Elsevier

Materials & Design

Volume 80, 5 September 2015, Pages 99-108
Materials & Design

Role of modification and melt thermal treatment processes on the microstructure and tensile properties of Al–Si alloys

https://doi.org/10.1016/j.matdes.2015.05.013Get rights and content

Highlights

  • High tensile strength applying the melt thermal treatment process.

  • Enhanced ductility by changing the Si particle morphology.

  • Control of the dissolution and precipitation of Mg2Si phase.

  • Establishment of the fracture mechanisms of Al–Si–Mg alloys.

Abstract

The present study was performed on an Al–7%Si–0.35%Mg alloy (A356 alloy) with the primary objective of improving the alloy performance through modification of the microstructure. Ultimate tensile strength (UTS) can be improved by the addition of strontium (Sr), superheating or Sr modified melt thermal treatment. The melt thermal treatment process alone has no apparent influence on the UTS. Both Sr-modified and Sr-modified melt thermal treatment can help to improve the percentage elongation of A356 alloy castings. A higher percentage elongation can be reached at a higher cooling rate. The effect of solution heat treatment on the tensile properties of various A356.2 alloy castings can be summed up as follows: (i) the yield strength of the A356.2 castings is significantly improved after 8 h solution heat treatment due to the precipitation of Mg2Si, (ii) the yield strength remains more or less the same with further increase in solution treatment time to 80 h, and (iii) the UTS is greatly improved within the first 8 h of solution heat treatment and continues up to 80 h, where this improvement is attributed to Mg2Si precipitation, dissolution of silicon within the Al-matrix and change in the Si particle morphology (spheroidization). The ductility of the A356.2 alloys can also be considerably enhanced with solution heat treatment (e.g. from ∼6% in the non-modified casting in the as-cast condition to ∼10% after 80 h solution treatment).

Introduction

As one of the main groups of aluminum alloys used in automotive applications, the mechanical properties of Al–Si–Mg alloys have been extensively investigated [1], [2], [3]. Among these works, those relating to the tensile properties of A356 alloys have been determined in terms of the secondary dendrite arm spacing (SDAS) or cooling rate, eutectic Si particle characteristics, alloying element addition (Mg, Fe, etc.), and casting defects such as porosity and inclusions [4], [1], [5].

The effects of modification and solution heat treatment, through their influence on the eutectic Si particle characteristics, can also control the tensile as well as the impact properties of the Al–Si based alloys [6], [7], [8], [9], [10]. The main results inferred from these references show that the addition of Ti and B greatly improves the alloy toughness, but only when the alloy is in a fully modified state. The improvements in toughness may be attributed to the change in Si particle morphology as well as to the dissolution and fragmentation of a number of the intermetallics formed during the T6 temper. The effect of strontium is more pronounced in the alloys cooled at a high cooling rate (dendrite arm spacing about 20 μm). However, strontium has a noticeable effect on the reduction of hardness in aged Mg-containing Al–Si–Cu alloys, in that it affects the precipitates containing Cu and Mg. In the Sr-modified alloys, solution treatment has varied effects depending on the cooling rate. The results also indicate that the morphology of fibrous Si in Sr-modified alloys enhances toughness because of its profound effect on crack initiation and crack propagation resistance. The combined addition of modifier and grain refiner leads to a 33% increase in the impact strength compared to the untreated alloy. The crack initiation energy in Al–Si alloys is greater than the crack propagation energy, reflecting the high ductility of Al–Si based alloys.

According to Hafiz and Kobayashi [11], Sr modification has little effect on the yield strength of the A356 alloy, but can moderately improve the tensile strength. The main impact of Sr modification is on the ductility, where additions of 240 ppm Sr to the A356 alloy melt can increase the elongation from 8.03% to 22.2% [12]. In comparison, there are few studies covering the effect of melt superheat and even less on that of melt thermal treatment (MTT) on the tensile properties of A356 alloys. The results of Jie et al. [12] show that superheating an A356 alloy melt to 810 °C can improve the tensile properties, especially the ductility (from 5.0% to 8.5%). The work of Wang et al. [13] shows that the MTT process can greatly improve the elongation, by almost 112%.

The Sr modified MTT (SrMMT) casting showed the best results in that the eutectic Si particle characteristics were the finest produced among all the casting types studied. It is to be expected, therefore, that the SrMTT casting samples will exhibit the best tensile properties, as well [14], [15]. The effect of solution heat treatment on the Si particle characteristics (and hence on the tensile properties) of A356 alloy has also been investigated [16], [17]. In the present work, the tensile test bars obtained from the various castings were solution treated at 540 °C for times ranging from 0 h (as-cast condition) to 80 h at intervals of 2 h for the non-modified (NM) and Sr-modified (SrM) alloy castings. Based on the results obtained, for the other castings, the test bars were solution treated at 540 °C, again for times of 0 h (as-cast condition), 8, 40 and 80 h, but at intervals of 8 h. Following solution treatment, all test bars were quenched in warm water and aged for 5 h at 155 °C (standard T6 treatment) before the tensile testing was carried out [18], [19].

The quality index concept was introduced by Drouzy et al. [20] as a means of better interpreting tensile test data. Rather than using ductility directly, they defined the quality index, Q, in terms of the UTS and percent elongation (El > 1%) and a coefficient, k, having a value of 150 MPa for a Al–7Si G06 alloy. This alloy is equivalent in composition to A357 alloy without beryllium. In view of the tensile properties of the different A356.2 alloy castings studied in the present work, it would be interesting to see how the different factors, viz., cooling rate, modification process, superheat and solution heat treatment would affect the ‘quality’ of the alloy or casting, in terms of the quality index. Accordingly, the Q values for the various samples were calculated based on the equation:Quality index=UTS(MPa)+150log(El%)with Q expressed in MPa units.

The current work presents the results of the tensile tests that were carried out for the non-modified, Sr-modified, superheated, melt thermal treated and Sr-modified thermal treated end-chilled castings. Tensile samples were obtained at three levels (of 10, 50 and 100 mm above the chill end) to incorporate the effect of cooling rate. In regards to the different parameters studied and their influence on the tensile properties, a high cooling rate (or fine DAS) is well known to enhance the tensile properties, due to the overall refinement of the microstructure and its constituents, including the Si particles.

Section snippets

Experimental procedure

Table 1 lists the chemical composition of the as-received alloy ingots. In a previous article [14], the present authors described the process of preparation of the alloys used in the present study. Table 2 summarizes the details corresponding to all the castings that were prepared. The chemical compositions of the corresponding melts are given in Table 3, obtained from spectroscopic analysis.

Castings were prepared using a rectangular end-chilled mold (64 × 127 × 254 mm). The four walls of the mold

Microstructural characterization

As can be seen from Fig. 3, the non-modified NM casting displays the typical acicular Si particles. Some amount of refinement due to cooling rate is observed in Fig. 3(a) compared to Fig. 3(b) and (c). In effect, the dendrite arm spacings of levels 2 and 3 are not that far apart (62 and 78 μm, respectively) and hence their microstructures would be more similar than different, particularly when compared to that of level 1, with a dendrite arm spacing almost half that of the others. With the

Conclusions

Based on the results obtained in the present study, the following conclusions could be drawn.

  • (1)

    Cooling rate and modification level seem to have no significant influence on the alloy yield strength in the as cast condition regardless the type of melt treatment used.

  • (2)

    The tensile strength can be improved by SrM, SH, and SrMTT treatments.

  • (3)

    The MTT process has no apparent influence on the UTS. Both SrM and SrMTT treatments can greatly improve the percentage elongation of A356 alloy castings. The SH and

Acknowledgement

The authors would like to thank Ms. Amal Samuel for enhancing the art work in the present article.

References (20)

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