Effects Of Cryogenic Treatment On Room Temperature Properties Of TC4 & TC18 Titanium Alloys Sheet

Cryogenic treatment(CT)is the extension of the conventional heat treatment.It can not only change the properties of materials,but also test the service limit of materials at alternating temperatures by cryogenic treatment.The cryogenic treatment process has been extensively studied abroad and applied in industry,especially in iron and steel materials.The relevant researches have been carried out in Al,Cu,Mg and Ti alloy,etc.However,little research has been done on the effects of cryogenic treatment on mechanical properties of titanium alloys.Titanium alloys are widely used in aerospace,automotive industry,chemical energy and medical industries due to excellent characteristics,e.g.,low density,small elastic modulus,high specific strength,low thermal conductivity,good heat and corrosion resistance,low temperature brittleness,good weldability and biocompatibility.It is known as an important strategic material.In this paper,TC4 and TC18 titanium alloys were treated by cryogenic treatment with liquid nitrogen after recrystallization annealing(ACT).Then the properties and microstructure of the alloys are characterized and analyzed by universal testing machine,Vickers hardness tester,X-ray diffractometer(XRD),optical microscopy(OM)and scanning electron microscopy(SEM).Main research contents and results are as follows:(1)The microstructural evolution of TC4 and TC18 titanium alloys after recrystallization annealing followed by cryogenic treatment was studied.It was found that cryogenic treatment after recrystallization annealing could further transform metastable ? phase into stable ? phase and ? phase,and dispersed on the matrix.That can improve the homogeneity of microstructure of the alloys.The peak strengths of main slip surfaces have changed significantly,and the grain rotation and preferred orientation have taken place in the grains.The changes of the hardness of TC4 and TC18 titanium alloys after cryogenic treatment were studied.The results show that the changes in the hardness are not obvious.(2)Tensile properties of TC4 and TC18 titanium alloys after cryogenic treatment were studied.It was found that the elastic modulus is decreased,and yield strength and tensile strength are declined slightly,whereas the ductility is increased significantly.(3)Punch shearing forming process was studied.It was found that the shear strength of the two titanium alloys is decreased significantly at room temperature or low temperature,and the shear strength of the samples at room temperature is slightly lower than that at low temperature.Moreover,the shear strength at room temperature declines more obviously.The height of the smooth zone is larger than that of the untreated.The rollover diameters are slightly larger than those of the untreated.The height of the burr shows a decreasing trend with the prolongation of cryogenic time,but the change is not significant.(4)The research on deep drawing process shows that the drawing depth of two titanium alloys is deeper than that of the untreated,the drawing load is reduced,and the decreasing tendency of the drawing load slows down with the increase of cryogenic time.