| The quick plastic forming (QPF) offers significant opportunity for forming complex, lightweight, highstrength sheet metal parts with almost no allowances, and it is increasingly used in automotive industry in recent years. Friction and lubrication play an important role during the forming process, so appropriate lubricant and lubricating area is of great significance to the improving of the performance and forming quality. For this purpose, this paper investigated the effect of tribological issues on the wall thickness distribution, forming ability, microstructure and surface quality by deforming the sheet into different molds under different lubricating conditions.The QPF of rectangle part with different friction coefficients were analyzed by using MSC.MARC. It was found that the smaller the friction coefficient was, the more uniform the thickness distribution was and the smaller the equivalent stress and strain for final forming area of the part. Reducing the friction coefficient between the sheet metal and mold can improve the wall thickness distribution, increase the forming ability, and prevent the premature rupture.The friction coefficients of no lubrication, boron nitride, graphite, and molybdenum disulfide were 0.56, 0.21, 0.26, 0.5 as measured under the ring upsetting process. The lubricants can reduce friction coefficients. Boron nitride had the most obvious effect, followed by graphite, and molybdenum disulfide was the worst.Lubricating conditions had little influence on the bulging height and wall thickness distribution of conical parts. The heights of the conical parts were 38.40mm, 38.84mm, 38.64mm, 38.62mm, respectively and the wall thickness inhomogeneities were 0.67, 0.66, 0.68, 0.69, respectively under four lubricated conditions (no lubrication, boron nitride, graphite, molybdenum disulfide).Lubricating conditions had great influence on the wall thickness distribution of rectangle pieces. The thickness inhomogeneities of bottom area were 0.35, 0.06, 0.13, 0.26 and the inhomogeneities of wall thickness were 0.27, 0.44, 0.39, 0.23 under four lubricating conditions (no lubrication, boron nitride, graphite, molybdenum disulfide).When the boron nitride was sprayed on the bottom, side wall and entrance filleting of the rectangle die respectively, the bottom of corner formed best under the lubricated condition of bottom. Spraying boron nitride or graphite on the entrance filleting led to a large number of local thinning and premature rupture. During the QPF process of rectangle parts, lubrication on some areas of the mold was good for forming, but lubrication on other areas of the mold was bad for forming.Lubricants could increase the forming ability of cylindrical parts. By using boron nitride, the cylindrical part with the diameter of 80mm and height of 30mm could be formed successfully in 300s, and rupture occurred near the bottom corner during forming process under no lubrication condition.The surface scratches and small gallings were avoided as well as the surface quality was improved by using lubricants. Boron nitride offered the best protection to the metal surface. Graphite had poor spreadability than boron nitride and was easily oxidized at high temperature. Molybdenum disulfide reacted with the metal surface and its spreadability was worst. Both graphite and molybdenum disulfide were not easy to be cleaned up from the surface.The grain size at the bottom center of rectangle piece under boron nitride lubrication was more uniform than that under no lubrication, but the situation was opposite at the bottom filleting.Spraying boron nitride on the bottom of the mold and taking multi-stage loading could form rectangle part with convex slot successfully in 300s. The thickness inhomogeneity of bottom was only 0.06. |
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