| Aluminium-silicon casting alloys are widely used in industry,such as aerospace engineering and automotive manufacture.Iron is the most common impurity in Al-Si alloys,and the needle-like iron intermetallic phase is very brittle and can significantly deteriorate the mechanical properties,especially the elongation.Thus,the purification or the transformation of the iron intermetallic phase in aluminum alloys are the key issues for the improvement of both the mechanical properties and the manufacturing process.In the paper,the effects of Li on the microstructure,phase assemblages of the Al-Si-Fe alloys were investigated by using XRD,OM and SEM and those of Li on the mechanical properties were determined by hardness and tensile properties.The transformation temperatures of primary α-aluminum,pro-eutectic β-AlFeSi and eutectic β-AlFeSi and Si during solidification of the alloys without and with Li additions were determined by DSC techniue.The mechanism responsible for the modification of both β-AlFeSi and Si were proposed based on the analysis of DSC.The addition Li into the Al-Si-Fe alloys can change the morphologies and sizes of both eutectic Si and β-AlFeSi phases.With the addition of Li,the eutectic Si is changed from plate-like structure to fibrous-like structure,theβ-AlFeSi phases are still needle-like but the size were greatly decreased,especially in the alloys with higher Fe content.DSC results indicate that the addition of Li decreases the crystallization temperature of eutectic Si andβ-AlFeSi,so as to increase the undercooling of ternary eutectic reaction.Meanwhile,the Li addition improves.the crystallization temperature of primary a-aluminum,the grown of primary a-aluminum dendritic crystal restrains the growth of primary β-AlFeSi,thus,to refine the eutectic Si and β-AlFeSi.Compared to the alloy without Li,the tensile strength and elongation of the alloys with Li additions were increased.The tensile strength and elongation of the alloys with 0.1%Li are 176 MPa and 7.0%,which are increased by 18%and 24%,respectively.Compared to those of the alloy without Li,149 MPa and 2.3%.The strength and elongation of the alloy with 0.2%Li are 183 MPa and 7.8%,which increased by 22.8%and 239%,compared to those of the alloy without Li.The decrease of the tensile properties of the alloy with 0.4%Li is due to the formation of a ternary phase,AlLiSi,which deteriorates the properties,particularly elongation,due to its brittleness.The eutectic silicon particles can dissolved and broke during following solution heat treatment and the effects increases with increasing temperature and holding time.With the additions of Li,the dissolution and broken process of the alloys were accelerated.After the process of 570℃ solution treatment,littleβ-AlFeSi melted.The β-AlFeSi melted more when Li element added to the alloys.Based on the changes of the hardness of the alloys after both solution and aging heat treatment,the optimum heat treatment parameters were determined.For the alloy without Li,there is no strengthening effect after aging heat.treatment.But for the alloys with Li additions,the yield strength were increased remarkably.When solutionized at 540℃ for 4h,the yield strength of the alloys with Li additions is about 83 MPa.After aging heat treatment,the yield strength of the alloys with 0.1%,0.2%and 0.4%are 118 MPa,122 MPa and 115 MPa,respectively,which are increased by 44%,43.5%and 40%.When solutionized at 570℃ for 40min,there is still not strengthening observed for the alloy without Li after aging heat treatment.But for the alloys with Li addition,the yield strength of the alloys with 0.1 wt.%,0.2%and 0.4%Li are 189 MPa,175 MPa and 180 MPa,which are increased by 133%,133%and 128%,compared to that,about 80 MPa,of the alloys after solution heat treatment. |
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