The Segregation Behavior,microstructure Evolution And Mechanical Properties Of Twin-roll Casting Al-Mg-Si Alloys

Aluminium alloys are the key materials to achieve lightweight,among which Al alloy sheet is widely used.The traditional preparation technology of Al alloy sheet has the disadvantages of complex processes and high cost.The two-roll casting could shorten the process of Al alloy sheet production,reduce production costs and reduce pollutant emissions,which is in line with the current economic development trend of low carbon and environmental protection.However,TRC Al alloy is prone to defects such as macrosegregation,especially for high-alloyed Al alloy,the center segregation is particularly serious.Therefore,it is of great significance to explore the microstructure evolution law and segregation influence mechanism of high alloying element content TRC alloys for developing high performance Al alloys and high efficient Al alloy preparation technology.In this paper,Al-Mg-Si alloy was taken as the research object,and Al-Mg-Si alloy sheets with different alloying elements were prepared by twin-roll casting（TRC）process,and some alloy samples were prepared by traditional permanent mold casting（TC）method for comparison.The solidification microstructure,segregation behavior,microstructure evolution and mechanical properties of TRC Al-Mg-Si alloys were studied.The main conclusions are as follows:（1）The influence of TRC process on solidification microstructure of Al-Mg-Si alloy was expounded.The high cooling rate of TRC process could refine the grain size and secondary dendrite arm spacing（SDAS）of Al-Mg-Si alloy,and reduced it to about1/4 of that of TC alloy.TRC process could refine the second phases of the alloy and improve its morphology and distribution.TRC process could improve the solid solubility of alloying elements,and reduce the solute content at the grain boundary,thereby reducing the microsegregation.The large temperature gradient（TG）formed by the contact between the surface of TRC alloy and the roller made the TRC alloy showed the nonuniform microstructure on the ND-RD plane,which was mainly composed of columnar dendrites near the surface of both sides and near-equiaxed grains in the center region.Due to the existence of roll separating force（RSF）in TRC process,the solute was enriched to the center region,thus forming the center segregation.（2）The influence of TRC process on the mechanical properties of Al-Mg-Si alloy was revealed.For Al-Mg-Si alloy with high solute content,there were a large number of coarse eutectic second phases in TC samples,which could not be completely eliminated by solution treatment,resulting in premature fracture of the alloy during the tensile process.The high cooling rate of TRC could refine the eutectic second phase,thus improving the plasticity of the alloy.（3）The influence of Mg/Si ratio on microstructure evolution and mechanical properties of TRC Al-Mg-Si alloys was revealed.Mg/Si ratio affected the eutectic reaction,and the solidification microstructures with different Mg/Si ratios had different kinds and distributions of eutectic second phases.The alloy with low Mg/Si ratio（Mg/Si=0.5）had severe center segregation due to the enrichment of eutectic Si in the center region.After cold rolling and solution treatment,the alloy with high Mg/Si ratio（Mg/Si=2.9）showed an elongated grain structure,due to the pinning effect of residual Mg₂Si particles.After artificial aging treatment,the volume fraction ofβ″phase of low Mg/Si ratio and high Mg/Si ratio alloys were 2.2%and 1.4%,respectively.Therefore,low Mg/Si ratio could increase the volume fraction of precipitates,thereby increasing the precipitate hardening.The yield strength of the alloy with low Mg/Si ratio was 84MPa higher than that of the alloy with high Mg/Si ratio.（4）The composition susceptibility coefficient S of center segregation was proposed to analyze the influence of element content on center segregation.The S value reflected the influence of alloying elements on the transverse growth rate of columnar dendrites during solidification.The smaller the S value was,the faster the transverse growth of columnar dendrites would be,thus preventing the enrichment of solute to the center region and reducing the aggregation degree of center segregation.For Al-Mg-Si alloys with the same eutectic reaction,the aggregation degree of center segregation decreased with the increased of Mg and Si contents.（5）The influence of Fe content and TiC nanoparticles on segregation behavior,microstructure evolution and mechanical properties of Al-Mg-Si alloys was revealed.The high cooling rate of TRC process maked the Fe-rich eutectic second phase change from coarse Chinese script shape of TC process to fine particle shape,thus improving the plasticity of high Fe Al alloys.Increasing Fe content would increase the content of Fe-rich eutectic second phase,thus aggravating the degree of center segregation.In the process of solution treatment,high Fe content could promote the precipitation of Fe-rich dispersoids and produce dispersion strengthening effect to improve yield strength and ultimate tensile strength.The addition of TiC nanoparticles reduced the center segregation,and increased the content of dispersoids in the center region during solution treatment,thus improving the tensile strength and uniform elongation of the alloy.（6）The formation mechanism of center segregation was revealed.The center segregation was mainly caused by the growth of columnar dendrites along the TG and the enrichment of solute along the dendrite to the center region under the RSF.Center segregation could be reduced by inhibiting the growth of columnar dendrites.The growth of columnar dendrites could be hindered by changing the contents of Mg and Si,TRC process parameters and introducing TiC,thus reducing the center segregation.