Elsevier

Materials & Design (1980-2015)

Volume 54, February 2014, Pages 544-552
Materials & Design (1980-2015)

An investigation on the tribological behavior of AZ91 and AZ91 + 3 wt% RE magnesium alloys at elevated temperatures

https://doi.org/10.1016/j.matdes.2013.08.073Get rights and content

Highlights

  • Wear behavior of AZ91 and AZ91 + 3RE alloys was studied at various sliding speeds.

  • AZ91 + 3RE alloy showed superior high temperature wear properties.

  • Influence of surface temperature on the alloys’ wear behavior was evaluated.

Abstract

The wear behavior of AZ91 and AZ91 + 3 wt% RE magnesium alloys was investigated under a normal load of 20 N at the wear testing temperatures of 25–250 °C and sliding speeds of 0.4 and 1 m s−1. As the sliding speed increased from 0.4 to 1 m s−1 at the wear temperature of 25 °C, the wear rates of AZ91 and AZ91 + 3 wt% RE alloys decreased by about 8% and 60%, respectively. With an increase in the wear temperature to 100 °C, the wear rate of AZ91 alloy was reduced by 58% at a sliding speed of 0.4 m s−1, while the wear rate was sharply increased at a sliding speed of 1 m s−1. At higher wear temperatures, the wear of the AZ91 alloy at both sliding speeds soared as a result of the softening of β-Mg17Al12 phase. However, the wear rate of AZ91 + 3 wt% RE alloy showed a minimum at the wear temperatures of 100 and 200 °C at sliding speeds of 1 and 0.4 m s−1, respectively. Superior wear behavior of AZ91 + 3 wt% RE at the elevated temperatures could be attributed to its higher thermal stability and strength. Furthermore, a rise in sliding speed led to a 55% reduction in the wear rate of AZ91 + 3 wt% RE alloy at the wear temperature of 100 °C due to the formation of stable oxide layers on the wear surface.

Introduction

The Mg–Al-based alloys have been the most frequently used commercial cast magnesium alloys since they were first developed during World War I [1]. The addition of Zn to Mg–Al magnesium alloys gives some strengthening that provides a series of magnesium alloys called AZ. Compared to other magnesium alloys, the use of AZ alloys is favored because of their reasonable price, good castability, suitable corrosion resistance and mechanical properties for room temperature applications [2]. AZ91 magnesium alloy (9 wt% Al, 1 wt% Zn, and 0.5 wt% Mn) is the most widely used of Mg–Al magnesium die-casting alloys, due to its excellent castability even for the most complex and thin-walled parts. However, AZ91 and other Mg–Al-based alloys are only utilized for working temperatures up to 110–125 °C because of their weak mechanical properties and creep resistance at higher temperatures [2]. The weak mechanical properties are attributed to the existence of β-Mg17Al12 phase in the interdendritic regions. In the as-cast condition, the β-Mg17Al12 phase can form in interdendritic regions of the alloys containing more than 2 wt% aluminum [1], [2]. The low melting point (between 437 and 458 °C) and low thermal stability of the phase [1], [2], [3] contribute to the reduction in the strength and creep resistance of the Mg–Al-based alloys as a result of coarsening, softening, and dissolution of the β-Mg17Al12 [4], [5].

Numerous investigations [6], [7], [8], [9], [10], [11], [12], [13], [14], [15] have been conducted to characterize the mechanical behavior of cast magnesium alloys and to increase their high temperature strength. Most of these studies have mainly focused on replacing discontinuous β-Mg17Al12 precipitate with more thermally stable intermetallics. It has been shown that the addition of rare earth elements to AZ91 magnesium alloy leads to the formation of a thermally stable Al11RE3 intermetallic compound, which improves creep [6], tensile strength [8] and stability [9] of the alloy at high temperatures. These works have also proven that the alloy containing 2 wt% RE shows the best thermal stability and high temperature mechanical properties. Nami et al. [10] reported that the addition of calcium and rare earth elements to AZ91 magnesium alloy ameliorates impression creep behavior of the alloy, resulting in the precipitation of thermally stable Al–RE and Al–Ca intermetallics and a reduction in the amount of the unstable β-Mg17Al12 phase. They also showed that an alloy containing both calcium and rare earth elements exhibits better creep properties than the alloys containing either calcium or rare earth elements. Jing et al. [11] studied the effects of calcium and strontium additions to an Mg–4Al based alloy. It was revealed that the formation of Al4Sr intermetallic compound in the alloy improves its creep properties at the temperatures of 150 and 200 °C. They also claimed that the addition of calcium, besides strontium, to Mg–4Al magnesium alloy contributes to the formation of other thermally stable intermetallics of Mg2Ca and Al2Ca. This study concluded that the precipitation of a higher amount of thermally stable intermetalics in the Mg–Al–Sr–Ca alloy provides the alloy with a superior creep resistance compared to the strontium containing alloy and base material. Influences of calcium, strontium and rare earth elements on the microstructure, creep and mechanical properties of magnesium alloys can also be found in other researchers’ works [12], [13], [14], [15]. These efforts have led to the creation of new types of alloys such as: Mg–Al–RE, Mg–Al–Ca, Mg–Zn–Al–Ca, and Mg–Al–Ca–RE [3].

The wear behavior of magnesium alloys is another crucial concern, which has become one of the main subjects of investigation during recent years. Chen and Alpas [16] studied the wear behavior of AZ91 alloy under different normal loads and sliding speeds. They showed that a mild wear regime can change to a severe wear as the surface temperature reaches 74 ± 15 °C. Aung et al. [17] also investigated the wear behavior of AZ91 alloy at different sliding speeds. They observed that abrasion, oxidation, and delamination are the main wear mechanisms at sliding speeds of 0.01, 0.1, and 1 m s−1, respectively. The variation of surface temperature under different normal loads and its effect on the wear behavior of AZ91 alloy was investigated by An et al. [18]. It was suggested that under normal loads of 150 and 200 N, melting of β-Mg17Al12 phase might occur due to a high surface temperature. Further increase in the normal load and, therefore, surface temperature could also contribute to the melting of α-Mg phase. The wear behavior of AZ91 magnesium alloy at wear testing temperatures of 25–200 °C was studied by Wang et al. [19]. They reported that under normal loads of 12.5–25 N, the wear rate of the alloy decreases as the wear testing temperature increases. The wear behavior of AZ91 alloy at elevated temperatures was also studied by Zafari et al. [20]. It was revealed that a transition from a mild to a severe wear might occur as the surface temperature reaches 140, 180, and 400 °C under normal loads of 40, 20, and 5 N, respectively. Blau and Walukas [21] compared the wear behavior of thixoformed and die cast AZ91 alloys in reciprocating and unidirectional sliding motions. They found that in the reciprocating sliding tests the wear rate of the thixoformed alloy was 25% lower than that of the die cast alloy, while the alloys showed similar wear rates in the unidirectional sliding tests. The effects of 1–3 wt% rare earth (RE) addition elements on the wear behavior of AZ91 magnesium alloy at room temperature (25 °C) were studied by Mashinchi Asl et al. [22]. They found that the wear resistance of AZ91 alloy is higher than the wear resistance of the RE containing alloys under normal loads of 10–50 N at sliding speeds of 0.25–0.5 m s−1. However, it was observed that under higher loads and sliding speeds the RE containing alloys show more resistance against surface damages. It was also revealed that the alloy containing 2 wt% RE exhibits surpassing wear resistance under the severe wear conditions. Ju et al. [23] studied the influence of 0.2–1 wt% RE elements on tribological behavior of AZ91 alloy at the wear temperature of 25 °C. It was found that the wear resistance of the alloy increases and coefficient of friction decreases as the RE content increases.

It was found in our work [24] on the effects of 1, 2 and 3 wt% RE additions on the tribological behavior of AZ91 alloy at a constant sliding speed of 0.4 m s−1 and different testing temperatures that the alloy containing 3 wt% RE was the most wear-resistant one. To further investigate and understand the wear behavior of AZ91 + 3 wt% RE alloy, this paper studies the influence of sliding speed on the wear properties of the alloy at a wide range of wear testing temperatures.

Section snippets

Experimental procedures

An AZ91 alloy with the composition of Mg–9 wt% Al–0.8 wt% Zn–0.3 wt% Mn and an addition of 3 wt% rare earth elements were produced from high-purity elements and a La-rich misch-metal (71.5 wt% La) via die-casting in an electrical furnace. Magnesium ingot was first covered with MAGREX 36 flux and melted at 750 °C. The additives were gradually added to the melt and mixed mechanically. The melt was held at 750 °C for 20 min, and then poured into a preheated steel die, 150 °C in temperature, through a

Results and discussion

The SEM micrographs of as-cast AZ91 and AZ91 + 3RE alloys are shown in Fig. 1. Fig. 1a indicates that AZ91 alloy consisted of a dark gray dendritic phase (region A) and a white secondary phase in the interdendritic regions (region B). Furthermore, EDS analysis of regions A and B, marked in Fig. 1a, revealed about 4 and 18 wt% of aluminum, respectively. The higher percentage of Al in region B could suggest the existence of β-Mg17Al12 phase while regions A could be α-Mg. Addition of RE elements to

Conclusions

  • (1)

    The tribological behavior of AZ91 and AZ91 + 3R magnesium alloys was investigated under a normal load of 20 N, at the wear temperatures of 25–250 °C and sliding speeds of 0.4 and 1 m s−1.

  • (2)

    The wear rates of the AZ91 and AZ91 + 3RE alloys decreased about 8% and 60% at the wear temperature of 25 °C as the sliding speed was increased from 0.4 to 1 m s−1 due to the formation of oxides on the worn surfaces.

  • (3)

    The AZ91 alloy showed a 58% reduction in wear rate at the wear temperature of 100 °C and sliding speed of 0.4

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