AN INVESTIGATION OF GALLING IN THE FORMING OF ALUMINUM SHEET

It has long been recognized that sheet metal formability is affected by the topography of the sheet and tools. Surface topography is important because: it determines how the asperities of the interacting surfaces interlock; it determines how the lubricant is supplied to the actual metal to metal interfaces during forming; some topographies may allow wear particles generated during sliding to be trapped in the downstream valleys, minimizing the damage done by the wear particles; and trapped lubricant affects the environmental pressure in the regions where asperities are being deformed.; The surface chemistry of the sheet is also important because of its effect on adhesion and the plastic deformation of the surface asperities.; In the present investigation the effects of the sheet and tool surface topographies, the thickness of the oxide layer on the sheet and the sheet surface magnesium content on the amount of metal build-up on the tools in the forming of 2036-T4 aluminum sheet were studied.; A draw bead simulator (DBS) test fixture was used to form aluminum strips. Aluminum build-up on the DBS tools was removed by dissolving in sodium hydroxide. Atomic absorption spectroscopy was then used to measure the amount of aluminum which had been on the tools.; Samples from seven batches of 2036-T4, with different surface topographies and surface chemistries, were DBS tested. A multiple regression analysis showed that build-up increased with increasing average peak width, measured at twenty-five per cent bearing area, and decreased with increasing surface magnesium content. Oxide layer thickness was shown to have no effect on build-up.; The use of a second DBS fixture in which the solid bead and shoulders were replaced by rollers allowed the measured deformation force to be separated into friction and bending components and the average normal and shearing stresses which acted on the tool to be calculated. A linear regression analysis of these data showed that the amount of build-up per unit contact area was proportional to the square of the shearing stress.; A hybrid upper bound model was developed to predict the effects of bead radius, shoulder radius, bead penetration, clearance and lubrication on the shearing stress due to friction for a strip flowing through a draw bead arrangement. This model was combined with the linear regression model and used to show that draw bead geometry has no effect on build-up.; The multiple linear regression and linear regression models developed were used to show that the variations in surface topography and surface magnesium content normally seen between batches of 2036-T4 sheet can result in differences in the total forming load in excess of ten per cent.