Characterisation of different lubricants concerning the friction coefficient in forging of AA2618

Abstract

In this paper a number of aluminium forging lubricants were tested with respect to their lubricating effect and their applicability in the production of parts with long flow paths. The tests were conducted on a custom built testing machine that simulates the parameters of industrial production. The lubricants were assessed considering the change of the coefficient of friction during the test and the influence on tool and workpiece surface. From the presented analysis, the most appropriate lubricant for the investigated process was found.

Introduction

Aluminium alloys are important construction materials in the automotive industry, in racing and aerospace due to their low specific weight, their corrosion resistance and their ability to achieve high strength with certain alloying additions. AA2618, e.g. is a heat treatable Al–Cu–Mg–Fe–Ni forging alloy developed for high temperature applications, especially for aircraft engine components (Oguocha, 1999). Due to the desired mechanical properties of aerospace components, parts made of AA2618 are typically forged. In the production of complex aluminium components by means of drop forging, besides the workpiece material, the lubricant is the most important factor to obtain a successful process.

There were two major tasks in this investigation: on the one hand, a testing method for characterising lubricants with respect to their lubricating effect and their applicability in the production of parts with long flow paths had to be developed. On the other hand, experiments with a representative selection of commercial lubricants had to be performed.

In the experimental determination of friction in metal forming, direct and indirect methods are distinguished: in direct testing methods, the friction stresses are acquired by using measurement pins locally in the tool–workpiece interface, whereas friction is determined via a deduced quantity such as force or deformation in indirect experiments. For the characterisation of lubricants, usually indirect methods are utilised because in indirect measurements the friction stress is averaged on the entire tool–workpiece interface and thus local inhomogeneities are compensated. Indirect testing methods, where friction is determined from specimen deformation, are very popular in industrial practice. The tests are easy to perform and represent the real process very well. However, most of the parameters cannot be controlled independently from each other, and no stationary state is present. Due to this these kind of tests are usually used in comparative investigations. The most popular friction test in this category is the ring-compression test developed by Kunogi (1954) and Male and Cockcroft (1965) (see Fig. 1(a)). This method was lately used for the evaluation of lubricants by Meiners and Hornhardt (2003) and Petrov (2007). In Fig. 1(b) the spike test utilized for lubricant characterization, e.g. by Sheljaskow (2001) is presented, and Fig. 1(c) shows the double-cup-extrusion test as depicted by Kim et al. (2004). All these experiments have in common that their parameters can be varied only in very limited ranges and the sliding distances are quite short.

Indirect experiments, in which friction is determined from force or torque measurements, allow a more detailed investigation of the tribological interactions in the tool–workpiece interface. Hansen and Bay (1986) performed backward-can-extrusion followed by a rotation of the container, where the torque transferred from the cup to the punch was measured (see Fig. 2(a)). Groche and Kappes (2004) utilise a sliding-upsetting test, in which at first a cylindrical specimen is compressed with a special tool in order to obtain a homogenous surface expansion on the bottom side of the specimen and then the lower tool is moved and the sliding force is measured (Fig. 2(b)). Doege et al. (2002b) developed a model experiment to determine friction and heat transfer in backward-can-extrusion by means of a lower punch equipped with strain gauges and thermocouples (Fig. 2(c)). Lately, two different research teams Ngaile et al., 2007, Daouben et al., 2007 evaluated lubricants by performing a sliding-upsetting test where relative motion is applied on an indenter penetrating a cylindrical billet (Fig. 2(d)).