Investigation On The Effect Of Heat Treatment And Hot Forming Strategy On The Formability Of Titanium During Incremental Forming Process

Titanium alloy is widely used in automotive,aerospace,medical and other fields because of good high temperature performance,corrosion resistance and high specific strength.Incremental sheet forming process,as a new type of die-less forming technology,is suitable for customized manufacturing of small batches and personalized titanium alloy products.However,the plastic deformation ability of titanium alloy at room temperature is poor.When using incremental sheet forming to manufacture titanium alloy products,there are some disadvantages such as large forming force,poor surface quality,and insufficient geometric accuracy.Therefore,how to improve the formability of difficult-to-deform materials such as titanium alloy is one of the current research hotspots.In order to solve this problem,this study takes two commonly used titanium alloys TA1 and TC4 as examples,and combines heat treatment with hot forming to improve plastic deformation ability of these two materials.And the research results were applied to the forming of skull prosthesis.The main contributions are summarized as follows.Firstly,the source of forming error was analyzed by orthogonal experiment,and the influences of step down size,tool diameter,feed speed and forming angle on the geometric accuracy of the forming parts were studied by range analysis method.It is found that forming angle has the most significant influence on the geometric accuracy of forming parts,followed by the tool diameter and step down size,while the feed speed has little effect on the accuracy.The geometric accuracy of parts decreases with the increase of the forming parameters.According to the experiment results,the combination of optimized parameters is:forming angle as 45°,tool diameter as 10 mm,step down as 0.5 mm and feed speed as 3000 mm/min.Secondly,based on the combination of optimized parameters,incremental forming experiments were conducted with TA1 sheets at different heat treatment temperatures and hot forming temperatures.The changes of microstructure and mechanical properties of sheets after heat treatment are discussed.The changing rule of forming force,geometric accuracy and forming limit of sheets under different heat treatment and hot forming temperatures are analyzed.The effects of oxide layer and lubrication conditions on surface morphology of parts are studied.Heat treatment and hot forming reduce the forming force,increase the forming limit of sheets and improve the surface quality of forming parts.But hot forming would reduce the geometric accuracy of parts.The forming force of as-received sheet formed at 500℃ decreases by 35.3%.The forming limit of 650℃ heat-treated sheet formed at 350℃increases by 22.86%.The experimental results show that heat treatment and hot forming have beneficial effects on improving the formability of sheet and surface quality of parts.Then,a variety of personalized heat treatment and hot forming process routes were designed for TC4 sheet.The effects of different process routes on sheet formability and hardness of forming parts are analyzed,and the changes of microstructure of forming parts are studied.The experimental results show that after solution and aging treatment,the forming force increases because of the increase of sheet strength.The oxide layer on the surface of the sheet metal is easy to produce microscopic cracks during the forming process,resulting in the rupture of the forming parts,which is not conducive to the improvement of the forming limit.However,the forming force decreases after annealing treatment,and the forming limit can be effectively increased by hot forming.The process route of "anneal-hot forming-solution treatment-aging treatment" can improve the formability of sheet and obtain forming parts with higher hardness.Compared with the parts formed by hot forming without heat treatment,the hardness value is increased by 21.6%.Finally,taking skull prosthesis as an example,the incremental sheet forming experiments were carried out,and the effects of heat treatment and forming temperature on the geometric accuracy and surface quality are analyzed.It is found that the process of"annealing+ hot forming" can guarantee the good geometric accuracy of forming parts and improve the surface quality at the same time.In order to solve the problem of poor geometric accuracy at the edge and bottom area of forming part,on the one hand,a pad with similar size to the opening of forming part is used to alleviate the deflection of edge area of sheet.On the other hand,after forming the parts,the bottom area is formed several times to make the sheet in this area fully deformed.Experimental results show that the geometric accuracy of forming parts can be improved obviously with the optimized forming strategy.