Cutting Aluminum Alloy Microstructure And Properties

The microstructure and properties of five kinds of free-cuttingaluminum alloys were investigated using mechanical test,XRD,SEMand EDS,DTA,TEM and OM analysis, and then corrosion property andmachinability of alloys were also studied by Huey test and on-the-spotmachinability experiments. The results show that:1) Pb was replaced by minor Sn, designed and developed 2011A alliybased on conventional lead-containing 2011 alloy, after optimization ofheat-treatment, the best aging processing of lead containing free-cutting2011 alloy after solution and cold-drawn is 170℃/8h, at this condition,the tensile strength,yield strength and elongation can get to 471MPa,378MPa and 10.1% respectively; and The best aging processing of Snand Bi micro-alloying lead-free free-cutting 2011A alloy after solutionand cold-drawn is 160℃/6h, in this condition, the tensile strength,yieldstrength and elongation can get to 473MPa,391MPa and 11.2%respectively; The tensile strength of 2011A alloy is equiverlant to that of2011 alloy, the yield strength and elongation increased by 13 MPa and1.1% respectively, and the synthesis mechanical property of 2011A alloyis better than 2011 alloy; Huey test and on-the-spot machinabilityexperiments show that corrosion resistence property of 2011A alloy wasequiverlant to 2011 alloy, and machinability property is a bit better thanlead-cintaining 2011 alloy.2) Designed three alloys of 6020,6020A,6020B based onconventional 6262 alloy, and Pb and Bi were replaced by Sn or Sn and Bi,after optimization of heat-treatment system, The best aging processing oflead-free free-cutting 6020 alloy after solution and cold-drawn is170℃/6h, at this condition, the tensile strength,yield strength andelongation can get to 351MPa,336MPa and 13.5% respectively; and Thebest aging processing of lead-free free-cutting 6020A alloy after solutionand cold-drawn is 170℃/6h, the tensile strength,yield strength andelongation can get to 314MPa,286MPa and 12.8% respectively, and the best aging processing of lead-free free-cutting 6020B alloy after solutionand cold-drawn is 170℃/10h, the tensile strength,yield strength andelongation can get to 348MPa, 339MPa and 12.5% respectively. And themechanical properties of this three kinds of alloy are equivelent to orbetter than lead containing 6262 alloy; on-the-spot machinabilityexperiments show that machinability of 6020A alloy is better than 6262alloy, and machinability of 6020 alloy is equiverlant to 6262 alloy, and6020B alloy is inferior to 6262 alloy.3) The technology principle of free-cutting aluminum alloy ofMicro-alloying Sn,Bi replace poisous Pb were that: in 2xxx series alloys,low-melting tissue substance Sn,Bi and SnBi formed to ensure goodmachinability, and the synthesis mechanical property was ensured bydispersion distributed CuAl₂ precipitates, in 6xxx series alloys,low-melting tissue substance Mg₂Sn and Sn with Mg₃Bi₂ and Bi wereformed by Sn,Bi additives to ensure good machinability, and thesynthesis mechanical property was ensured by dispersion distributedMg₂Si precipitates.4) Compared with 2011 alloy, the aging time that need to attain peakstrength at 170℃of 2011A is decreased by 2/3, this is because the add ofSn have changed the aging sequence of alloy, compress the formation ofGP zone andθ″, and promote nucleation ofθ′which lead to thedecrease of aging time under the same temperature; moreover, Sn canrefine CuAl₂ phase and prevent its growth.5) Mg₂Sn in 6020 alloy with hot extrusion treatment mainly exist inthe form of hexagonal and then change into Cubic structure after solutiontreatment, this is because hexagonal of Mg₂Sn is a metastable state, andeasily transform into stable cubic structure.6) Bi is more likely to form Mg₃Bi₂ with Mg in Al-Mg-Si alloy whichwill decrease or prevent the formation of main strengthen phase Mg₂Si,and deteriorate the strengthen effect of Mg₂Si.