Abstract
The microstructure and mechanical properties of the optimised Al–0·35Mg–7·0Si cast alloys subjected to solid solution treatment followed by aging treatment have been analysed. The results indicate that the overheating treatment temperature of optimised alloy is 508°C. The mechanical properties of alloy with 495°C ×5 h and 500°C ×4 h solid solution treatment are better than that of other experiment conditions. Tensile strength is 239–243 MPa. Hardness is ∼104 HB. Elongation is 4·4–4·8. The main precipitation phases may have Si, Al2Cu, Al6Cu3Ni and AlxCuCe phases whose morphology may be long strip, round, ellipse, flake and block. All specimens were fractured in a mixed mode fracture character consisting of a ductile and brittle manner, and the fractographs are in good agreement with the variations in fracture modes and mechanical properties of different samples.
Introduction
Recently, the use of lightweight metals, such as aluminium alloys and magnesium alloys, has increased particularly in the automotive and aerospace industries, where weight saving is a huge consideration.1,2 Compared to the 2xxx and 7xxx Al alloys, the 6000 series Al–Mg–Si cast alloys have been widely used in architectural and structural applications for the past years due to their excellent combination of strength, ease of manufacturing (i.e. extrudability, formability and weldability) and resistance to stress–corrosion cracking.3,4 Shah et al. 5 reported that the cast Al–Si–Mg alloys had good wear resistance; moreover, the silicon content and aging temperature significantly affected the wear resistance. Ease of manufacture (including extrudability, formability and weldability) and good resistance to stress–corrosion cracking of Al–Mg–Si cast alloys had been reported by Yao et al. 6 The composition optimisation of Al–Mg–Si cast alloys was obtained by Zhang et al. 7 as following: 7·0Si–0·35Mg–2·0Cu–0·2Mn–0·2Ni–0·1V–0·8RE–89·35Al. Zhang et al. 8 had also studied the effect of modification treatment on the microstructure and mechanical properties of Al–0·35Mg–7·0Si cast alloy.
In Al–Mg–Si casting alloys, coarse plate-like or acicular eutectic silicon phases are the initiators of fracture, resulting in poor mechanical properties, particularly the elongation. In order to achieve good tensile properties, eutectic silicon should be modified and heat treated. Osorio et al. 9 studied the effects of eutectic modification and heat treatment on mechanical properties of Al–9·0Si casting alloy. Li et al. 10 studied the influence of yttrium and heat treatment on microstructure and tensile properties of Al–0·5Mg–7·5Si cast alloy. Liu 11 studied heat treatment and mechanical properties of Al–0·7Mg–12·7Si cast alloy.
The optimised composition of Al–0·35Mg–7·0Si cast alloys had been obtained. Therefore, the present study is focused on investigating the microstructure and mechanical properties of Al–0·35Mg–7·0Si cast alloys subjected to solid solution treatment followed by aging treatment. The mechanical properties and fracture mechanism of optimised Al alloys at different solid solution treatment processes were also analysed.
Experimental
Based on the authors’ previous research, the chemical composition was determined through the study of the orthogonal test theory, and the composition optimisation of Al–Mg–Si cast alloys was Bal. Al–0·35Mg–7·0Si–2·0Cu–0·2Mn–0·2Ni–0·1V–0·8RE. 7 The alloy was melted in an SG2-5-12 high temperature pot resistance furnace and cast in a metal mould (shown in Fig. 1a) with a lid to prevent magnesium from oxidising. The temperature of the molten alloy was measured with the help of a thermocouple immersed into the melt and a temperature indicator. Two key interactions between the liquid aluminium and its environment are the dissolution of hydrogen and the formation of oxide films. 12 At primary alloy production, the hydrogen gas and oxide films become incorporated into the melt, which introduce defects into castings, and many forms of degassing and fluxing operations to minimise these pre-existing problems are performed, which can affect the properties of the alloy such as elongation, ductile dimples and brittle planar features. 13 In the present paper, we use a traditional hexachloroethane tablets method for degassing. The dross was removed from the melt surface. The holding time of the melt was ∼3 min. The crucible was removed from the furnace at a melt temperature 750°C for pouring. The temperature was logged every 0·2 s during the solidification of the casting. Specimens were then heat treated at the required solid solution temperature for different times, quenched in water and aged at 175°C for a specific aging time as based on the factorial design of experiments. The aging temperature in the prior work included 155, 165, 175, 185, 195, 205 and 215°C with different aging times. The mechanical properties of alloy with 175°C ×6 h are better than those of other experiment conditions.
Schematic diagram of tensile testing specimen
A Dupont 2000 thermal analyser fitted with a 910 differential scanning calorimetry (DSC) module was used to examine the precipitation and melting behaviours of alloys in a continuous heating process. The protective atmosphere was argon during DSC process. Tensile tests were carried out at room and elevated temperatures using an Instron Universal testing machine at a strain rate of 2×10−4 s−1 to obtain the ultimate tensile strength and per cent elongation El at fracture. The sample dimension for tensile testing is shown in Fig. 1b. The machine was calibrated before each test was conducted. The test data were collected automatically using a data acquisition system attached to the Instron machine. Since the maximum limit of the extensometer used was 10, the extensometer was removed from the sample when the elongation exceeded this value. In such cases, the final elongation was measured manually using a micrometer. The average TS and El value obtained from the three samples tested per alloy/condition level was considered to represent that condition. Hardness was determined through a universal Brinell hardness tester. Hardness was measured at seven points, and the average was considered for the study. The fracture surfaces of selected specimens were investigated using a JEOL JSM-5400LV scanning electron microscope (SEM).
Results and discussion
Process principle of solid solution treatment
In order to obtain the best solid solution hardening for optimised Al–Mg–Si alloys, higher heating temperature in solid solution treatment is better. However, solid solution treatment temperature cannot excess critical point (overheating temperature). Differential scanning calorimetry and optical micrograph can confirm accurate solid solution temperature. 14 The DSC curve and optical micrographs of alloys under study in as cast and heat treatment are shown in Figs. 2 and 3 respectively. From Fig. 2, there is an endothermic peak at 508°C, which indicates that the ternary eutectic phases [α (Al)+CuAl2+Al2CuMg] of low melting point have melted. From Fig. 3, the microstructure of alloy at 500°C is normal, but the grains at 510°C had grown larger than that at 500°C. According to DSC curve of alloy (shown in Fig. 2) and microstructure at different heat treatment temperatures (shown in Fig. 3), the overheating temperature of Al–Mg–Si cast alloys is 508°C. In DSC curve of alloy, there are two other endothermic peaks, one is binary eutectic phase [α (Al)+Si] transformation point at 583°C, another is solid–liquid phase transformation point at 610°C, which is the melting point of alloy.
Differential scanning calorimetry curve of Al–Mg–Si cast alloy
Optical micrographs of cast samples for solid solution treatment temperature at a 500°C and b 510°C
Effect of solid solution treatment on mechanical properties
It is known that the mechanical properties of alloys play critical roles in many advanced application. As can be seen, Table 1 demonstrates the influence of solid solution treatment on mechanical properties of Al–Mg–Si cast alloys. In order to know the obvious relationship between solid solution treatment (i.e. temperature and time) and mechanical properties (i.e. tensile strength, hardness and elongation), Fig. 4 shows variations of mechanical properties with different holding times at 485°C solid solution treatment temperature. Figure 5 shows variations of mechanical properties with different solid solution treatment temperatures for 5 h holding time.
Relation between mechanical properties and different holding times at 485°C solid solution treatment temperature
Relation between mechanical properties and different solid solution treatment temperatures for 5 h holding time
Relation between solid solution treatment and mechanical properties
From Table 1 and Fig. 4, as can be seen, the mechanical properties (i.e. tensile strength, hardness and elongation) of alloys first increase and then decrease with the increasing of holding time. When the holding time is 4 or 5 h, the mechanical properties of alloys are the highest. When the holding time is <4 or 5 h, the strengthening phases do not precipitate or precipitate little, which results in the lower strength and hardness. Especially, this phenomenon is obvious when the heat treatment temperature is low and the holding time is short. Otherwise, when the heat treatment temperature is high and the holding time is long, the grains begin to coarsen gradually, thereby causing strength and hardness character to decrease.
It is worthy to notice the principle of solid solution strengthening. The precipitations of solute atoms and other strengthening phase (i.e. Mg2Si and Al2Cu) in Al matrix cause lattice distortion of matrix, thus causing stress field. 15 The interactions of stress field near dislocations can hinder dislocation movement, which results in the increasing of strength and hardness. From Fig. 5, when the holding time is 5 h, the mechanical properties of alloy with 495°C solid solution treatment temperature are better than those of other experiment conditions. From Table 1, it can be seen clearly that the mechanical properties of alloys with 505°C solid solution treatment temperature have no obvious variation trend. Because the overheating temperature of alloys is 508°C, 505°C is very near to 508°C. The temperature control is not accurate and the actual temperature may be over 508°C when the solid solution treatment temperature is 505°C. This resulted in the unstable mechanical properties.
Effect of solid solution treatment on microstructure
It is worthy to notice that solid solution treatment at such a high temperature and such long time did change the size and morphology of precipitation phases, and the Al matrix grains in a considerable manner. After solid solution treatment, the morphology of precipitation phases may have some characteristic morphologies (i.e. long strip, round, ellipse, flake and block). 11 These characters can be observed in Fig. 6. From the energy dispersive spectroscopy analysis of these precipitation phases (shown in Fig. 6), Si, Al2Cu, Al6Cu3Ni and AlxCuCe phases may form. The morphology, size and distribution of Si phases, including primary Si and eutectic Si, affected the mechanical properties of alloys greatly. 16 The Al2Cu phase is precipitation phase, showing solid solution strengthening and dispersion strengthening, which has good heat resistance. The Al6Cu3Ni phase locates in grain boundary and interdendrite, hindering dislocation movement and lattice slipping, which can have strengthening effect and improve heat resistance of alloy. The AlxCuCe phase is a complex compound and has excellent high temperature stability, which can improve mechanical properties of high temperature of alloy.
Morphology (SEM) and energy dispersive spectroscopy analysis of particles at 495°C solid solution treatment temperature and 5 h holding time
Al–7·0Si–0·35Mg cast alloy is a heat treatable hypoeutectic alloy. During solution treatment, eutectic silicon undergoes fragmentation, spheroidisation and coarsening. Figure 7 presents the size and morphology of eutectic silicon after T6 heat treatment (solid solution treatment+artificial aging). In Fig. 7, the ellipse dendritic microstructure is α phase (Al solid solution), and the interdendrite is eutectic silicon. 15 When the holding time of solid solution treatment is from 3 to 5 h, the Al matrix phase gradually spheroidised and homogenised, thereby causing strength and hardness character to increase (see Table 1). Furthermore, if the holding time increases to 6 h, the morphology of some Al matrix phases has changed from spheroid to strip block when comparing Fig. 7c and d. Especially, from Fig. 7e and f, it is clear that the eutectic silicon has changed from spheroid and ellipse to large long strip, thereby causing strength, hardness and elongation character to decrease (see Table 1). As is known, the microstructure, mechanical property and process mutually influence each other. Therefore, the analysis of microstructure change with different processes can help to explain the variation of mechanical properties.
Optical micrographs of alloy at 495°C solid solution treatment temperature and different holding times
Effect of solid solution treatment on fracture morphology
Figure 8 presents SEM fracture surfaces of as cast Al–7·0Si–0·35Mg alloys with different solid solution treatment. It can be clearly seen that a little cleavage planes and many dimples are apparent on fracture surface. 12 This indicates that all the specimens were fractured in a mixed mode fracture character consisting of a ductile and brittle manner. The dimples in Fig. 8b are more than that in Fig. 8a and c. Moreover, the fracture planes in Fig. 8a and c are flat, and there are some cleavage planes and tearing ridges. However, the cleavage planes and tearing ridges mostly disappear, and many equiaxed dimples are more homogenous and deeper in Fig. 8b. It results in higher mechanical properties of specimens in Fig. 8b (see Table 1). The number of dimples in Fig. 8d–f decreases compared with that in Fig. 8b; meanwhile, the number of cleavage plane increases, thereby causing strength, hardness and elongation character to decrease (see Table 1). Especially, the fracture surface in Fig. 8f is very flat, and the dimples decrease largely, causing mechanical properties to decrease largely (see Table 1). The reasons for this may be due to grain coarsening, because the overheating temperature of alloys is 508°C; 505°C is very near 508°C. As a whole, the findings from the fractographs are in good agreement with the variations in fracture modes and mechanical properties of different samples.
Fracture morphology of tensile samples at different solid solution treatments
Conclusions
The present work is focused on investigating the microstructure and mechanical properties of the optimised Al–0·35Mg–7·0Si cast alloys subjected to solid solution treatment followed by aging treatment. The mechanical properties and fracture mechanism of alloys at different heat treatment processes were analysed. The results can be summarised as follows.
According to the DSC curve of alloy, the overheating temperature of optimised Al–0·35Mg–7·0Si cast alloys is 508°C.
The mechanical properties of alloy with 495°C×5 h and 500°C×4 h are better than that of other experiment conditions. Tensile strength is 239–243 MPa. Hardness is ∼104 HB. Elongation is 4·4–4·8.
The main precipitation phases may have Si, Al2Cu, Al6Cu3Ni and AlxCuCe phases whose morphology may be long strip, round, ellipse, flake and block.
A little cleavage planes and many dimples are apparent on the fracture surface. All the specimens were fractured in a mixed mode fracture character consisting of a ductile and brittle manner.
Footnotes
Acknowledgement
This project is supported by open foundation of the Key Laboratory of New Processing Technology for Nonferrous Metals and Materials (grant no. GXKFJ09-04) and the Scientific Research Foundation of Guang Xi University (grant no. XBZ090765).