Technical paperEvaluation of environmentally friendly lubricant for aluminum cold forging using friction test based on spline extrusion
Introduction
The applications of cold forged aluminum alloy parts have been increasing because of the important needs for weight reduction of automobile. Aluminum alloys have a remarkable tendency to pickup on a die surface. An appropriate lubricant is necessary for aluminum alloy cold forging process. An aluminum fluoride coating, which is a kind of conversion coating, has been used as a popular lubricant film for aluminium alloy cold forging in Japan [1]. Although it has good lubrication performance, it is environmentally hostile, because a lot of wastes are generated during the coating process. Furthermore, the conversion coating process has a weak point in cost. It has been required to replace the aluminum fluoride coating with some environmentally friendly lubricants with the raise of the consciousness to the nature.
Recently, double-layer-type environmentally friendly solid lubricant films have been developed. They have the similar lubrication performance to a zinc phosphate conversion coating for steel cold forging [2]. The aluminum fluoride coating is more harmful than the zinc phosphate coating, due to the fluorine included in the wastes. The appliance of the double-layer-type lubricant film will bring large merit to aluminum alloy cold forging. However, it is necessary to evaluate the lubrication performance before adoption. Aluminum alloys are often forged under large surface expansion conditions due to their good formability. Therefore, the evaluation of friction condition for the cold forging of aluminum alloys should be conducted under a similar friction conditions to the actual forging. The authors proposed the friction test based on combined forward spline-backward can extrusion [3]. The test can realize the large surface expansion of spline extrusion, and it also enabled us to control the surface expansion ratio by changing die shape. In the present paper, the double-layer type lubricant film and aluminum fluoride coating are applied to a precipitation hardened aluminum alloy. In addition, the lubrication performances under various surface expansions are evaluated by the friction test. Some various surface treatments are applied to the workpieces before coating, in order to investigate the effect of surface treatment on the lubrication performance.
Section snippets
Friction test based on forward spline-backward can extrusion
Nakamura et al. proposed a friction test based on combined forward rod-backward can extrusion [4]. It is able to evaluate the friction condition at the conical forward extrusion die. Because the forward edge of the workpiece makes line contact with the die, the film at the edge is subjected to high pressure and long sliding distance. Therefore, this test is suitable for examining the endurance of the lubricant films at the region. However, the surface expansion ratio is about 3 at the most,
Double-layer-type environmentally friendly lubricant
The aluminum fluoride coating needs large number of processing steps and long processing time. They increase production costs. In addition, waste fluid and sludge including fluorine are generated during the coating process. The disposal of wastes also increases environmental risks and costs. Recently, some environmentally friendly lubricant films have been developed as a substitute of the conventional conversion coatings [8]. These films are formed by dipping the workpiece in a bath and drying
Experimental setup
The friction tests were carried out using a 1.5MN servo press. For the motion of the punch, a crank motion was used in order to reproduce the practical forging condition. The ram velocity was 80 mm/s at the stroke point 10 mm above the bottom dead centre. The punch was made of high-speed steel (SKH51 in JIS). The dies were made of cemented carbide (V10 in JIS). The tool surfaces were polished to obtain the maximum height of profile of 0.3 mm.
Harder materials make the test severer for lubricant
Comparison of lubricants
Lubricant films applied on machined workpieces were evaluated by four kinds of dies. Fig. 7, Fig. 8 show the mDs estimated by the Type I and Type II dies. It was confirmed that mDs had good reproducibility when SP was 13 or 15.5 mm. However, mDs tended to disperse when SP attains to 17.5 mm. The experiments with SP = 17.5 mm were performed twice unless severe seizure was observed. In case of the Type II die with large projection, severe seizure was observed in the grooves and high mD was obtained
Conclusions
The lubrication performance of a double-layer-type environmentally friendly lubricant was compared to an aluminum fluoride coating. The evaluations were conducted under various surface expansions by using friction test based on forward spline-backward can extrusion. The effect of surface treatment was also investigated. Obtained conclusions were as follows:
- 1.
The frequency and grade of pickup relate to the surface expansion. However, the friction do not absolutely relate to the surface expansion.
- 2.
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Present address: Hamamatsu Technical Support Center, Industrial Research Institute of Shizuoka Prefecture, 1-3-3, Shin-miyakoda, Kita-ku, Hamamatsu 431-2103, Japan.