Influence of Friction during Forming Processes – a Study Using Numerical Simulation Technique

Friction has an important influence in metal forming operations as it contributes to the success or failure of the process. In the present investigation, the effect of friction on metal forming was studied by conducting compression tests of cylindrical Al-Mg alloy using Finite Element Method (FEM) technique. Three kinds of compression tests were performed. For the first test, a constant coefficient of friction (i.e., 0.65) was employed at both upper die – work-piece and lower die work-piece interfaces. For the second and third tests, the coefficient of friction between the upper die and work-piece interface was same as that of previous test (i.e., 0.65). However, at the lower die work-piece interface, two different coefficient of friction values, namely 0.65 and 0.35, were employed at different locations (i.e., each halves and each quarters of the cylindrical work-piece for the second and third tests, respectively) in the tests. Simulation results showed that a difference in metal flow occurs near the interfaces owing to the difference in coefficient of friction. It was concluded that the variations in coefficient of friction between the dies and work-piece directly affect the stress distribution and shape of the work-piece. This would directly affect the microstructure of the material being processed INTRODUCTION Friction between the tool and work-piece has a significant effect on material deformation, forming load, component surface finish, and die wear. It is also an essential input parameter for the ever-increasing use of Finite Element (FE) simulation for metal forming. The coefficient of friction, if controlled properly, could generate the required stresses to deform the metal to the required shape. It could also lead to fracture of the sheet if not controlled properly. It was reported earlier, that surface texture of the die indeed plays an important role on coefficient of friction during forming [1-2]. Lakshmipathy and Sagar [1] studied the influence of die grinding marks orientation on friction in open die forging under lubricated conditions. They used commercial pure aluminium as the work-piece and H11 steel, as the die materials. Two sets of dies, one with unidirectional grinding marks and the other with criss-cross grinding marks were employed. It was found that, for the same percentage of deformation, the dies with the criss-cross ground pattern required lesser forging loads when compared with the situation prevailing with dies having uni-directionally ground pattern. The friction factor was also lower during the forging process when the die with the criss-cross surface pattern was used. The authors [1] concluded that compared to the crisscross ground die, the lubrication breakdown tendency is more when pressing is done with unidirectionally ground die. In another work, Malayappan and Narayanasamy [2] conducted upset forging experiments to study the bulging effect of aluminium solid cylinders by varying the frictional conditions at the flat die surfaces. Different machining processes like grinding, milling, electro-spark machining, and lathe turning with emery finish were produced on the flat dies to vary the frictional conditions. The authors [2] concluded that the degree of barreling depends on friction and thus on surface finish. Further, ring compression tests have been conducted for evaluating the frictional effects in bulk metal forming [3, 4]. The result [3] showed that under frictionless conditions, the hole size of the hollow cylinder increases proportionately to the outer diameter. However, with increasing frictional constraint the rate of expansion of the hole decreases and eventually the compressive hoop stress developed at the hole causes the hole to contract. Earlier, Menezes et al. [5-10] studied the effect of surface texture on coefficient of friction and transfer layer formation during sliding under both dry and lubricated conditions. Various kinds of surface textures – namely unidirectional grinding marks, 8-ground, and random were prepared using simple metallographic techniques. Roughness, represented by Ra, of surfaces was varied over a range and these were prepared using different grit emery papers or abrasive powders. Figure 1 shows the variation of coefficient of friction with surface textures when Al-Mg alloy pin slid on steel plate of different surface roughness [5]. Here U-PD and U-PL represents sliding direction perpendicular and parallel to the unidirectional grinding marks, respectively. It was found that the sliding perpendicular to unidirectional grinding marks (U-PD) gives maximum friction force contributed by higher plowing component, and at the other extreme, the random texture resulted in lower friction values. For 8ground and U-PL surface textures, the coefficient of friction lies in between these two extremes. It was observed that the roughness as given by Ra within the test range does not significantly affect the friction values. Further, it was suggested that the results obtained provide a basis for controlling the coefficient of friction across various locations along the interface between die and work-piece in metal forming process. These results may be employed to obtain a particular die surface finish in a particular area of the die so as to obtain the desired coefficient of friction. The coefficient of friction maybe controlled in the following ways: U-PD surface texture maybe machined on the die surface to obtain a high coefficient of friction and Random surface texture maybe generated when a low coefficient of friction is required. Figure 1: Variation of coefficient of friction and surface roughness (Ra) with types of surface textures. In the simulation work reported in literature for sheet metal and other forming operations [11-15], the coefficient of friction was assumed to be a constant at the interface or was set at different values at various locations. The criterion for choosing different values of coefficient of friction at different locations was made either arbitrarily or based on intuition. By controlling the surface texture of the die, the friction at the interface and final shape can be controlled. This would affect the stresses of the workpiece. Based on the research work done earlier [5], attempts have been made in the present investigation, to study the state of stress by implementing the obtained different coefficient of friction values at various locations between the die and work-piece during metal forming by using Finite Element (FE) analysis. In this simulation approach, the surface textures of various types as reported in the earlier work [5] were not attained on the die surfaces. Instead, the coefficient of friction values, generated for these surface textures, were employed directly at the die – work-piece interfaces to study the state of stress in the work-piece and the deformed shape by conducting compression test using FE simulation. EXPERIMENTAL DETAILS In the present investigation, the influence of coefficient of friction between the die and work-piece on the metal flow behavior was studied by simulating compression tests using FE analysis. Process simulation of compression tests was performed using commercially available non-linear finite element code, DEFORM 3D. The package is capable of simulating metal flow during forging, extrusion, rolling, drawing, and stamping operations. In this study, the dies (upper and lower) were modeled as a rigid body, while the work-piece in the shape of a cylinder was modeled as a rigid-plastic body. The formulation assumes that the material stress increases linearly with strain rates until a threshold strain rate, referred to as the limiting strain rate. The material deforms plastically beyond the limiting strain rate. The plastic material behavior of the object is specified with a material flow stress function or flow stress data. The dimensions of the work-piece were 5 mm in diameter and 2.5 mm in height. It was reported earlier that the coefficient of friction varies from 0.35 to 0.65 for different surface textures even though for the same roughness (Ra) values when Al-Mg alloy slid against steel counterface [5]. Hence, Al-Mg alloy material properties were assigned to the work-piece. The material parameters were obtained from the database library of DEFORM. Thus, process conditions were assumed such that they are close to actual experiments. Interface friction conditions were modeled using shear friction. Three kinds of compression tests were performed. A constant coefficient of friction (i.e., 0.65) was employed between upper die and work-piece interface for all the three tests. However, the friction values were varied between lower die and work-piece interface for other tests. Thus, to vary the friction between lower die and work-piece interface, the lower die was divided into four zones, namely L1, L2, L3 and L4. A schematic diagram of the lower die with four zones is shown in figure 2. Table 1 shows the different coefficient of friction values employed at different zones between lower die and work-piece interface for different tests. Figure 3 shows the schematic diagram of compression test with same friction values between all contacting surfaces (first test) Figure 2: Schematic diagram of lower die (top view) Tests Friction at different zones