The Effects of Molding Materials on Microstructure and Wear Behavior of A356 Alloy

Introduction

Cast A356 (Al–Si–Mg) alloys are widely used in industrial applications because of their low density, high electric and thermal conductivities, good corrosion resistance and castability. Besides these superior characteristics, since aging heat treatment can be applied, it is a material preferred in the automotive industry. The Mg₂Si intermetallic phase formed in structure through the aging heat treatment improves mechanical properties of this alloy. Furthermore, the Fe-based intermetallic phases formed in structure during solidification of the alloy increases hardness, while they adversely affect the % elongation [1, 2, 3]. Mechanical properties and microstructure of the cast Al–Si–Mg alloys can be controlled by means of various parameters such as chemical composition, modification and grain refinement, molding conditions, solidification and cooling conditions [4, 5]. Wear encountered in service conditions is the most extensively experienced damage type observed in the industrial applications. In addition, damage of parts due to wear is one of the expensive problems. In general, wear is defined as material loss occurred as a result of relative movement of parts in contact with each other [6]. In other words, wear is one of the important mechanical properties and occurred as a surface damage during the operation of the parts. This is one of the major problems in the reduction of the efficiency of the established system. A lot of work has been made to investigate the wear behavior of Al–Si–Mg alloys [7, 7, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 1717]. Wear behavior depends on many parameters such as applied load, sliding speed and environmental conditions. Chemical composition and varying solidification conditions due to mold materials, core and chiller affect the mechanical properties of cast Al alloys [18, 19, 20]. The microstructure is also affected by mold material which is an important parameter. Heat transfer coefficient of the mold material is very important for local solidification time and secondary dendrite arm space (SDAS). In conjunction with SDAS, the morphology of eutectic silicon (Si) and grain size are the other effective parameters affecting the mechanical properties. Therefore, many studies are available in investigating the effects of these parameters on wear behaviors of the A356 alloy [21]. Ardakan et al. [4] reported that increasing Mg content in hypereutectic Al-Si-Cu-Mg alloys increased the formation of Mg₂Si intermetallic compounds and this, in turn, increased the hardness and decreased the weight losses of the alloy. Prasada Rao et al. [6] reported that grain refinement in Al and Al7Si alloys increased wear resistance. Grain refinement processes are performed by the addition of Al–Ti–C into the molten alloy. Grain refinement process can improve hardness and mechanical properties of the alloy. However, the grain structure can be improved by decreasing the solidification time (increasing cooling rate) as well [22]. Therefore, this study aims at investigating the effects of microstructural changes on wear behavior of A356 alloy. Different microstructures in A356 were obtained using molds made from different materials.

Materials and method

In the experimental studies, three different mold materials (60–65 grade quartz sand AFS (SiO2), 50–55 grade AFS chromite sand and insulated ceramic Lod 607 which is known with high thermal–shock resistance and very low thermal conductivity) were used. Two percent alkaline phenolic resin and hardener were used for the quartz sand and chromite molds, while 90 % water was used in the preparation of ceramic molds. After the insulated ceramic molds were prepared, they were sintered at 1,000 °C for 8 h. Quartz sand, chromite sand and insulated ceramic molds were hold at atmospheric environment (at room temperature) for 24 h before the casting process. The shape, dimensions and gating system of the cast blocks are given in Figure 1. In order to prevent irregular flow of the liquid metal, thinned diffuser and runners were used [3].

Figure 1:

The shape, dimensions and gating system of the cast block.

The chemical composition of A356 alloy used in the experimental studies is given in Table 1. The melting process of A356 alloy was carried out in an electric resistance furnace with 40 kg Al melting capacity. The molten A356 alloy (750 °C) was degassed within the furnace at 1 bar pressure using argon (5 min). After the casting process was completed, their gating systems were cut and the cast blocks were homogenized at 540 °C for 6 h. Then, the samples prepared for characterization and wear tests were aged. The solid solution process was carried out at 540 °C for 8 h and then quenched. All the samples were artificially aged at 170 °C for 10 h (T6) after 24 h natural aging.

For characterization studies, the aged samples were subjected to standard metallographic processes and etched 95 ml H₂O, 2.5 ml HNO₃, 1.5 ml HCl and 1 ml HF (Keller) solution for 15 s. The prepared samples were characterized through optical microscope (OM; MEIJI), image analysis system (MSQ Plus 6.5), scanning electron microscope (SEM + EDS/FEI Quanta 250) and hardness measurements (Affri System VRSD-251/HV2). Hardness values were determined by taking averages of five individual hardness measurements from each sample. Wear tests were carried out on pin-on-disc-type wear equipment according to the ASTM-G99-05 standard. Before the wear tests, in order to ensure full contact between the abrasive steel disc and sample surfaces, all the samples were ground using 1,200 grade SiC sand paper. The wear tests were carried out at room temperature and dry sliding conditions, 1 ms⁻¹ sliding speed, 20 N load and four different sliding distances (500–2,000 m). Following the wear tests, the worn surfaces were examined with SEM.

Results and discussion

Microstructure examinations

In Figure 2, OM images of the cast A356 alloy poured into different mold materials are shown. As it can be understood from the OM images that microstructure of the alloy is comprised of α-aluminum and Al–Si eutectic phase. Since the thermal conductivities of molds are different, a difference was observed among the sizes of α-aluminum grains. SEM images of A356 alloy, produced into different mold materials, were shown in Figure 3.

Figure 2:

Optical microscope (OM) images of cast A356 alloy poured into different mold materials; (a) quartz mold, (b) Lod 607 mold and (c) chromite mold.

Figure 3:

SEM images of A356 alloy produced by different mold materials.

Size and distribution of Si particles, situated in Al–Si eutectic structure, are shown in SEM images. It was observed that Si particles in bar shape are distributed close to each other in the insulated ceramic and quartz molds while they are distributed as fine and globular shape in the chromite mold. Dendrite arms were found to be coarser for the alloy poured into the Lod 607-insulated ceramic mold whose cooling rate was the lowest, while they were found to be finer for the alloy poured into chromite mold whose cooling rate was highest. It is seen from the images of the alloys poured into the quartz sand mold that the sizes of dendrite arms are in between those of the other two alloys. This situation can be clearly seen from the SDAS measurement results given in Figure 4.

Figure 4:

SDAS results of A356 alloy poured into different mold materials.

Average SDAS measured from the OM images (five different images) decreased with increasing cooling rate. While SDAS was measured as 32 µm for the A356 alloy poured into the Lod 607-insulated ceramic mold, it was determined for quartz and chromite molds as 24 and 15 µm, respectively. Thermal conductivities of Lod 607-insulated ceramic, quartz sand and chromite sand molds are known as 0.21, 0.52 and 0.63 W/mK, respectively [23, 24]. It can be seen that the results obtained as a result of experimental studies are in agreement with the thermal conductivity values. It was observed that the SDAS value for cast A356 alloy poured into the mold with the lowest heat conductivity value increases, whereas it decreases when it is poured into chromite and mold with the highest heat conductivity value. In a previous study, similar results were reported [22]. In Figure 5(a) and 5(b), SEM images and EDS (mapping) results of artificially aged (T6) A356 alloy poured into quartz sand mold are given.

Figure 5:

SEM image (a) and EDS analysis (b) of cast A356 alloy poured into quartz sand mold.

The SEM image in Figure 5 shows the presence of Mg₂Si intermetallic phase which increases the strength of the alloy and was formed through artificial aging. Moreover, oxides can also be formed during melting/pouring processes or solidification of the alloy. From the EDS analysis, it is understood that Fe-rich intermetallic phase was formed during casting processes. It was reported in the previous studies, Fe-rich intermetallic compounds were formed within structure of cast A356 alloy [25, 26, 27]. Salleh et al. [28] reported that Fe-rich intermetallic compounds have detrimental effects on mechanical properties of hypo-eutectic Al–Si–Cu alloys. In Figure 6, hardness variations of A356 alloys poured into different molds are given.

Figure 6:

Hardness variations of cast A356 alloy poured into different molds.

According to the hardness results given in Figure 6, it is seen that cooling rate of cast A356 alloy has not only effect on the SDAS (Figure 4), but also on the hardness of the alloy. Depending on the cooling rate, hardness of the alloy also increases. While hardness of the cast A356 alloy poured into Lod 607-insulated mold was measured as 87 HV, hardness values of the alloys poured into quartz sand and chromite sand were measured as 114 HV and 118 HV, respectively. According to these results, solidification practice of the alloy is substantially significant parameter on the microstructure and hardness. Additionally, the presence of Mg₂Si and Fe-rich intermetallics formed by artificial aging is also effective on the hardness of the alloy. In the solidification process of the alloy, the parameters such as cooling rate, speed of solidification and chemical composition play an important role in formation of dendritic structure (formation of either coarse or finer structure). As the dendritic structure and the secondary phase compounds in the alloy make the movement of the dislocation difficult, they improve the mechanical properties [25].

Wear tests

Weight losses and wear rates of cast A356 alloy poured into the different molds are given in Figure 7. The wear tests were carried out under 20 N loads and for four (500–2,000 m) sliding distances.

Figure 7:

Weight losses and wear rates of cast A356 alloy poured into the different molds.

It can be clearly understood from Figure 7 that microstructural changes in cast A356 alloy depending on the thermal conductivities of the mold material have also influence on the wear behavior of the alloy. There is a close correlation among the weight losses and wear rate results; SDAS results are given in Figure 4 and hardness results are given in Figure 6. Concerning the SDAS results given in Figure 4, the lowest SDAS value (15 µm) was measured with the alloy poured in chromite sand mold, and the one with the highest hardness value was measured again with the same mold material. According to the weight loss results given in Figure 7, the lowest weight loss is obtained for the alloy poured into chromite sand mold. It is also observed that weight loss and wear rate of the alloy poured into Lod 607 mold increase due to its lowest hardness. These obtained results are also confirmed with OM images given Figure 2. Concerning the alloy poured into chromite sand mold, it is seen from OM images given in Figure 2 that Al–Si eutectic phase is thinner. Sharma et al. [5] reported that Al–Si coarse eutectic phase in the structure is broken during wear tests and accordingly this increases weight loss. The results obtained during the wear tests confirmed this finding. In Figure 8, SEM images of worn surface cast A356 alloys poured into different mold materials are given.

Figure 8:

Worn surface SEM images of quartz mold (a), Lod 607 ceramic mold (b) and chromite mold (c).

In the worn surface SEM images given in Figure 8, sliding direction can be seen clearly on the samples poured into Lod 607-insulated ceramic material. It is understood that on the worn surfaces of all samples, adhesive wear mechanism is dominant and certain level of oxidation emerged on surfaces during sliding process. Previous studies in the literature report that oxide layers occur on the worn surface during sliding as a result of thermal effects and these oxide layers facilitate sliding [29, 30]. Additionally, adhesive and partially abrasive wear mechanism occurred in A 356 alloy poured into the mold prepared using Lod 607-insulated ceramic material.

Conclusion

In this present study, the effect of the molding materials of the cast A356 alloy on microstructure, hardness and wear behaviors was investigated. The results of the experimental studies are summarized below:

1. Microstructures of the Al–Si–Mg alloy differ depending on the mold materials. SDAS decreases proportionally with increasing cooling rate.

2. Based on the cooling rate, hardness values of the alloy also differ. As the cooling rate increases, hardness of the alloy increases.

3. According to the wear test results, it was observed that as hardness of the alloy increases, weight loss and wear rate decrease.

4. It was determined as a result of the molding and aging processes that Mg₂Si intermetallic and Fe-rich intermetallic compounds were formed within the structure.

5. From the SEM images, it was observed that the wear surface of the cast A356 alloy poured into different molds, dominant wear mechanism is adhesive. However, abrasive wear mechanism is also seen for the sample poured into the Lod 607-insulated ceramic mold.