| Since titanium alloy owed great mechanical properties,including outstan ding properties of intensity and fatigue resistance,it also suffered high tem perature and corrosion,made it widely used in aerospace/aviation,biological science,rail transit and so on.TC4 Ti alloy owed typically microstructure?+?,it had great combination properties,which made it widely used in the world and has the highest production.But,TC4 had low thermal conductiv ity,and it can easily come up chemical reaction with other products,which made TC4 hard to process and had a long production cycle.Addictive manufacture offered TC4 a new way to develop.But,the biggest problem that addictive manufactured TC4 titanium alloy exist,was the anisotropy in its microstructure,it had a great influence on the productions.So,we had to understand why the anisotropy happened and how it worked,offered useful reference to the processing,then we can make addictive manufacture better and better.This issue designed two different experiments:the addictive manufactured experiment and the fatigue crack propagation experiment.We researched the microstructure characteristics of the product produced by addictive manufactured TC4 alloy and the average hardness after different heat treatment,find the reasons.Refer to GB/T 6398-2000,we did the crack propagation experiment,used Paris formula to create ?K-da/dN mathematic models.Compared the differences between addictive manufactured product and as-forged product,with the microstructure,paths of crack propagation and the ?K-da/dN mathematic model,discussed the reasons.The main results of this issue included:(1)The macrostructure of TC4 alloy was coarse ? columnar grains which vertical to the deposited direction,the dimension of the columnar grains crossed several deposited layer.The microstructure of alloy was typical widmanstatten structure,the boundary of ? grains was clear,? phase exist in the grain boundary consecutively,the a phase located in ? grains was regular arranged in coarse sheet.When the cooling speed was fast,the ? phase became thinner.When the cooling rate was high,the acicular martensite ?' exist.Widmanstatten structure had low tension moulding and fatigue property.(2)With the temperature of heat treatment growd,structure of TC4 product got coarsen,hardness of the product got higher first,then it got lower,the hardness got peak at 800?.Used 800?,1 hour,and air cooled heat treatment to produce crack propagation samples.The paths of crack propagation had great differences between intergranular fracture and transgranular fracture samples,intergranular fracture had stable paths and high crack rate,but transgranular fracture had tortuous paths and secondary crack,high energy loss and low crack rate.(3)The crack directions were different in zone I of intergranular fracture,grain boundary could be found in zone II and there were much fatigue belts in the same direction;transgranular fracture had much fatigue belts in zone I&II in different directions and many secondary crack.Used Paris formula to discuss the crack propagation,found intergranular fracture had higher crack rate than transgranular fracture.(4)The microstructure of as-forged ti-alloy was widmanstatten structure,coarse phase changed the crack direction,made crack rate unstable and stress forced,created secondary crack,dispersed the energy of main crack.The secondary crack and crack closure effect could also made crack rate unstable. |
PDF Full Text Request