficiency of Al alloys.However,there are seldom studies that focused on the SRS in Al alloys,especially on the accurate relationship between solute redistribution,microsegregation,microstructure evolution and mechanical properties,which brings great difficulties and challenges to understand the SRS behavior and corresponding mechanical properties of Al alloys.Therefore,the effects of Mg contents on microstructure evolution,phase selection,microsegregation and mechanical properties of Al-Mg-Si-Fe alloys were studied under sub-rapid solidification at 200℃/s.The microstructure evolution,microsegregation and solute interaction in the liquid Al-0.7Mg-1.0Si-0.2Fe alloy were systematically studied at different cooling rate between 30–350℃/s.Finally,the influences of solution temperature on the solute clusters,precipitation evolution,and precipitation strengthening of the wrought Al alloy in the as-quenched and artificially aged states were also revealed.The main research contents and conclusions are as follows:（1）The effects of Mg contents on the columnar-to-equiaxed transformation（CET）, Fe-bearing phase selection,microsegregation and mechanical properties of Al-x Mg-1.0Si-0.20Fe alloys were investigated under the SRS.The results show that in the alloy with Mg content of 0.15 wt.%,the sub-rapid solidified microstructures were fully columnar and the Fe-bearing phase wasβ-Al Fe Si,while a small number of equiaxed grains（area fraction～9%）appeared and the Fe-bearing phase became α-Al Fe Si as the Mg content increased to 0.73 wt.%.Moreover,as the Mg content further increased,the proportion of equiaxed grains also rised markedly（≥35%）,and the Fe-bearing phase was stillα-Al Fe Si.A large number of Mg₂Si phases were generated at the dendrite boundaries in the high Mg content alloys.Finally,under this SRS condition, the alloy with 0.73 wt.%Mg had the weaker microsegregation and the better matching of mechanical properties.Therefore,the optimal composition for this SRS process is Al-0.7Mg-1.0Si-0.2Fe（wt.%）.（2）The microstructure evolution of the Al alloy dring the cooling rate of 30–350℃/s was investigated and the relationship between partition coefficient and cooling rate was quantitatively established.Furthermore,the ability of being trapped byα-Al matrix for solute elements were revealed at high cooling rate conditions.As the cooling rate elevated from 30℃/s to 350℃/s,the coarse Mg₂Si phase was completely suppressed. he Fe-bearing phase transformed fromβ-Al Fe Si toα-Al Fe Si and the proportion decreased from～3.3%to～0.4%.In addition,the enhancement of partition coefficient for Mg element was 0.22,which was higher than that of Si and Fe（0.09 and 0.06）.It can be concluded that the partition coefficient of Mg is the most sensitive to the change of cooling rate,i.e.,with the increase of cooling rate,the partition coefficient of Mg has the largest enhancement,which indicates that Mg element is prone to be trapped by theα-Almatrix during the SRS.（3）Based on the BCT model,a free dendrite growth model for multi-component dilute alloy systems was established.The results show that the crystal growth rate of the Al-Mg-Si-Fe alloy was between 0.05 m/s and 0.68 m/s in the range of 30–350℃/s.The theoretical partition coefficients of Mg,Si,and Fe elements were gained through the Aziz equation.It was found that the theoretical partition coefficients of Mg and Si wereclose to the measured values.However,the theoretical partition coefficients of Fe deviated from the measured results to a great extent.The reason is that Si significantly promotes the diffusion of Fe in liquid Al.（4）The effects of solution temperature on precipitation strengthening of sub-rapid solidified Al alloy were studied.When the solution temperature was 400℃,there was no obvious precipitation strengthening during artificial aging at 170℃,with the micro-hardness of 38.6 and 40.8 HV at as-queched and artificial aged states.While the Al alloy showed precipitation strengthening effect（the micro-hardness increases from 47.9 HV to 58.4 HV）after artificial aging for 4 h as the Al alloy was solution heat-treated at 450℃. Besides,when the solution temperature got to 550℃,the Al alloy showed precipitation strengthening after artificial aging for only 0.5 h,with the enhancement of 1.6 HV for micro-hardness.It can be concluded that the precipitation strengthening during artificial aging can be accelerated by raising the solution temperature.Futhermore,with the increase of solution temperature,the strength and micro-hardness of the Al alloy both in as-quenched and artificial aged states obviously increased.At as-queched state,the increment of micro-hardmenss was 44.9,as the solution temperature increasing from 400℃to 550℃.And it was 77.6 HV after being artificial aged.Therefore,the high solution temperature can also enhance the precipitation strengthening of Al alloy.（5）The influences of solution temperature on solute clusters and precipitates in Al alloy were revealed.There was no solute cluster or precipitate in the as-quenched and artificil aged（170℃,11 h）Al alloy after being solution heat-treated at 400℃,while a large number of Mg-Si clusters with radius of 0.8±0.3 nm and number density of 1.86×10²⁴ m^-3 existed in the as-quenched Al alloy when the solution temperature rised to 550℃. At the artificial ageing state,metastable precipitateβ"formed with the quantity density of～1.64×10²³ m^-3.Moreover,the contents of Mg and Si in theα-Al matrix after being solution heat-treated at 400℃were about 0.22 and 0.52 at.%,while the contents were significantly raised to～0.64 and 1.15 at.%when the solution temperature was elevated to 550℃.Therefore,the solid solubility of solute in theα-Al matrix can be improved by elevating the solution heat-treatment temperature.As the solution temperature increasing from 400℃to 550℃,the yield strength of as-quenched Al alloy increases by about 98 MPa.The increment is mainly provided by solid-solution strengthening（～41 MPa） and cluster strengthening（～57 MPa）,and the latter contribution is higher.In summary,the effects of Mg contents on the microstructure and mechanical properties of sub-rapid solidified Al-Mg-Si-Fe alloys were studied,and the optimum chemical composition system for SRS was acquired.Furthermore,the SRS behavior of Al alloy and influences of solution heat-treatment temperature on solute cluster,precipitate and corresponding mechanical properties were systematically investigated.The results of this work provide a foundation for deeply understanding the SRS behavior of the Al alloy,optimizing the microstructure and mechanical properties,and developing high-performance Al-Mg-Si alloys efficiently based on the SRS technology.