Grain-Refined AZ92 Alloy with Superior Strength and Ductility
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Grain-refined AZ92 (GR-AZ92) alloy with superior tensile properties is developed by adding 1 wt% Zn and a very small amount of SiC (0.17 wt%) to commercial AZ91 alloy for enhancing the solid-solution strengthening effect and refining the crystal grains, respectively. The homogenized GR-AZ92 alloy with an average grain size of 91 μm exhibits a tensile yield strength (TYS) of 125 MPa, ultimate tensile strength (UTS) of 281 MPa, and elongation of 12.1%, which are significantly higher than those of AZ91 alloy with a grain size of 420 μm (TYS of 94 MPa, UTS of 192 MPa, and elongation of 7.0%). The peak-aging time of GR-AZ92 alloy (8 h) is significantly shorter than that of AZ91 alloy (32 h) owing to a larger amount of grain boundaries in the former, which serve as nucleation sites of Mg17Al12 precipitates. A short-aging treatment for less than 1 h of the GR-AZ92 alloy causes an effective improvement in its strength without a significant reduction in its ductility. The 30-min-aged GR-AZ92 alloy has an excellent combination of strength and ductility, with a TYS of 142 MPa, UTS of 304 MPa, and elongation of 8.0%.
KeywordsMetals Casting Aging Grain refinement Tensile test
With the tightening regulations on the fuel efficiency and carbon dioxide emissions of vehicles, efforts are actively underway to reduce the weight of the vehicle body in order to conform to these regulations . In this context, magnesium, which has the lowest density among commercially available structural metal materials, has been attracting increasing attention for application in the transportation industry. Cast Mg alloys have been applied to various automobile components such as seat frames, instrument panel structures, radiator supports, transmission cases, and engine blocks [1, 2]. Mg-9 wt% Al-1 wt% Zn (AZ91) alloy is known to have excellent castability and relatively superior mechanical properties and corrosion resistance to other cast Mg alloys; for this reason, more than 90% of cast Mg products are used in the form of AZ91 alloy . However, because Mg alloys generally have lower strength than steels and Al alloys—which are the predominantly used metals in automobiles—it is imperative to improve the mechanical properties of Mg alloys to enable them to replace steel or Al components.
The strength of cast Mg alloys can be improved by the addition of rare earth (RE) elements [4, 5, 6, 7] or reinforcements [8, 9, 10, 11, 12]. Zhang et al.  reported that the addition of Y-rich misch metal (Ymm) effectively improves the tensile yield strength (TYS) of cast AZ91 alloy and showed that AZ91-0.8Ymm (wt%) alloy exhibits excellent tensile properties, with a TYS of 160 MPa, ultimate tensile strength (UTS) of 272 MPa, and elongation of 11%. Furthermore, Wang et al.  demonstrated that cast AZ91 alloy with added 1.0 wt% Nd has considerably higher tensile properties (TYS of 120 MPa, UTS of 172 MPa, and elongation of 3.9%) than cast AZ91 alloy (TYS of 100 MPa, UTS of 161 MPa, and elongation of 3.2%). Cast AZ91 alloys containing these RE elements (e.g., Y, Nd, Ce, La, and Gd) exhibit superior tensile properties, with a TYS of 102–161 MPa, UTS of 152–272 MPa, and elongation of 3.1–14.0% [4, 5, 6, 7]. However, the addition of these expensive RE elements to Mg alloys—whose material price is already higher than that of competitive material Al alloys—causes an increase in the cost of the final products; the consequential lower cost competitiveness of these products reduces their practical applicability. The addition of reinforcements such as silicon carbide (SiC), aluminum borate, and Cu particulates is also highly effective in improving the strength of the cast material [8, 9, 10, 11, 12]. Ho et al.  showed that the addition of 15 wt% SiC to AZ91 alloy improves the TYS and UTS of the cast alloy by 257 and 289 MPa, respectively. Zheng et al.  also developed a high-strength cast AZ91 alloy with a TYS of 266 MPa and UTS of 352 MPa through the addition of 30 wt% Al18B4O33 whiskers. The strength of the cast AZ91 alloy can be improved by the addition of these reinforcements (TYS of 122–299 MPa and UTS of 152–385 MPa), but its tensile elongation decreases considerably owing to a loss of ductility by the addition of hard and/or brittle reinforcements (elongation of 0.7–1.38%) [8, 9, 10, 11, 12]. Such low elongation makes it difficult to use reinforced cast AZ91 alloy as a structural component that requires certain levels of ductility and toughness.
Therefore, in order to expand the application range of cast Mg alloys, it is necessary to improve both their strength and elongation without increasing the cost of the final products. This study attempted to develop RE-free Mg–Al–Zn alloy with superior tensile properties to commercial AZ91 alloy through multiple applications of various strengthening mechanisms. For this purpose, the Zn content of AZ91 alloy was optimized to enhance the solid-solution strengthening effect and the grain size was refined by adding a very small amount of SiC to achieve grain-boundary strengthening. A short-aging treatment was then performed to induce precipitation strengthening without a large reduction in ductility. The end product was a grain-refined AZ92 (GR-AZ92) alloy with excellent strength and ductility. This paper discusses the enhancement of the tensile properties of the alloy through Zn addition and grain refinement and the control of its tensile strength and elongation via a short-aging treatment.
2 Experimental Procedure
Cast ingots of Mg–9Al–0.2Mn alloys with different Zn contents—i.e., 1, 2, 3, and 4 wt% (hereafter referred to as AZ91, AZ92, AZ93, and AZ94, respectively)—were prepared by first melting them in an electric resistance furnace at 750 °C under an inert atmosphere containing a mixture of CO2 and SF6 gases, then holding the molten metal at 700 °C for 15 min to stabilize it, and finally pouring it into a steel mold preheated to 210 °C. A cast ingot of GR-AZ92 alloy was fabricated by adding a small amount (0.7 wt% melt) of Al-SiC master alloy (20 vol% SiC) to molten AZ92 at 750 °C during the casting process; the SiC particles (10 μm in size) included in the Al-SiC master alloy serve to enhance the heterogeneous nucleation of the primary Mg phase and refine the microstructure of the cast billet [13, 14]. All cast ingots were homogenized at 410 °C for 24 h and then water-quenched. For aging treatment, the homogenized AZ91, AZ92, and GR-AZ92 alloys were machined to rectangular samples with dimensions of 20 × 20 × 10 mm3; aging heat treatment was performed at 200 °C for up to 64 h.
The microstructures of the as-cast and homogenized samples were analyzed by optical microscopy (OM), and the average grain size of the homogenized samples was measured by averaging the values obtained from five OM images through the linear intercept method. The hardness of the homogenized and aged samples was measured using a Vickers microhardness tester with a load of 0.2 kgf and a dwell time of 10 s. All hardness values were determined by averaging values excluding the maximum and minimum values measured at 10 different positions for each sample. Tensile tests of the homogenized and aged samples were performed at room temperature by using an Instron 4206 universal testing machine with a strain rate of 1.0 × 10−3 s−1. The tests were performed three times using dog-bone-shaped samples (gage section: Ø6 mm × 25 mm) by means of an extensometer, and the average values of these measurements were used in this study.
3 Results and Discussion
Owing to the high growth restriction factor (GRF) of Zr and similar crystal structure to Mg, Zr has been considered as a highly effective grain refiner for Al-free Mg alloys such as pure Mg and Mg–Zn-based alloys [17, 18]. However, Zr is an ineffective grain refiner for Mg alloys containing Al, Si, and/or Mn, because the strong affinity of Zr with Al, Si, and Mn causes the formation of intermetallic compounds [19, 20]. Carbon inoculation is regarded as the most effective way to refine the grains of Mg–Al alloys, and it is commonly known that aluminum carbide plays an important role of a potent nucleant that promotes heterogeneous nucleation [21, 22]. Carbon can be introduced into the melt in various forms such as C2Cl6, Al4C3, MgCO3, MnCO3, and graphite powders [21, 22, 23, 24, 25, 26]. One method of carbon inoculation is the use of SiC—which has been widely used as a reinforcement in Al- and Mg-based composites—as an effective grain refiner for Al-bearing Mg alloys [27, 28, 29]; it has a particularly remarkable grain refining effect when added to Mg–Al alloys containing less than 9 wt% Al . When a large amount of SiC is added, the Si decomposed from the SiC particles reacts with Mg to give a Mg2Si phase with “Chinese-script” morphology; this leads to a deterioration in the tensile properties of the cast material . For example, a considerable amount of Mg2Si and/or residual SiC particles are present in Mg–3Al–10SiC (wt%)  and AZ91-5SiC (wt%)  alloys. In the present study, to refine the grain size of the AZ92 alloy without the formation of the Mg2Si phase, a very small amount of SiC (0.17 wt%) was added to the melt in the form of an Al–SiC master alloy.
Tensile properties of homogenized and aged AZ91, AZ92, and grain-refined AZ92 (GR-AZ92) alloys
Aged for 10 min
Aged for 30 min
Aged for 60 min
Peak-aged for 8 h
With the aim of manufacturing a cast Mg alloy with superior strength and ductility, 1 wt% Zn and a very small amount of SiC (0.17 wt%) were added to commercial AZ91 alloy for enhancing the solid-solution strengthening effect and refining the crystal grains, respectively. The homogenized GR-AZ92 alloy with an average grain size of 91 μm exhibits significantly higher tensile properties (TYS of 125 MPa, UTS of 281 MPa, and elongation of 12.1%) than the homogenized AZ91 alloy (TYS of 94 MPa, UTS of 192 MPa, and elongation of 7.0%). In addition, because the refined grains accelerate the precipitation behavior induced during aging treatment, the strength of the GR-AZ92 alloy can be effectively improved, without a major reduction in its ductility, through a short-aging treatment less than 1 h. The GR-AZ92 alloy short-aged for 30 min has an excellent combination of strength and ductility, with a TYS of 142 MPa, UTS of 304 MPa, and elongation of 8.0%.
This work was supported by the National Research Foundation of Korea Grant funded by the Korea government (MSIP, South Korea) (Nos. 2016R1C1B2012140 and 2017R1A4A1015628).
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