Accuracy Control Of Superplastic Forming For TC4 Titanium Alloy

With the application of superplastic forming is becaming more and more widespread, more hign accuracy was demanded, mainly including thickness distribution and dimensional accuracy shape distortion. Direct-Reverse superplastic forming is an effective process to improve the thickness distribution of component. This process utilize the reverse preforming to disperse the deformation and then the final forming was performed to achieve the shape of female die. Although this technique was applied widely, the important effect of friction condition on the thickness distribution was often neglected. To the dimensional accuracy, because the coefficient of thermal expansion (CTE) of refractory steel differ form that of compomemt, the linear contraction of dies differ from that of the component when cooling after superplastic forming. This leads to the dimensional difference between the die and compomemt. Further more, plastic deformation of workpiece was created by the constrained force of die when cooling, leading to the shape distortion of component. To the component with complex shape, it is more easier to generate the shape distortion.In this paper, the TC4 deep cylinder's aero ratio of deformation zone and original plate reaches 3, this means that the thickness will be more non-nuiform by simple female superplastic forming. Based on the direct reverse superplastic forming, the influence of friction condition on final thickness distribution was analyzed, and the friction changing to control the thickness distribution was proposed. The result show that higher friction coefficient of preforming die can efficiently reduce the thickness of the regions which need to be thinned, results in a more uniform final thickness distribution along the component. The surface of preforming die and final forming die were treated, and the aerospace TC4 deep cylinder with 300% deformation ratio was fabricated successfully by friction changing direct-reverse superplastic forming experiment,the actual thickness distribution meet the requirement of 1.6±0.2 mm.The dimensional accuracy and shape distortion caused by the contraction of ZG35Cr24Ni7N refractory steel die when cooling after superplastic forming were analyzed using MSC.MARC. And the influence of different processing parameters on the shape distortion was also studied. The result show that during the cooling process from 930 to room temperature, when the temperature decreases to 915℃, the shape distortion begin to generate, and increasing with the reducing of the temperature up to 800℃. The shape distortion increased with the rising of the rate of cooling, because the shape distortion generates in the range of 915→800℃, the rate of cooling in this range made much influence on the plastic deformation.To solve the problems caused by the difference of CTE between refractory steel die and TC4 component, the ZrO₂-TiO₂ ceramic with adjustable CTE was developed. The influence of different volume fraction of TiO₂ and relative density on the linear CTE of ZrO₂- TiO₂ ceramic was investigated. To measure the dimensional accuracy of the compomemt superplastically formed by the ZrO₂-TiO₂ ceramic die with same CTE with the TC4, the impression experiment was performed. The result show that the dimensional inaccuracy of TC4 impression is not more than 59μm. To evaluate the service performance of the ZrO₂-TiO₂ ceramic die with the same CTE with the TC4, the cylinder ceramic die was sintered. The TC4 alloy cylinder was superplastically formed using this ceramic die at 930℃. At the room temperature, the TC4 cylinder was easily separated from the ZrO₂-TiO₂ ceramic die surface because of their identical expansion coefficient, and the component can be put into the ceramic die again without gap between them. Based on the oral gypsum model, the ceramic die with the complicated oral shape was fabricated by impression and sintering. Using this ceramic die, the TC4 denture base was superplastically formed at 930℃. The dimension of the denture base was matched with the cramic die. The thickness of denture base is in the range of 75%→81% of the initial thickness of the sheet, the maximal deviation is 0.05mm, and the thickness is close to be uniform.