by Erfan Abbasi, Young Researchers Club, Saveh Branch, Islamic Azad University, Saveh, Iran and Mohammadreza Salmani, Material Science and Engineering Department, Islamic Azad University of Saveh Branch, Saveh, Iran
The three-dimensional surface topography of electrogalvanized steel sheet affects its press-forming behavior and also influences the appearance of the end product. In this study, the three-dimensional surface topography of several steel sheets processed by a stochastic surface texturing method has been considered both theoretically and experimentally. In the presence of liquid lubricant, results indicate an increase in rate of mixed lubricated regime rather than boundary lubricated regime as well as improvement of stretchability of textured samples in comparison with non-textured samples.
Stretch forming is one the most significant processes for sheet metal forming (SMF). SMF consists of deformation processes where a steel sheet is shaped by tools or dies. Stretch forming is used in particular for forming body panels, such as doors, roofs or fenders for the automotive and other industries. The performance of SMF processes depends on characteristics of the forming process, the sheet metal, the tool material, frictional conditions at the tool or steel interface and other product requirements.1
In SMF processes, the sheet surface structures have the function of storage, transport and distribution of the lubricant as well as the take-up and transport of wear particles. In order to fulfill these functions, they can be separated as follows: (a) stochastic surface texturing and (b) deterministic surface texturing.2
It has been demonstrated that the frictional conditions during testing are controlled via lubricant viscosity, film thickness, surface roughness and deformation velocity.3,4 Often, the friction force in a lubricated tribo-system is described as a function of one or even more of the operational parameters. Depending on the value of operational parameters, a tribo-system can operate in the following lubrication regimes: (a) hydrodynamic lubrication, (b) mixed lubrication or (c) boundary lubrication.5
In practice, the operational parameters of the three-dimensional surface topography are modified by some methods such as: (a) shot blast texturing, (b) electro-discharge texturing, (c) electron beam texturing and (d) laser beam texturing.6 The interaction of surface texturing with coated and uncoated steel sheets during SMF has been considered in previous studies.7 Evaluation of the tribo-system during the SMF processes has been carried out via different theoretical equations.
In the stretch forming process, the upper limit of the punch load is determined by the maximum tensile load that can be transmitted by the workpiece material, the angle γ between the sheet metal at the grippers and the direction of material of the punch (Fig. 1), the ultimate tensile stress of material UTS, the total width of the clamped sheet b and sheet thickness s, while frictional effects can be considered using a corrective friction factor c, per Equation 1.1,8