A study of lubrication impact on mechanical characteristics of formed alloy.
In most instances friction is preferably reduced to zero by the introduction of a lubricant. The lubrication problems are one of the most delicate problems in cold forming. The influence of lubrication on wear, friction, forming force, temperature, material and geometrical properties and finally costs are very important.(Lange, 1998)
In the majority of metalworking processes the workpiece is deformed by means of a contacting die. The pressure required for deformation generates a normal stress to the die surface, and movement of the workpiece relative to the die surface generate a shear stress at the interface (Wilson, 1997). Thus a classical tribology situation arises, with friction at the die-workpiece interface, and with potential for wear of both die and workpiece materials. Mitigation of these effects then calls for the introduction of a lubricant.
The choice of the right lubricant and its proper application is very important. Despite the extremely wide range of conditions under lubricants must function, some systematic approach to selection can be made (Tiernan et al., 2005). Final selection is almost always a matter of compromise, but there are some fairly general, desirable attributes. Many scientific papers deal with impact of lubrication in metal forming, but a majority of them focused on pressures, forces and tool wear (Takhautdinov et al., 2001, Tiernan et al., 2005, Fereshteh-Saniee & FatehiSichani, 2006) or material microstructure (Le & Sutcliffe, 2005). Only few of these papers describe an impact of coefficient of friction on material characteristics (Campos & Cetlin, 1998).
2. EXPERIMENTAL WORK
In the frame of the experimental work the process of cold forward extrusion of a cylinder from the copper alloy CuCrZr was analyzed. This is a copper-chrome-zirconium alloy with high electrical and thermal conductivity and excellent mechanical and physical properties also at elevated temperatures.
It is used as electrode material in spot, seam and butt resistance welding of low carbon steel sheets. Further it is used for manufacture of various components for resistance welding equipment. The cylinders of dimension [PHI]22 mm x 32 mm were extruded in a special tool for forward extrusion (Fig.1) at 20[degrees]C temperature and four different lubricants coefficients of friction ([mu] = 0,05, 0,07, 0,11 and 0,16). The effective strain was in all cases [[epsilon].sub.e] = 1, 29.
The measurements of coefficient of friction for each of the four different lubricants were carried out by using the ringcompression test. Increasing friction presents increasing resistance to free expansion of the ring, resulting in a decrease of the ring internal diameter. To determine the influence of the coefficient of friction on mechanical and electrical properties of cold forward extruded pieces tensile tests and electrical conductivity measurements were carried out. Tensile strength, yield strength, reduction of area and elongation were determined with the tensile tests. Electrical conductivity of extruded samples was measured by the Sigmatest instrument. Many experiments were done to provide reliable results.
3. RESULTS AND DISCUSSION
The diagrams on the Fig. 1 and Fig. 2 present the change of tensile strength [R.sub.m], yield stress [R.sub.p02], reduction of area Z and elongation [A.sub.5] as a function of lubrication's coefficient of friction [mu] at the constant tool speed [v.sub.tool] = 12 mm/s. The results at different coefficients of friction are very similar and the difference between them is less then 5 %.
Although this difference in mechanical properties could be of importance in some specific cases, in general it is possible to say that the value of coefficient of friction does not affects significantly the measured mechanical properties of the cold extruded alloy. Of course this conclusion can be made only for lubricant friction factor interval.
[FIGURE 1 OMITTED]
[FIGURE 2 OMITTED]
If we compare the results for tensile stress Rm and yield stress [R.sub.p0,2] measured at different values of coefficient of friction very interesting thing can be observed. By using lubricator with the lowest coefficient of friction ([mu] = 0, 05) the highest tensile and yield stresses were measured. With increasing coefficient of friction ([mu] = 0, 07 and [mu] = 0, 11) both, tensile and yield stress decreases, although insignificantly. We could expect a further decrease of both stresses when using lubricant with the highest value of coefficient of friction ([mu] = 0, 16). But tensile and yield stress increase for about 2% compared with stress values when coefficient of friction [mu] = 0, 11 was used.
The same phenomenon can be observed with elongation [A.sub.5] and reduction of area Z on Fig. 2. But the change of values for elongation and reduction of area measured at different coefficients of friction is even smaller (less than 5%) than the change of tensile and yield stresses.
Measurement of the electric conductivity was carried out on the cylinders taken from the root of the cold extruded alloy. Dimension of these cylinders was [PHI] 11mm x 16mm and the conductivity was measured by special Sigmatest instrument with measuring frequency of 120 kHz. Many measurements were done to provide reliable measuring results of electric conductivity. The influence of coefficient of friction on the electrical conductivity of the deformed material is shown in Fig. 3.
[FIGURE 3 OMITTED]
Electrical conductivity is decreasing with higher strain. When measured on the specimen which was extruded with lubricant's coefficient of friction [mu] = 0,05, the electrical conductivity is about 20% lower than the electrical conductivity of unformed material (0 on x-axis in the Fig. 3). It is obvious that the electrical conductivity increases with increased coefficient of friction, although the difference between electrical conductivity values when using smallest and highest ([mu] = 0,16) coefficient of friction is small (less than 5%).
The magnitude of friction needs to be known for several reasons. Pressures, forces and energy requirements can be calculated only if interface conditions can be described by shear strength or friction factor. For this, a numerical value must be established. Reducing friction is often the main criterion in choosing a lubricant. For this, comparative values are often sufficient. The ultimate choice of the lubricant may have to be based on full--scale operating experience, but such tests can be disruptive and expensive. The values of coefficient of friction can also have effects on the material properties during and after forming process. Although this influence in our experiments was very small, it could be of importance when very precise information of mechanical properties of the extruded part is necessary. One of the reasons for very small difference in mechanical properties when using different lubricators with different coefficients of friction could be the fact that we have done our experiments with lubricant with rather low coefficients of friction [mu] (between 0,05 and 0,16). By using lubricants with higher coefficients of friction, the influence on the mechanical properties could be more significant. But, of course, new experiments and measurements must be performed to confirm or reject such a statement.
In our future research the emphasis will be on the modeling of material characteristics by means of evolutionary methods like genetic programming method. On this way it will be possible to generate accurate mathematical models for change of different mechanical properties.
Campos H.B.; Cetlin P.R. (1998). The influence of die semi-angle and of the coefficient of friction on the uniform tensile elongation of drawn copper alloys, Journal on Materials Processing Technol. 80-81, pp.388-391
Fereshteh-Saniee F.; Fatehi-Sichani F. (2006). An investigation on determination of flow curves temperature and under forming at room conditions, Journal on Materials Processing Technology 177, pp.478-482
Lange, K (1998). Handbook of Metal Forming, McGraw Hill, New York, ISBN 0-07-036285-8
Le H.R.; Sutcliffe M.P.F. (2005). The effect of surface deformation on lubrication and oxide scale fracture in cold metal rolling; Metallurgical and Materials Transections B, Vol. 35, pp. 919-928
Takhautdinov R.S.; Latypov R.T.;. Spirin S. Yu; Antipenko A.I. (2001). Efficient new lubrication technologies for the production of cold--rolled sheet; Metallurgist, Vol. 45, 5-6, pp. 241-244
Tiernan P.; Hillery M.T.; Draganescu B.; Gheorghe M. (2005). Modelling of cold extrusion with experimental verification, Journal on Materials Processing Technol. 168, pp. 360366
Wilson W.R.D. (1997). Tribology in Cold Metal Forming, Journal of Manufacturing Science and Engineering Vol. 119, pp.695-698
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|Publication:||Annals of DAAAM & Proceedings|
|Date:||Jan 1, 2009|
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