Research On Electric Hot Incremental Forming Of Ti-6Al-4V Titanium Sheet

Titanium alloys have been widely used for aerospace applications because of superior mechanical properties, and Ti-6Al-4V is the most commonly used alpha/bata titanium alloy in the world. Low-speed forming techniques, such as superplastic forming, are usually used to process Ti-6Al-4V at present, but complex heat-resistant dies and expensive heating equipment are needed. In recent years, single point incremental forming (SPIF) has emerged as a flexible forming method that allows manufacturing of 3D sculptured sheet metal parts without the need for expensive, dedicated tools, and the process is economical to form complex parts in small to medium batches.In order to apply the single point incremental forming technique in the forming of Ti-6Al-4V titanium sheet, in the current work, a novel technique named electric hot single point incremental forming (EHSPIF) was proposed to process hard-to-form sheet metals, and a processing system was developed. With the help of theoretical analysis, numerical analysis and experimental study, the research presented in the dissertation includes the forming mechanism, the numerical analysis and optimization of the technology, forming experiment of Ti-6Al-4V titanium sheet, the mechanism of friction and wear, and abrasion and lubrication test. The research provides a theoretical and technical basis for the practical application of this technology. The results and the main contributions of the dissertation are as following.(1) EHSPIF is a kind of shear deformation, the change of thickness obeys the cosine law, and basketwave microstructure with high performance can be formed on the formed side. By analyzing the forming mechanism, key factors including electric hot, heat transfer and hot plastic deformation were pointed out, and factors of material and processing were also given.(2) Electric hot incremental forming of Ti-6Al-4V titanium sheet was simulated by using MSC.Marc software, and the simulation method and process were given. The distribution of voltage and current was analysed, and influencing regularities of every factor on temperature fields, force, and stress and strain of sheet metal were also studied. According to force status of sheet, the forming region is divided into three parts: the bending deformation, shear forming, reverse bending forming. In order to aquire a workpiece with high combination property, larger current and step down, middle diameter tool were proposed to adopt.(3) Orthogonal experiments were designed to verify results of numerical analysis, factors were arranged according to the significance level of impact on temperature field and forming limit, and the influencing regularity of every factor was also given. The results show that the current and feed speed are important technological factors, and the best forming temperature range is 500～600℃. The influencing regularity of every factor on forming accuracy was also studied and given. Surface bulging occured when a small angle workpiece was negative formed, and the solution was given. The influence of each processing factor on the surface topography was studied, and online detection and current control were proposed to control temperature accurately. The material property of the forming workpiece was studied, and methods to improve the property were found.(4) Friction mechanisms of SPIF and EHSPIF were studied, and the mechanical model can be described by adhesive friction theory. The friction of EHSPIF is a mixed friction, and is divided into adhesive dry friction and boundary friction. The formula for calculating friction was proposed, and the way to improve lubricating effect was found.(6) Influences of process parameters on friction and wear were investigated, and a molybdenum disulfide self-lubricating coating with appropriate thickness was proposed to improve the lubricating effect and property of forming sheet, at the same time, to decrease the oxidation and wear of sheet metal. The electrolytic codepositon of molybdenum disulfide with nickel was proposed as an ideal lubrication used in EHSPI, and a set of processes and optimal parameters were suggested.