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The Effects Of Si ?Cu Addition And Heat Treatment On The Microstructure And Properties Of Al-30Zn Aluminum-High Zinc Alloy

Posted on:2018-02-06Degree:MasterType:Thesis
Country:ChinaCandidate:Y S HuFull Text:PDF
GTID:2321330515970562Subject:Materials science
Abstract/Summary:PDF Full Text Request
The outstanding advantages of Zn-Al cast alloys are cheap,low cost,high hardness and strength,good dimensional stability and natural aging strengthening,which can be used to manufacture sleeve,bearing wear parts and replace the traditional wear resistant materials(Sn based or Pb based bearing alloy,tin bronze alloy and nodular cast iron)directly without heat treatment.At the same time,Zn-Al alloys owe low density,moderate thermal conductivity and electrical conductivity,high tensile strength,ultimate bearing resistance,no magnetism and collision does not produce sparks,and other unique properties.However,Zn-Al alloys are easier to occur inter granular corrosion,which aggravates the corrosive wear and the natural aging.Besides,poor high-temperature mechanical properties and thermal stability,and the increased friction coefficient sharply,which limit its application greatly at so long time.In order to improve the defects of Zn-Al alloys and obtain excellent corrosion resistance and high-temperature wear resistance,this paper decides to use the Al-30 Zn based alloy and the Si,Cu alloying elements,which combined with the hardness test,tensile test and thermal stability test,impact toughness test,friction and wear test,immersion test and electron-chemical test system so as to study the effects of Si and Cu elements on the Al-30 Zn aluminum-high zinc alloy about the microstructure,mechanical properties,thermal stability,wear resistance and impact corrosion resistance.We also investigate the effects of homogenization treatment on the microstructure,mechanical properties and wear resistance of Al-30Zn-3Si-2Cu aluminum-high zinc alloy.The main conclusions are as follows:The microstructure of Al-30 Zn aluminum-high zinc cast alloy is composed of primary rich aluminum ? dendrites and eutectoid phase(?+?).With the increase of Si content,the hard phase eutectic Si starts to appear foremost,then the eutectic Si phase changes to be coarse more than 3.0wt.%Si which leads to micro segregation and precipitation,and at last the bulk primary silicon phase appears which the number and shape tend to be deterioration.With the increase of Si content(Si content is less than 3.0wt.%),the micro hardness increases,high temperature strength has beenimproved greatly,the size is more stable under high temperature and the wear rate decreases which reflects the wear resistance improvement of the alloy.However,the toughness and plasticity of the alloy decreases observably.At the same time,the corrosion potential of Al-30 Zn aluminum-high zinc alloy shifts positively and the corrosion tendency decreases with the increase of Si content,but will increase the amounts of corrosion micro-cells which increases corrosion rate and aggravates intragranular corrosion that the electron-chemical performance shows a decreasing trend.Al-30Zn-3Si aluminum-high zinc alloy will precipitate the rich Cu phase?(CuAl2)that disperses in the vicinity of grain boundaries and grain boundaries with adding the moderate Cu element,which is conducive to refine grain and increase the degree of uniformity of the microstructure.When the Cu content is more than2.0wt.%,the rich Cu phase ?(CuAl2)increases so lot that changes the particle size,which is unable to disperse and starts to be reticular distribution.With the increase of Cu content(Cu content is less than 2.0wt.%),micro hardness,tensile strength and wear resistance of the alloy are improved greatly,but will weaken the toughness and plasticity of the alloy.At the same time,the corrosion potential of Al-30Zn-3Si aluminum-high zinc alloy shifts positively and the corrosion tendency decreases with the increase of Cu content,and the corrosion rate will reduce when the Cu content is moderate.However,it will aggravate the electrode potential difference of each phase which increases the amounts of corrosion micro-cells that increases the corrosion rate and exacerbates intragranular corrosion when the Cu content is excess.The toughness and plasticity of the Al-30Zn-3Si-2Cu aluminum-high zinc cast alloy is relatively low,though the micro defects decrease obviously after 400?/10 h homogenizing annealing which is propitious to improve the toughness and plasticity,such as crack,shrinkage,dislocation accumulation,intragranular segregation,dendrite segregation and so on.In addition,the fine flake eutectoid appears,eutectic Si phase and ?(CuAl2)phase break to be nodulizing that disperse in the vicinity of grain boundaries and grain boundaries,the low melting point eutectic almost disappears,which are conducive to refine grain and increase the degree of uniformity of the microstructure.The strength and micro hardness are lower than the cast,but theplasticity and impact toughness have been greatly improved,for example,the elongation(5.59%)is about 1.67 times as cast(3.35%).Besides,the wear resistance and the comprehensive performance of the alloy are much better after 400?/10 h homogenizing annealing.
Keywords/Search Tags:aluminum-high zinc alloys, high-temperature performance, dry sliding friction and wear, corrosion resistance, heat treatment
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