Study On Pulse Current Heattreatment Technology For TC4 Alloy With A Complicated Shape And Its Mechanism

With the rapid development of modern industry,the service environent and performance requirements of the metal materials are becoming more and more stringent.In order to meet the needs of industrial production,plenty of complicated metal components with different performances in different working surfaces.Therefore a new high efficiency heat treatment used to modification research for metal materials has been the most urgent study topic.TC4 is the most widely used??+??type titanium alloy of Ti-Al-V series.In this series,there are four structural types including equiaxed microstructure,duplex microstructure,net-like microstructure and Widmannstatten structure.The major factor in determining the performance of TC4 alloy is microstructure and Morphology,which is determined by heat treatment process.Generally,electrical pulse modification plays an important role in electromigration,electro plastic,solidification process and recrystallization process of metal materials.And the pulse current density is related to cross sectional area of metal materials.Thus,In this experiment,the pulse current heattreatment were adopted in complicated TC4 alloys to explore the influence law of pulse current heattreatment on microstructures and properties of TC4 alloys with complicated shape.The results show that complicated shape TC4 alloys exhibit various microstructure characterizations through different pulse currents heattreatment.There are three microstructures with increasing temperatures:?1?equiaxed structure in ? single-phase regions;?2?Sub-micron scale lamellar microstructures in??+??two-phase regions;?3?needle-like ?'phases in ? single-phase regions????' in cooling process?.The microhardness and electrical resistivity experience a rising trend taken along ? single-phase regions,??+??two-phase regions and ? single-phase regions.Three geometrical TC4 alloys?triangular prism,rectangle and cyclinder?were estigated through the same pulse parameter 700 V,20Hz,2×10-3s.In rectangle section TC4 alloys,part A and B illustrate refined rod-short equiaxed microstructures different from bimodal microstructures in part C;In cyclinder section TC4 alloys,there are three microstructures,refined equiaxed microstructures in part A,lamellar microstructures in part B,needle-like ?'phases,respectively;In triangular prism section TC4 alloys,all the three parts show coarse equiaxed microstructures.The changes of microhardness abide by the rules: HV_cylinder >HVrectangle > HV_{triangular prism};meanwhile,the electrical resistivity is as followed: ?_cylinder >?_rectangle>?_{triangular prism}.In ? and ? single-phase regions of TC4 alloy subjected to pulse current heattreatment still retain intrinsic orientation relation,namely,the ? phase with the ? phase in the Burgers relation:?0001?_?//?110?_?,[11???0]_?//[1???1]_?.After pulse current heattreatment at 850-1000?,many lamellar ? phases are observed to coexist inside the coarse ? phase.They are in differentorientations and these ntersecting lamellae subdivide the original ? phase into many blocks,resulting in a new orientation relation:?1???00?_?//????12?_?,[0001]_?//[110]_?.In summary,this work investigates the effect of pulse current heattreatment on microstructure and properties of TC4 alloy with a complicated shape,which to some extent proposes a new mothed for controlling multi phase microstructure of metal and alloy with complicated shapes.