Improvement in Tribological Properties of Surface Layer of an Al Alloy by Friction Stir Processing

Soheyl Soleymani, Amir Abdollah-zadeh, Sima Ahamd Alidokht

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DOI: 10.4236/jsemat.2011.13014                

Abstract

An innovative technique, friction stir processing (FSP) was employed to modify the surface layer of Al5083 alloy. The FSP passes of 1 to 4 were applied on alloy samples. The processed samples were subjected to microstructural analysis and dry sliding wear test. FSP resulted in microstructural refinement and improvement in wear resistance of Al5083. Moreover, the results indicated that the more number of FSP passes were found to be more effective in improvement of wear resistance, due to more microstructural refinement. It was also found that the load bearing capacity of samples significantly improved with increasing the number of FSP passes

Keywords

Friction Stir Processing, Wear, Grain Refinement, Friction Coefficient

1. Introduction

Surface properties such as wear resistance can determine the service life of components in many industrial applications. Components which are produced by aluminum and its alloys, exhibit poor tribological properties leading even to seizure under detrimental conditions. Hence, There is a strong drive to develop new Al-based materials with greater resistance to wear and better tribological properties [1,2]. Recently, much attention has been paid to FSP that is known as a surface modification technique [3,4]. FSP which was developed based on the principle of friction stir welding (FSW), is remarkably simple. A rotating tool with a pin and shoulder is inserted into a single piece of material and results in significant microstructural changes in the processed zone, due to intense plastic deformation. FSP has been proved to be an effective way to refine the microstructure of aluminum alloys, and thereby improve the mechanical properties [5,6]. Previous investigations [7,8] have indicated that a fine grain structure affects the tribological properties of surface layer. Prasada et al. [9,10] reported that the grain refinement leads to the improvement of wear resistance and load bearing capacity of Al-7Si alloy. Chandrashekharaiah and Kori [11] also reported similar results for different Al-based alloys. One of the major problems associated with Al5083 like other aluminum alloys, is their relatively poor wear resistance which limits their tribological performance [12].

The aim of this study is to investigate the effects of multiple FSP passes on microstrutural and tribological properties of Al5083 alloy. Besides, the effects of the applied normal force during sliding have been studied in order to find out the wear mechanisms in details.

2. Experimental Procedure

The base metal was commercially Al5083 rolled plates of 3 mm thickness with a nominal composition of 4.3Mg –0.68Mn–0.15Si–bal. Al (in wt pct). FSP was applied on the surface of the alloy by a tool made of steel H-13 with a shoulder of 20 mm diameter and a pin of 6 mm diagonal length and 2.8 mm height.

Samples were subjected to 1 to 4 FSP passes along the same direction with rotation rate of 1250 rpm and travel speed of 50 mm/min in room temperature.

Microstructural observations were performed on specimens by transmission electron microscopy (TEM). The hardness of the surface composite layers was measured using 31.25 KgF. Wear behavior of the specimens was evaluated by using a pin-on-disk tester at room temperature. Pin specimens with 5 mm diameter were cut from the processed zone of each sample and ground on emery paper up to grade 320. Counterparts were Discs made of AISI D3 steel with hardness of 58 HRc. The tests were carried out at normal loads of 1 to 5 KN and the rotation speed of 60 rpm. The friction force was recorded automatically against sliding distance by the tester software. The wear weight loss was measured with an accuracy of ±0.01 mg. The worn surfaces of samples were studied using scanning electron microscopy (SEM).

3. Results and Discussions

Figure 1 shows typical TEM microstructure of processed zone for the 1-pass and 4-pass samples. Figure 2 also shows the variations of grain size of base metal and samples processed at 1 to 4 FSP passes. The results indicated that FSP has led to the grain refinement. Moreover, increasing the number of FSP passes has resulted in decreasing grain size of processed zone. Previous investigations [13,14] have indicated that in FSP/W, a continuous dynamic recrystallization phenomenon occurs due to the tool pin disruptive mechanical action and the frictional heat produced. This phenomenon can lead to intensive microstructural refinement. Increasing the number of FSP passes probably has led to occurrence of more dynamic recrystallization.

Conflicts of Interest

The authors declare no conflicts of interest.

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