Tribology in Hydrostatic Extrusion of Metals A review

Tribology in Hydrostatic Extrusion of Metals A review P. Tomar*, R. K. Pandey, Y. Nath Mechanical and Automation Engineering Department G.G.S. Indraprastha University, Delhi-110403, India *Corresponding author: pankaj_12343@rediffmail.com Indian Institute of Technology Delhi, New Delhi- 110 016, India rajpandey@mech.iitd.ac.in, ynath@am.iitd.ac.in ABSTRACT Hydrostatic extrusion process is used in the forming of hard materials which are difficult to deform. In this forming process, lubrication plays vital role. Existence of lubricating film between die and billet ease the forming. Moreover, lubricating film reduces the extrusion pressure and improves the life of die and quality of the products. Therefore, awareness and challenges of tribological related problems in the hydrostatic extrusion process is highlighted in this paper through literature review. Mainly papers dealing with minimum film thickness and friction at the interface of die and billet are discussed in the present paper. Authors believe this paper may be useful to the researchers working in the area of hydrostatic extrusion. been widely recognized that relatively thick films at the interface of work piece and die exists in many of the metalforming operations. Such films are highly effective in reducing the interface friction and surface damage. However, in the presence of thick lubricating film, unconstrained deformation of the work piece causes unacceptable products. Therefore, study and development of improved lubrication in hydrostatic extrusion process are essential issue. Thus, hydrostatic extrusion studies carried out by the researchers in the area have been reviewed and discussed herein for awareness of readers both from industry and academics. Discussions have been presented in this paper considering three aspects i.e. lubricating film thickness, hydrostatic pressure and friction at die/billet interface. Keywords: Friction force, extrusion pressure, minimum film thickness, viscous dissipation. INTRODUCTION Proper lubrication at the interface of billet and die in the hydrostatic extrusion process reduces extrusion pressure and enhances the tooling life. Existence of thin lubricating film at the die/billet interface improves the tolerances of the product. In the hydrostatic extrusion process billet is completely surrounded by pressurized lubricant in a hydrostatic container as shown in Figure 1. Hydrostatic pressure in the surrounding fluid/lubricant is controlled externally using a ram. The deformation of billet in the contact zone is achieved by the hydrostatic pressure and the pressure developed by the dynamics at the interface. The idea of hydrostatic extrusion was first proposed by Robertson [1] but method was first attempted by Bridgman [2]. Hydrostatic extrusion process involves less friction in comparison to conventional extrusion processes (forward and backward extrusions). Lowering the friction at die/billet interface by proper choice of lubricants in the hydrostatic chamber has a beneficial effect in the perspective of energy saving. The entrainment of lubricant films between the die/billet interface and their subsequent transport and break down have been the key points of the studies since last couple of decades. In last four decades, it has 1 Figure 1: Schematic diagram of Hydrostatic extrusion FILM THICKNESS To study the variation of film thickness at the interface of die and billet as functions of operating parameters, the authors [3] have presented inlet zone analysis of hydrostatic extrusion process. Film thickness expression in inlet zone as a function of back pressure is reported by them. It has been concluded that reduction in the interface pressure takes place by the application of back tension. A transient hydrodynamic lubrication model of hydrostatic extrusion is presented by Wilson [4] to investigate the nature of the lubricating film during the initial stage of extrusion process. He has concluded that there is temporary breakdown of the lubricating film at high extrusion pressure during the starting of the extrusion operation. An isothermal hydrodynamic lubrication model of

hydrostatic extrusion is presented by the Wilson and Walowit [5] with Newtonian fluid by ignoring pressure gradient in the work zone. The film thickness expressions have been derived by the authors for the entire domain (inlet zone, work zone & outlet zone). They have also concluded that film thickness decreases sharply in the inlet and work zones at elevated operating conditions. A hydrodynamic lubrication in hydrostatic extrusion is presented by Snidle et al. [6]. The authors have investigated the effects of elastic deformation of billet on temperature rise along the extrusion direction. Figure 2 demonstrates the variation of temperature for reduction ratio of 44.4 during deformation of aluminum. It can be seen in Figure 2 that billet surface temperature θ b, die surface temperature θ d and mean temperature of the lubricant film θ m (assumed to be same) is less than the billet core temperature. The film thickness of lubricant is strongly dependent on the boundary temperature of billet and it reduces sharply in the inlet zone. Linear variation of film thickness is observed in the work zone only for a particular case. thickness to isothermal minimum film thickness) with the corrected viscous parameter L at various semi die angle is studies by the authors (Figure 4). A good correlation between theoretical and experimental results can be seen in Figure 4. Tomer et al. [4] have presented a mathematical model for the study of thermal minimum film thickness in inlet zone with various operating parameters. They have concluded that minimum film thickness decreases with increase in semi-die angle, viscous thermal parameter, and material parameter. However, it increases with increase in extrusion pressure. Extrusion direction Figure 3: Inlet film thickness with billet speed and extrusion pressure, Wilson and Madhavan [8] Figure 2: Temperature variation of lubricant film along extrusion direction, Snidle et al. [6] A hydrodynamic lubrication model of hydrostatic extrusion with thermal consideration has been developed by Wilson and Madhavan [8]. They have included the effect of viscous heating on the lubricating film thickness in the inlet and work zones. The authors have concluded that viscous heating significantly reduces the lubricating film thickness in the inlet and work zones. The experimental observations on the inlet zone film thickness at various billet speeds and extrusion pressures are studies by the authors as shown in Figure 3. They have concluded that film thickness increases linearly with increase in billet velocity. However, at higher velocity reverse trend in film thickness is reported. Variation of thermal correction factor C (ratio of thermal minimum film Figure 4: Comparison of experimental film thickness with theoretical film thickness, Wilson and Madhavan [8] 2

PRESSURE The experimental investigation of maximum pressure required to extrudee aluminum billet with lubricants of varying viscosities are reported by Kulkarni and Schey [10]. The authors have reported that lowest extrusion pressure does not always produce better surface finish products. The extrusion pressure for tube and rod are compared by Matsushita et al. [11] with same geometry of die and operating parameters for various materials. They have concluded that 3 to 20% higher extrusion pressuree is required to extrude round tube than rods of aluminum, aluminum alloy, copper, carbon steel, zircaloy and stainless steel. Extrusion pressure exceeds 10 to 40% for complicated profiles shaped tube. The hydrodynamic lubrication theory in relevancee with thermal effect due to viscous dissipation and strain hardening parameter was developed by Mahdavian [12] and author has reported that thermal and strain hardening effects have inverse effects on the extrusion pressure. The effects of die angle, extrusion ratio, hydrostatic medium and draw stress with breakthrough pressure have been presented by Loh and Cheung [13]. The conclusions listed by the authors are: (i) Optimum die angle depends on extrusion ratio, (ii) Breakthrough pressure depends on extrusion ratio, (iii) Breakthroughh pressure depends on the size of wire, (iv) Draw stress enables a higher extrusion ratio, and (v) Higher extrusion ratio can be obtained with higher viscous hydrostatic medium. The theoretical and experimental investigationss of the hydrostatic extrusion process with respect to various die angles and reduction ratio have been carried by Elkholy [ 14]. The main findings of the author are; (1) Extrusion ratio increases with decrease of reduction ratio ( 2) The extrusion pressure is minimum for an optimum cone angle at given reduction ratio (3) The optimumm cone angle does not depend upon the billet material (4) The optimum cone angle inversely varying with reduction ratio. The hydrostatic extrusion of aluminum rod has analyzed experimentally by Hung and Hung [15]. They have reported that the extrusion pressure attains a peak and then decreases and latter on remains at this value. The maximum pressure is resulted from the static friction between billet and die. The hydrodynamic lubrication theory of hydrostatic extrusion of tungsten alloy in relevance with critical speed at which hydrodynamic lubrication prevails has been studiedd by Wang et al. [16]. It has been concluded that extrusion pressure has inverse behavior to critical speed. The roles of lubricant and lubrication in cold extrusion processes are very important with respect to the reduction of extrusion force and die wear. The cold extrusion process with four lubricants (one mineral oil and three synthetic oils) is analyzed by Caminaga et al. [17]. It has been reported by the authors that semi-synthetic oils are the good alternative for the phosphated billet. The four lubricants (mineral oil, semi synthetic oil, powder soap and 3 wheat flour) are analyzed experimentally by Caminaga et al. [ 18]. They have observed that at one deformation stage wheat flour is the best lubricant and for two deformation stages mineral oil is the best choice. FRICTION The friction effect in extrusion process has been investigated using energy approach for small and large semi die angles by Avitzur [19-20]. Assuming constant friction stress and constant co-efficient of friction over the die length, authors have concluded that the cylindrical portion of die (die land) increases friction force and reduces extrusion ratio. The experimental studies for the coefficient of friction in the hydrostatic extrusion of aluminum and aluminum alloy are carried by Pugh [21]. Moreover, the coefficient of friction has been evaluated experimentally by Erans and Avitzur [22] during the investigation of extrusion force, flow pattern, tool wear and surface finish of the product. Using hydrodynamic lubrication theory, Wilson [23] has proved that friction is higher in well lubricating conventional extrusion process in comparison to hydrostatic extrusion process for same operating conditions. The coefficient of friction based on the hydrodynamic lubrication theory in hydrostatic extrusion process has been evaluated theoretically by Snidle et al. [24]. They have described that the coefficient of friction is much higher (Figure 5) at low extrusion ratio. The coefficients of friction in different regimes namely thick film regime, thin film regime, mixed regime and boundary regime are investigated theoretically by Wilson [25]. Figure 5: Co-efficient of friction at die interface with extrusion ratio for different semi die angle, Snidle et al. [6] The effect of additives on the coefficient of friction is investigated by Osakada and Asada [26]. The authors have

suggested that the additives such as zinc dialkyl dithiophosphate and chlorinated paraffin with machine oil are effective in reducing the friction. A friction model for mixed lubrication is presented by Vidal-Salle et al. [27]. It has been concluded that the lubricant film thickness and the leakage pressure are the two parameters that affect the change of the local friction. The friction modeling of different tribological interfaces in mixed lubrication of extrusion process for large contact ratio at the billet/die interface is presented by Hsu and Huang [28] using average Reynolds equation. CONCLUSIONS Based on the literature review, the following conclusions have been drawn: Dearth of design relations exist pertaining to parametric understanding of the performance parameters in the hydrostatic extrusion process. In spite of hydrodynamic lubrication, huge power loss in the hydrostatic extrusion operation is reported. Dearths of experimental results exist using the surface modification techniques applicable to dies. No film thickness relation has been noticed as functions of various operating parameters at elevated operating conditions incorporating starvation effects in the lubricating film. The friction force at the interface of container and billet in hydrostatic extrusion has not been highlighted and investigated accurately. Energy conservation issues during the hydrostatic extrusion operation are not addressed properly. REFERENCES [1] J. Robertson, Method of an apparatus for forming metal articles, British Patent No, 19356 (1893), US Patent No, 524504 (1894). [2] P. W. Bridgman, Studies in large plastic flow and fracture, McGraw-Hill, New York (1952). [3] S. Thiruvarudchelvan and J. M. Alexander, Hydrodynamic lubrication in hydrostatic extrusion using double reduction die, Int. J. Mach. Tool, Vol. 11, (1971), pp. 251-268. [4] W. R. D. 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