Laws And Mechanisms For The Effects Of La, Pr On The Microstructure And Elevated Temperature Properties Of The Cast Al-Cu Alloys

In recent years, for reducing fuel consumption, saving resources and protectingenvironment, the structural material with high performance and lightweightincreasingly urgent is required in the manufacturing industry. Aluminum alloymaterial which possess light weight, high strength and corrosion resistance, havebroad application prospects in the automotive, aerospace and defense industry.However, the strength of aluminum alloy at elevated temperature is low, whichlimited the scope of its application. In cast aluminum alloy, the casting Al-Cu alloypossesses good mechanical properties at room temperature. Compared with wroughtaluminum alloy, the casting Al-Cu alloy has the some advantages. However, thestrength and creep resistance of aluminum alloy is low at elevated temperature. Inthis research, we study the effect of single rare earth La, Pr and La+Pr compoundmodification on microstructure, mechanical properties and creep resistance castAl-Cu alloy, and discusses their mechanism of rare earth. In this paper, the results areas follows:1. It reveals that the strength of the modified casting Al-Cu alloy by La and Pr ishigher than that of the unmodified alloy at room and elevated temperatures. Thestrength and elongation at room temperature,180qC and220qC of the unmodifiedalloy is475MPa and8.4%,283MPa and9.6%,220MPa and10.4%. The strength and elongation at room temperature,180qC and220qC of the modified alloy by0.3wt.%La （or Pr）520MPa and13.6%（514MPa and11.7%）,320MPa and14.2%（307MPa and12.8%）,270MPa and14.7%（263MPa and13.6%）. The strength andelongation at room temperature,180qC and220qC of the modified alloy by La+Prcompound modification536MPa and14%,335MPa and14.5%,273MPa and15.1%.2. It is revealed that the age behavior of the casting Al-Cu alloy by La or Pr isobviously strengthened. The peak hardness of the modified alloy by0.3wt.%La（or Pr） at165°C,185°C and250°C is respectively187HV,180HV and138HV（183HV,174HV and132HV）, than those of the unmodified alloy （172HV,157HVand116HV）, increased by8.7%,14.6%and19%（6.4%,10.8%and13.8%）. Thehardness of the modified alloy by0.2wt.%La+0.1wt.%Pr at165°C,185°C and250°C is191HV,185HV and142HV, respectively, compared with the unmodifiedalloy, increased by11.05%,17.83%and22.41%. At250°C, the aging peakhardness time of the modified alloy by La or Pr are respectively4h and5h,respectively, that of Al-Cu alloy is6h.3. It is revealed that the creep mechanism of the unmodified and modified alloy byLa or Pr is dislocation climb. The creep resistance of the modified alloy by0.3wt.%La （or Pr） at180°C,200°C and220°C and stress of60MPa,70MPa,80MPa and90MPa are increased by3⁴times （2³times）, compared with the unmodifiedalloy. The creep resistance of the modified alloy by0.2wt.%La+0.1wt.%Princreased5⁶times. The apparent activation energy of the unmodified alloy is205KJ/mol at60MPa, those of the modified alloy by0.3wt.%La, Pr and0.2wt.%La+0.1wt.%Pr are274KJ/mol,249KJ/mol and307KJ/mol respectively.4. It is revealed that the microstructure of the casting Al-Cu alloy is refinement dueto La or Pr addition. The dendrites size of.–Al of the unmodified alloy is100¹80Pm, the dendrites size of.-Al of the modified alloy by0.3wt.%La orPr0.2wt.%La+0.1wt.%Pr is65⁸5Pm,80⁹5Pm and60⁸0Pm, respectively. In the modified Al-Cu alloy by La, Al20Ti2La phase is formed, which acts asprimary.-Al nucleation heterogeneous core, reducing the dendrite size of theprimary.-Al. In the solidification process, La and Pr induce solute redistribution.They gathered at the solid-liquid interface, increasing in undercooling insolidification process, resulting in reducing the dendrite size.5. It is revealed that the T cprecipitates size of the modified alloy by La, Pr isreduced. The number and thermal stability of T pcrecipitates of the modified alloyby La, Pr is increased. the T cprecipitates size of the unmodified alloy at165°C×10h,185°C×10h and250°C×6h aging treatment is140nm,180nmand240nm, respectively. The T cprecipitates size of the modified alloy by0.3wt.%La （or Pr） and0.2wt.%La+0.1wt.%Pr at165°C×10h,185°C×10h and250°C×4h （Pr modified alloy250°C×5h） aging treatment is100nm （110nm）,120nm （135nm） and145nm （165nm）, respectively. La and Pr enhance theinteraction between Al and Cu, which is conducive to the promotion the form ofT pcrecipitates.6. It is proposed that the strengthening mechanism of the modified alloy by La, Pr atroom temperature:.-Al dendrite grain refinement, precipitate precipitationhardening; strengthening mechanism at elevated temperature and creepmechanism of the modified alloy by La, Pr: La and Pr enhance the interactionbetween Al and Cu, improve thermal stability of T cprecipitates, inhibit thedecomposition and grown of T cprecipitates at elevated temperature, whichimpede dislocation movement. Al₁₁La₃phase and Al₁₁Pr₃phase have highmelting point, high hardness and high thermal stability. They are the distributionat the grain boundaries. Al₁₁La₃phase and Al₁₁Pr₃phase inhibit the grainboundary movement in the creep process. Therefore, it could improve theelevated temperature strength and creep resistance.