Microstructure And Mechanical Properties Of Recycled 6061 Aluminum Alloy By Rare Earth Microalloying

Because Aluminum and its alloy good corrosion resistance,wide use,100%recyclable,which is implied as the new century"energy bank".With the concept of"green development"and"replacing steel with Aluminum"deeply rooted in the hearts of the people,the demand for Aluminum products continues to increase,and the recycling and reuse of waste Aluminum products is imminent.However,impurities（H,Al₂O₃,etc.）in the recycled Aluminum lead to low mechanical properties,resulting in degraded use of recycled 6061 Aluminum alloy.At present,the effects of rare earth on the microstructure and properties of Aluminum alloys have been studied extensively.However,most researchers are concerning about the effect of Sc,Er and other expensive Rare Earth on the microstructure and mechanical properties of Aluminum alloy,which greatly increases the production cost for practical production.Therefore,the development of Rare Earth recycled 6061 Aluminum alloy with low price and excellent performance has far-reaching significance for promoting the use of recycled6061 Aluminum alloy.In this paper,with the recovery of 6061 Aluminum alloy as the research objective,low cost La,Ce and Y mixed Rare Earth were added by means of microalloying for melt purification,clutter control and microstructure refinement to obtain high quality alloy.The microstructure was further regulated by extrusion deformation and optimization of heat treatment process,and the microstructure and mechanical properties before and after metamorphism were quantitatively analyzed.To reveal the mechanism of rare earth microalloying.The recycled 6061 Aluminum alloy has serious defects of shrinkage porosity and oxidation inclusion.The shrinkage porosity and oxidation inclusion size are 30～60μm and 20μm respectively.When 0.15 wt.%La,0.1 wt.%Ce and 0.1 wt.%Y were added,no obvious shrinkage porosity and oxidation inclusion defects were found.Moreover,rare-earth elements can promoteα-Al grain refinement from 171.32μm to 127.71μm.The main reason is the increase of the undercooling degree after the addition of rare earth elements and the formation of the nano-rare earth phase intoα-Al heterogeneous nucleation particle to promote grain refinement.Compared with rare earth La and Ce,the effect of rare earth Y is better.The main reason is that the formed Al₃Y and Al atoms have a lower mismatch degree（5.6%）,so it is easier to become the nucleation particle ofα-Al phase.So as to play a better refinement effect.The acicular Fe-rich phase formed fishbone and short rod-like morphology.The SEM results showed that rare earth elements were enriched on the surface of Fe-rich phase,and the enrichment of rare earth atoms hindered the growth of Fe-rich phase and weakened the preferred growth trend.This microstructure makes the as-cast regenerated 6061 alloy with rare earth have excellent mechanical properties.Compared with unmodified regenerated6061 aluminum alloy,the tensile strength and elongation of the alloy are increased by40.02%and 60.6%,respectively.After the homogenization and extrusion process at 525℃/18 h,the second phase morphologies are short rod-like and spherical,with short rod-like size of 10～20μm and spherical size of 5μm.Compared with the unmodified 6061 regenerated alloy,the tensile strength and elongation of the alloy are increased by 16.6%and 6.6%,respectively.For the as-cast or extruded alloy,the triple tensile strength of the alloy is greatly improved after adding trace rare earth Y.The main reason is that the solid solubility of rare earth Y in aluminum is 5 times that of rare earth La and rare earth Ce（at room temperature）,and its solid solution strengthening effect is better.Meanwhile,for Fe-rich phase,La and Ce can only play a refining effect,while rare earth Y can promote the transformation ofβ-Fe toα-Fe,which makes the alloy containing rare earth Y have excellent mechanical properties.After solution treatment at 570℃/2h,the hardness of the regenerated 6061aluminum alloy is 89.4HV,and that of the alloy containing rare earth（La,Ce,Y）is97.1HV,an increase of 8.61%.The optimal aging system of unmodified 6061 alloy is570℃/2 h+175℃/6 h,and the tensile strength and elongation are 321.56 MPa and15.2%,respectively.The optimum aging system of the alloy containing dilute（La,Ce,Y）is 570℃/2 h+175℃/4 h,and the tensile strength and elongation are 348.26 MPa and 16.5%,respectively.Compared with alloy 1,the tensile strength and ductility are increased by 8.3%and 8.5%after the addition of rare earth.Rare earth elements can effectively promote the precipitation of the reinforced phase of the recycled 6061 alloy,greatly reducing the time for the alloy to reach the peak aging.Meanwhile,the uniform and fine distribution of the precipitated phase leads to a significant increase in the alloy strength.Rare earth microalloying can promote the alloy to reach the peak strength in advance,thus reducing the alloy aging cost.The results show that the recycled 6061 by rare-earth composite modification（La,Ce,Y）can achieve melt purification,grain refinement,Fe-rich phase modification and other microstructure control targets.This microstructure characteristic makes the alloy have good mechanical properties.At the same time,rare earth can promote the precipitation of Mg2Si strengthened phase in the aging process,which can effectively reduce the aging cost.The research results provide theoretical support for the grade protection utilization of recycled 6061 aluminum alloy.