Effect Of Nb And Zr Alloying On Microstructure And Properties Of TC4 Alloy By Laser Melting Deposition

In recent years,people use additive manufacturing(AM)technology to achieve the near net forming of titanium alloy structural parts in the aerospace field has achieved rapid development,but AM forming samples often need heat treatment to meet the high standard requirements of the current aerospace field.Therefore,in order to avoid the problems of high manufacturing cost of heat treatment process,long production cycle and difficulty in heat treatment of large structural parts,the as-deposited state microstructure can be regulated by adding alloying elements according to the characteristics of titanium alloy,so as to further improve its mechanical properties.In this study,a certain mass fraction(2%,4%,6%,8%)of Nb and Zr elements were added to TC4 by laser melting deposition(LMD)technology,and the effects of Nb and Zr on the microstructure and mechanical properties of LMD TC4as-deposited state in XY direction were studied by means of optical microscopy,scanning electron microscope,X-ray diffractometer,Vickers hardness tester,tensile testing machine and pin-disc friction and wear testing machine,etc.,which provided a theoretical basis for further improving the comprehensive properties of laser additive manufacturing titanium alloy.The microstructure analysis shows that the as-deposited state samples with Nb or Zr elements additions still consist of close-packed hexagonal structureα-Ti and body-centered cubic structureβ-Ti phase,but both Nb and Zr can increase theβ-Ti content in TC4 alloy to a certain extent.The metallographic microstructure of TC4,TC4+Nb and TC4+Zr as-deposited state is all composed of equiaxed crystals,and the microstructure is all composed of primary needle-likeα-Ti and net basket structure.Adding Nb or Zr respectively can significantly refine the originalβgrains,which has the effect of fine grain strengthening;The average width of primaryα-Ti decreases from about 5.5μm(TC4)to about 1.5μm(TC4+8%Nb)with the increase of Nb content;And with increasing Zr content,the average width of primaryα-Ti decreases to about 3.2μm(TC4+8%Zr).In addition,the energy spectrum analysis shows that both Nb and Zr are extensively clustering inβ-Ti,which have the effect of solid solution strengthening,and Zr can reduce the solid solubility of Al inβ-Ti.The results of mechanical properties show that the hardness of TC4 alloy can be improved by adding Nb or Zr under the effect of fine grain strengthening and solid solution strengthening.Compared with TC4(about 339.9HV),the alloys adding 8%Nb(about 405.4HV)and 8%Zr(about 420.7 HV)respectively have the highest hardness value.Both the tensile strength(σ_b)and elongation(δ)of TC4+Nb as-deposited state samples at room temperature increase firstly and then decrease,while the tensile strength of TC4+Zr as-deposited state samples increases slowly,and elongation increased firstly and then decreased.However,they are generally higher than TC4 as-deposited state samples(σ_b≈928.6MPa,δ≈9.7%).After comparative analysis,the alloy with 6%Nb has the best tensile properties,the maximum tensile strength is about 1069.7 MPa,and the maximum elongation is about12.3%;The maximum tensile strength of the alloy with 8%Zr is 962.4 MPa,and the maximum elongation of the alloy with 4%Zr is 10.8%.The friction and wear tests show that adding Nb or Zr can improve the wear resistance of TC4 alloy,and the alloys adding 8%Nb and 6%Zr respectively have the best wear resistance.