UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

618 IMMC 2016 | 18th International Metallurgy & Materials Congress

Eff ect of SiC Grain Refi ning on Wear Resistance of Mg-Al Alloys

Erdem Karakulak¹, Norbert Hort², Yusuf Burak Küçüker1

¹Kocaeli University,²Helmholtz-Zentrum Geesthacht - Türkiye,Germany

Abstract:

Different amounts of SiC particles were added to Mg-Al alloys to refine grain size of the samples cast with permanent mold direct chill casting technique. Microstructures of the cast materials investigated and grain size measurements were conducted. Vickers hardness tests and dry sliding wear tests were realized on the specimens with different grain size. Addition of SiC and increasing aluminum content in the alloy caused a decrease in the grain size of the specimens. Refinement of microstructure increased hardness and wear resistance of the cast samples. Worn surfaces of the specimens were investigated by scanning electron microscope to understand wear mechanisms.

1. Introduction

Magnesium is the lightest structural metal with a density of 1,738 g/cm3 [1,2]. Low density and good castability of magnesium and its alloys make them candidate materials for numerous applications especially in automotive industry to lower fuel consumption and CO2 emissions [3,4]. However limited mechanical properties of magnesium alloys are the main problems to overcome to widen the usage of these materials in different applications [5]. Wear damage is one of the main problems for moving parts in automotive production. In general magnesium and its alloys show poor wear resistance [6]. There are different ways to increase wear resistance of Mg alloys like plasma electrolytic oxidation (PEO) [7], surface mechanical attrition treatment [8], ceramic coatings with physical vapour deposition (PVD) [9]. Grain refining is a process where hardness and wear properties of material can be improved without any alloying addition or coating. In this study wear tests were conducted on different binary Mg-Al alloys with

different average grain sizes. Grain refinement of Mg-Al alloys were obtained with SiC inoculation. SiC addition is an effective way of refining grain size of magnesium alloys, especially alloys containing aluminium as alloying element or impurity. Effect of grain refinement with SiC on microstructure, hardness, wear resistance was investigated in detail.

2. Experimental

Melting of different Mg alloys were realized using an electric resistance furnace under protective atmosphere of 0,3 % SF6 +Ar. Chemical compositions of the used alloys are given in Table 1. When the magnesium is molten alloying elements and SiC (average particle size 2 μm) was added to the melt. Then liquid metal is transferred to a cylindrical steel mold (100 mm in diameter and 230 mm in height). This mold is then placed into the permanent mold direct chill casting machine. In this device the melt is kept at 680 °C for 30 minutes for all alloys. After waiting for 30 miutes two TP1 samples were taken and solidified according to the standard for grain size measurements. After that the mold with remaining melt inside slowly lowered in to water to be solidified. Grain size measurements were conducted using line intercept method. Hardness tests were realized using a Vickers hardness tester, using 5 kg load and 30 s of loading duration. All given hardness values are an average of 10 measurements. Wear tests were conducted on a Nanovea ball-on-disc type tribometer at room temperature, using 25 N normal load and AISI 52100 steel ball with 5 mm diameter as counter surface. Sliding distance was kept constant at 500 m for all tests. Specimens were cleaned with alcohol and weighed before and after wear tests to obtain weight loss data. Obtained weight loss data is used to calculate wear rate of the specimens. The fallowing equation was used to

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

61918. Uluslararas ı Metalurj i ve Malzeme Kongresi | IMMC 2016

calculate wear rate of specimens: W = M/ D, where W is the wear rate (mm3/m) M denotes mass loss (g), and (g/mm3) and D (m) are the density and sliding distance respectively [10]. Worn surfaces of the specimens after wear tests were investigated under SEM to understand wear mechanism operated during wear tests.

Table 1. Results of chemical analyses of the cast samples

Element Al Fe Mn Zr Mg Pure Mg 0,11 0,02 0,03 0,004 Bal. Mg-1Al 0,92 0,03 0,03 0,004 Bal. Mg-3Al 2,83 0,03 0,03 0,003 Bal.

3. Results

3.1. Grain Size Measurements

Addition of SiC to pure Mg and Mg-Al alloys refined the grain size of the material. Representative microstructures and measured grain size values are given in Fig. 1 and Fig. 2 respectively. Increasing SiC in the alloy decreases the grain size of the cast materials. But the grain refining effect is almost faded after % 0,4 SiC addition. Addition of higher amount of SiC has little effect on grain size. Also increasing aluminium content of the alloy decreases grain size of the material, which is a result of growth restriction effect of aluminium in magnesium alloys [11].

(a)

(b)

(c)

Figure 1. Microstructures of (a) Pure Mg, (b) Mg-1Al and (c) Mg-3Al without SiC addition

Figure 2. Effect of SiC addition on grain size of pure Mg and Mg-Al alloys

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

620 IMMC 2016 | 18th International Metallurgy & Materials Congress

3.2. Hardness Tests

To understand the effect of aluminium addition and SiC grain refining on the hardness of the materials hardness tests were conducted on all specimens. Results of hardness tests can be seen on Fig. 3. With increasing SiC and Al content hardness of magnesium increases as expected. Addition of SiC dramatically decreases grain size of the material especially for pure magnesium. Decrease of grain size causes an increase in the hardness of material. Addition of aluminium to pure magnesium also increases hardness both with grain refining and alloying effects.

Figure 3. Effect of SiC and Al content on hardness

3.3. Wear Tests

Weight loss data of the specimens were obtained by weighing cleaned specimens before and after wear tests. Weight loss data is used to calculate wear rate values of the specimens. Variation of wear rate values of alloys with different SiC addition is given in Fig. 4. As can be seen on the image wear tests results are in good harmony with hardness test results. With increasing hardness wear rate of alloys decreases resulting with a lower wear rate.

Figure 4. Change of wear rate of alloys with SiC addition

3.4. Worn Surface Investigations

Worn surfaces of the specimens were investigated under SEM to understand the wear mechanism. Representative images of worn surfaces can be seen on Fig. 5. Wear mechanism operated during the wear tests of specimens were mainly abrasion. Small amount of plastic deformation was also reported especially on the edges of the wear track. Another important finding was the cracks formed on the worn surface during wear tests. The reason of these cracks is the low deformation ability of magnesium because of its hexagonal lattice structure.

TMMOB Metalurj i ve Malzeme Mühendisleri Odas ıBildir i ler Kitab ı

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Figure 5. SEM images of worn surface of Mg-3Al alloy with 0,6 SiC addition in different magnification

4. Conclusions

Effect of SiC on grain size, hardness and wear properties of three different magnesium alloys were investigated in this study. Fallowing conclusions were drawn according to the results of the experimental work.

• Addition of SiC to pure Mg and Mg-Al alloys decreases grain size. After 0,4 % addition grain refining effect becomes less effective.

• Grain refining of pure Mg and Mg-Al alloys results with an increase in the hardness of material. The increment of hardness is much higher in pure Mg, because of higher grain refining effect.

• Increasing hardness with grain refining also increases wear resistance of alloys. Wear rate of all alloys were decreased with SiC addition.

• Main wear mechanism was abrasion but crack formation and propagation was also reported on the worn surfaces of the specimens.

Acknowledgment

Authors thank to The Scientific and Technological Research Council of Turkey (TUBITAK) for their financial support under 2219 program.

References

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