A new method for evaluation of friction in bulk metal forming

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Abstract

In this investigation a simple theoretical analysis of the cylindrical compression test has been developed so that the constant friction factor can be estimated quantitatively by using cold/hot compression test. The compression tests have been carried out on the Ti-IF-steel without lubricant, with mica sheets and with glass powder lubricants the commercial aluminum with oil lubricant to confirm this analysis. This new method for obtaining the constant friction factor may be called “The Barrel Compression Test”. The advantages of the proposed method are: (i) very simple method, (ii) high sensitivity of geometrical shape of cylinder to friction condition, (iii) does not need force measurement, and (iv) the constant friction factor can be determined for forming processes corresponding to actual processing condition.

Introduction

In metal forming processes, friction plays a significant role in determining the life of the tool, the formability of the work material and the quality of the finished product such as, surface finish, internal structure, and product life. Excessive friction leads to heat generation, wear, pick-up and galling of the tool surface, which contribute to the premature failure of the tools. Friction can increase the inhomogeneity of deformation, leading to defects in the finished products.

In order to reduce the detrimental effects of friction, lubricants are used extensively. Nevertheless, it should be noted that it is not always the practice to reduce the interfacial friction to a minimum value. Friction can also be used beneficially to manipulate the material flow to achieve the desired end product with a minimum effort. For example, in processes such as rolling, conform extrusion and extrusion forging, the beneficial effects of friction is favorable in achieving the end product. A number of studies have already been made in an attempt to obtain quantitative data on friction in metal processing by using the actual metal forming operation or by using simulative laboratory tests.

Frederiksen and Wanheim [1] used friction-testing methods based on the geometrical changes in order to adjust the frictional conditions in the simulation to the condition of the real process. Bay [2] studied the application of the friction model when analyses of bulk metal forming process are given. In addition to measuring of friction with general methods like ring test [3], some researchers proposed methods to determine friction factor with metal forming processes. For example Venugopal et al. [4] developed a simple theoretical analysis of the solid compression test and estimated the friction factor from the reduction-capacity test. Shen et al. [5] proposed a method for estimating the value of the shear friction factor using a backward-extrusion type forging. This method used forging load multiplied by the bottom thickness in a backward-extrusion test as a measure to calibrate the friction factor. Bushhausen et al. [6] proposed a friction test based on a double backward-extrusion process and examined in order to obtain information on lubrication quality. Bay et al. [7] presented the development of an indirect friction test by a combined forward-rod/backward-can extrusion.

In this study by means of the upper-bound theory, the constant friction factor is determined by using the barrel compression test. With this method we can calculate the constant friction factor (m), only by measuring the degree of barreling (maximum radius, RM, and height of cylinder after deformation). Also Lee and Altan [8], Kulkarni and Kalpakjian [9], and Narayanasamy et al. [10] have found out that the barreled shape can be reasonably characterized as the arc of a circular curvature and the shape of the barrel is influenced by the initial height-to-diameter ratio and by the friction conditions. In hot compression test, application of a suitable lubricant may reduce friction but, will never eliminate it completely. So it is not possible to estimate the true stress–strain curves from a simple compression test. Cook and Larke [11], for instance, proposed an extrapolation approach, whereas, Polakowski [12] developed a method in which repeated compressing and machining of cylindrical specimens was involved. Neither of these methods is practical, however, when testing at high temperatures is considered. It is much more preferable to measure the interface frictional conditions in one experiment. The material’s flow strength may then be derived from the barrel compression test with the help of theoretical analysis. The major advantage of using the barrel compression test for the evaluation of friction is that, it involves only the physical measurement of shape change, unlike some other tests, which require the mechanical properties of the material and the forming loads. The measurement of these parameters presents a major difficulty due to the cumbersome nature of the experiments, especially those at high temperatures and high strain rates.

Section snippets

Analysis

The upper-bound theorem was first formulated for rigid perfectly plastic materials, and states that among all the kinematically admissible strain rate fields, the actual one minimizes the power. Druker and Providence [13] extended this theorem to include the velocity discontinuities.:J=23σ0V12ε̇ijε̇ijdV+SKΔVidS+SmmKΔVidS−STTiVidSis a minimum for the actual velocity field.

The first term in Eq. (1) expresses the internal power of deformation, due to the strain rate field. The second and

Experimental procedure

In this study hot compression test have been carried out on Ti-IF-steel. Table 1 shows the composition of this steel. The compression test specimens with heights of 7.5 mm and diameter of 5 mm (the height to diameter ratio is 1.5) were machined from a roughed slab. The flat end of the samples were machined with spiral groove of 0.1 mm depth. The experiments carried out at temperature of 1100 °C and strain rate of 0.1 s−1 without lubricant, with a mica sheet lubricant and a glass powder lubricant. A

Results and discussion

Fig. 2 shows the true stress–strain curves obtained from hot isothermal compression test at temperature of 1100 °C and strain rate of 0.1 s−1 in different friction conditions. The true stresses in all curves have been reduced to steady state after a peak stress. The reason of the drop in stress is the dynamic recrystallization occurrence. Fig. 4 illustrates the microstructure before and after deformation. The fine grain structure after deformation and stress–strain curves confirm that dynamic

Conclusion

  • 1.

    Barreling occurs due to friction between the work piece and the die in isothermal hot compression test. In hot compression test, application of a suitable lubricant may reduce friction but will never eliminate it completely.

  • 2.

    The barreling compression test is a new useful method for evaluation of friction in cold or hot bulk metal forming. This method can calculate the constant friction factor, m, only by measuring the degree of barreling (maximum radius, RM, and height of cylinder after

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