Investigation On Forming Law And Properties Of Ti And Ti Alloys By High-velocity Compaction

Titanium and its alloys have been widely used in aerospace, automotive, marine, chemical, biomedicine and other areas because of their excellent combination of elastic modulus, corrosion resistance, specific intensity and biocompatibility. Powder metallurgy technology has the characteristics of high efficiency, high precision, high performance and low cost. Meantime, it has unique advantages in environmental protection, green manufacturing and producing false alloy. As a new powder metallurgy technology, high velocity compaction(HVC) technology enables to manufacture high density and high performance powder metallurgy parts with low cost. The compacts pressed with HVC have higher relative density than that formed by conventional compaction methods. In the paper, pure titanium, titanium alloys and titanium matrix composites were regarded as raw material and the green compacts were fabricated using the mechanical energy stored type powder high velocity compaction machine. Then the effects of impact energy, the contents of TiB2 and sintering holding time on density and performance were discussed, and the densification mechanism of the process was investigated preliminarily. The results provide guidance for the application of titanium and titanium alloys.The green density can be increased by increasing impact energy. As impact energy rises up, the green density of pure titanium increases when the increment quality of density reduces gradually. At the maximum impact energy of 1805 J, the highest green density of pure titanium samples reaches up to 4.37 g/cm3(relative density 96.9%). The HVC experimental datas of pure titanium are compared with Guo Shiju equation, Chuanbei equation and Huang Peiyun equation, and found to fit well. Furthermore, the green strength and the radial spring back of pure titanium compacts also increase with increasing impact energy. The densification mechanism of high velocity compaction was investigated preliminarily. The results reveal that the high velocity compaction is one of dynamic compaction whose suppression effect is more obvious than static compaction, and the forming effect increases with increasing impact energy. During the course of pressing, grains rearrangement, grains breakage and grains deformation occur. Meantime, the temperature effect caused by friction, plastic deformation and heat insulation leads to cold welding, which is beneficial to the densification of the powders.With impact energy increasing, the green density and radial spring back of the Ti-13Nb-13 Zr and Ti-29Nb-13Ta-4.6Zr alloys increase. When the impact energy is 1805 J, the Ti-Nb-Zr compact obtains the maximum green density of 4.72 g/cm3(relative density 94.0%), and the green density of Ti-Nb-Ta-Zr compacts reaches up to 5.63 g/cm3(relative density 94.1%). Since element composition and particle size are different, the radial spring back represents different. With the increase of impact energy, the increasing trend of radial spring back of Ti-Nb-Zr green compact increases when that of Ti-Nb-Ta-Zr slows down. TiB2 is added in titanium alloys, the green density and the radial spring back of the TiNbZr/Ti B2(TNZs) and TiNbTaZr/TiB2(TNTZs) alloys increase with increasing impact energy, but have no obvious regularity with the content of TiB2.After vacuum pressureless sintering, the relationships between the density, hardness, tensile of the sintered compact and the impact energy, sintering holding time, the quality of TiB2 were investigated. As impact energy increases, the sintered density, hardness and tensile strength of pure titanium samples increase. Meantime, the sintered density and hardness increase with sintering holding time prolonging. When impact energy is 1805 J and sintering holding time is 2.5 h, the highest sintered density reaches up to 4.50 g/cm3(relative density 99.8%) and the highest hardness is 298 HV0.30. However, too long sintering holding time is bad for improving tensile strength of pure titanium. When impact energy is 1805 J and sintering holding time is 1.5 h, the sintered compact exhibits a maximum tensile strength of 638 MPa. With the increase in impact energy, the sintered density, hardness, tensile strength and compression strength of the Ti-Nb-Zr and Ti-Nb-Ta-Zr alloys increase when the sintering shrinkage of the alloys reduces. After sintered, the sintered density of Ti-Nb-Zr increases slightly and that of Ti-Nb-Ta-Zr reduces. At impact energy of 1805 J, the maximum sintered density of Ti-Nb-Zr alloy reaches up to 4.79 g/cm3 and the highest hardness is 456 HV0.30, when the maximum sintered density of Ti-Nb-Ta-Zr alloy reaches up to 5.53 g/cm3 and the highest hardness is 325 HV0.30. Too long or too short sintering holding time is not benefit to improve tensile strength of the titanium alloys. When pressed at 1805 J and sintered holding for 2.0 h, the highest tensile strength of Ti-Nb-Zr is 1167 MPa when that of Ti-Nb-Ta-Zr is 630 MPa. The hardness of the TiNbZr/TiB2 and TiNbTaZr/Ti B2 samples increases significantly with increasing impact energy. At impact energy of 1805 J, the TiNbZr+12%Ti B2(TNZ4) and TiNbTaZr+12%TiB2(TNTZ4) samples have the maximum hardness, which are 558 HV0.30 and 490 HV0.30 respectively. The tensile strength of the TiNbZr/TiB2 and TiNbTaZr/TiB2 alloys increases with increasing impact energy but decreases with increasing reinforcement content. At impact energy of 1805 J, the TiNb Zr+3%TiB2(TNZ1) and TiNbTaZr+3%TiB2(TNTZ1) samples have the maximum strength, which are 765 MPa and 702 MPa respectively.