Strain Induced Surface Change in Sheet Metal Forming: Numerical Prediction, Influence on Friction and Tool Wear

Abstract

In sheet metal forming, free deformation of the sheet takes place frequently without contact with forming tools. The pre-straining resulting from the free deformation leads to a surface roughening of the sheet metal. It is assumed that the roughening has an influence on friction and wear behavior of the following forming process as well as the painting quality after the manufacturing. In this paper, a numerical prediction based on a polycrystalline model is first proposed to predict the effect of surface roughing based on the material data of the as-received state of the sheet metal. Different states of strain are analyzed and the numerical result is validated through experimental evaluation. Besides the numerical prediction, the friction behavior after pre-straining is evaluated in strip drawing tests and the coefficient of friction (COF) is calculated. For interpretation of the measured COF, the surface roughness after the friction test and the surface image are evaluated by a transparent toolset. It is shown that the surface transformation as a result of pre-straining has a negative influence on the lubricating effect of the sheet metal and degrades the friction behavior. Finally, the influence of the strain-induced surface roughening on wear is discussed based on wear testing in strip drawing test with draw bead geometry.

Method for the Estimation of Wear Resistance Curves in Sheet Metal Forming with Uncoated Tools

“…In former researches of the authors, the obtained wear development shows a similar development with three phases [3,9,10,31]. Therefore, the damage development describing fatigue strength and wear is the first similarity.…”

“…In addition, in the field of forming technology, a method for wear prediction using the life span estimation through wear resistance diagrams (WRD) was developed [3,9]. These diagrams have been used to characterize the wear resistance of different tribological systems in laboratory tests [10]. The WRD illustrates the life span of tools under different contact stress, quantified by the sliding length reachable in the strip drawing test.…”

Combined effect of inhomogeneous strain-induced and anisotropy on friction in deep drawing of DC06EK sheet metal

“…These two parameters have a signi cant in uence on the COF and the surface roughness [19][20][21][22][23]. Trzepieciński and Fejkiel [19] used a strip drawing test to evaluate the COF for different specimens taken from steel sheets at 0° and 90° to the rolling direction (RD).…”

“…These two parameters have a signi cant in uence on the COF and the surface roughness [19][20][21][22][23].…”

“…This is explained by the increase in the sheet hardness as a function of the plastic strain that results from the work hardening phenomenon. In the work of Wu et al [20], the frictional behavior was also evaluated on pre-strained specimens in strip drawing tests. The measured COF and surface roughness increase with plastic strain level.…”