Effect of united refining and modification on mechanical properties of A356 aluminium alloys

ZUO Xiu-rong(左秀荣), ZHONG Zhi-guo(仲志国)

Key Laboratory of Material Physics, Ministry of Education, School of Physics Science and Engineering,

Zhengzhou University, Zhengzhou 450052, China

Received 28 July 2006; accepted 15 September 2006

Abstract: The refining and modification effect of Ti (from GRAI), B, Sr and RE (cerium-riched mixtures of rare earth) on the mechanical properties of A356 aluminum alloys under T5 and T6 treatment condition were studied by OM, SEM, EDAX, etc. It is found that the addition of RE to A356 alloys containing Ti and/or B and Sr makes strength and elongation increase in T6 treatment, but make elongation decrease in T5 treatment, at the same time, the long axis of α(Al) grain structure decreases and the mean diameters of silicon particles increase with RE additions increasing. Grain refining with 0.01%Ti plus 0.03% B makes the dendrite α(Al) grain structure transform into equiaxed structure, resulting in obvious increase of elongation percentage. The mean diameters of silicon particles in T5 treatment are smaller than that in T6 treatment. Roundness of silicon particles in T5 treatment is higher than that in T6 treatment. A356 alloys modified and refined with Ti, B and Sr obtain the best mechanical properties in T5 treatment, however, the alloys with Ti, B, RE and Sr additions obtain the best mechanical properties in T6 treatment.

Key words: A356 aluminium alloys; grain-refining aluminium alloys (GRAI); modification; refining; mechanical properties

1 Introduction

A356 aluminium alloys are widely used to fabricate structural castings for automotive and other applications. A356 alloys usually adopt Al-Ti master alloys refining α(Al) in the range 0.08%-0.20%Ti, resulting in bulk use of Al-Ti master alloys and higher production cost. In the resent years, a number of attempts have been tried to find new grain refiner other than Al-Ti master alloys[1]. KORI et al [2] suggested that Al-B and Al-1Ti-3B alloys were more efficient grain refiners than Al-Ti in Al-7Si alloys, when adding 0.03%B, fine equiaxed structure were obtained. The modification of Al-Si eutectic from a flake-like to a fine fibrous silicon structure can be achieved by addition of Sr[3-5]. The modification effect of Sr is good, but fluidity decreases and inspiration increases because of Sr addition, resulting in mechanical properties decreasing. There have also been reports that the addition of RE in A356 alloy has the function of modification, refining and degassing[6-7], and the modification effect last longer, because the radius of RE atom is larger enough to produce modification effect [8], and RE can react with H and O in A356 melt.

At present, in order to improve the mechanical properties and decrease the production cost, looking for adequate modification and refining methods of A356 alloys is urgent question. Grain-refining aluminium ingots (GRAI) containing 0.01%-0.20%Ti are produced by electrolysis, through adding TiO₂ into aluminium electrolyzing cell, under the same production technology condition as pure Al. It remains desirable to GRAI, which can provide the benefits of refining combined with low production cost, simple operation and persistence of the refined structure on repeated melting and long-time melting practice[9-11].

Production costs of A356 aluminium alloys produced with GRAI are low because of Ti addition by electrolytic method. The properties of A356 alloys produced with GRAI were also studied [12-13]. But few investigations about A356 aluminium alloys produced with GRAI, at the same time the addition with RE, B and Sr, was conducted. So the aim of the present work is to evaluate the influence of united refining and modification with Ti (from GRAI), B, Sr and RE on the mechanical properties of A356 aluminium alloys.

2 Experimental

The commercial A356 alloys were prepared using pure aluminium(Al 99.70, A) and GRAI(XAl 99.70, A-4), modified with Sr or RE, or both, and inoculated with Ti or B, or both. The commercial A356 alloys with a nominal composition of 7%Si, 0.35%Mg, 0.12%Fe were melted in a 7 kW resistance furnace. Sr was added by a commercial Al-10%Sr (mass fraction) master alloy. RE was added by a commercial Al-10%RE master alloy. B was added by a commercial Al-4.43%B master alloy. The melt was degassed with high purity argon gas for 12 min through a graphite rod immersed into the melt to ensure low hydrogen content. The surface of the melt was skimmed periodically. A foundry ingot was cast into tensile test die at 715 ℃. The samples in T5 treatment condition were placed in a furnace at 535 ℃ for 3 h and were then quenched in water at 60 ℃, naturally aged for 24 h and artificially aged for 3 h at 165 ℃. The samples in T6 treatment condition were placed in a furnace at 535 ℃ for 8 h and were then quenched in water at 60 ℃, natural aged for 24 h and artificially aged for 6 h at 160 ℃. Tensile samples were machined, with parallel gauge length of 55.0 mm and diameter of 10.0 mm. Tensile testing was done using a computer-controlled 100 kN closed-loop servo-hydraulic MTS 810 testing machine, at a cross head speed of 1.5 mm/s, with an extensometer attached. Microstructure was observed using an optical microscope (Olympus BX51). The quality index Q was used to represent the comprehensive mechanical properties of alloys. The microstructural examination was carried out in a JEOL, JSM-6700 SEM with energy dispersive X-ray (EDX) spectrometer.

3 Results and discussion

The mechanical properties of alloys in T5 and T6 treatment conditions are shown in Fig.1.

1) The RE additions in alloys containing Ti and/or B and Sr with T6 treatment make an average elongation fracture value 42.5% higher than that without RE addition. But the RE additions in alloys containing Ti and/or B and Sr with T5 treatment make elongation 28.0% lower than that without the RE addition. The RE additions in alloys with T5 and T6 treatment don’t make strength decrease.

2) Grain refining with 0.01%Ti plus 0.03%B makes an average elongation fracture value increase obviously.

3) Solid treatment from 3 h (T5) to 8h (T6) make σ_0.2 increase between 3.9% and 16.7%. But the average elongation fracture value decreases.

4) The alloys refined and modified with the Ti, B and Sr in T5 treatment conditions obtain the best elongation (up to 13.3%). The alloys refined and modified with Ti, B, Sr and RE in T6 treatment conditions obtain the best elongation (up to 9.6%).

5) The alloys with the Ti, B and Sr additions in T5 treatment conditions and alloys with the Ti, B, Sr and RE additions in T6 treatment conditions have almost the same quality index.

Fig.1 Mechanical properties of alloys in T5 and T6 treatment conditions: (a) T5 treatment condition；(b) T6 treatment condition

Fig.2 shows the SEM fractural images of tensile specimens. Fig.3 shows the microstructures of eutectic Si. Table 1 lists the parameters of α(Al). Table 2 shows the parameters of silicon particles. The SEM fractural images of alloys refined and modified with 0.1%Ti plus 0.45%RE consist of quasi-cleavage and dimples, showing brittle and ductile mixing fracture feature. It is obvious that the mean diameters of Si particles in alloys modified and refined with Ti and RE are larger than those in other samples, and the roundness of Si particles is smaller than that in other samples, showing the partial modification microstructure(Figs.3(c), (h), Table 2). The fracture surfaces consist of dimples in other alloys, showing plastic deformation and tear ridge patterns. It is evident from Tables 1 and 2 that the RE addition to alloys 1 and 4 makes the long axis of α(Al) grain structure decrease and the mean diameters of Si particles increase in T6 and T5 treatment condition. With the RE addition to alloys 1 and 4, eutectic reactions L→ α(Al)+Al₄La and L→α(Al)+Al₄Ce occur at 642 and  637 ℃, respectively. When the temperature reaches the liquidus of A356 alloys, the solid grows from α(Al) obtained from the eutectic reaction, resulting in the decrease of long axis of α(Al) grain. The addition of RE to alloys makes the diffusion coefficient of Si increase. The Si particles grow larger during the same solution time.

Fig.2 SEM fractural images of tensile specimens in A356 alloys: (a) T5 treatment, Ti+RE; (b) T6 treatment, Ti+RE;  (c) T5 treatment, Sr+Ti; (d) T6 treatment, Sr+Ti

Fig.3 Microstructures of eutectic Si in A356 alloys: (a) T5 treatment, Sr+Ti; (b) T5 treatment, Sr+Ti+RE; (c) T5 treatment, Ti+RE; (d) T5 treatment, Sr+Ti+B; (e) T5 treatment, Sr+Ti+B+RE; (f) T6 treatment, Sr+Ti; (g) T6 treatment, Sr+Ti+RE; (h) T6 treatment, Ti+RE; (i) T6 treatment, Sr+Ti+B; (j) T6 treatment, Sr+Ti+B+RE

In A356 alloys with RE addition, the reactions ([La]+2[H]=LaH_2, [Ce]+2[H]=CeH₂, 2[La]+Al₂O₃(s)= 2Al(l)+La₂O₃(s), 3[Ce]+2Al₂O₃(s)?=4Al(l)+3CeO₂(s)) result in the decreasing of [H] and Al₂O₃ in melt[7]. So the alloys have little inclusions and porosities. These are of advantage to plasticity.

Fig.4 shows the SEM images of alloys. Table 3 shows the results of EDAX analysis. The alloys with the RE addition contain white particles. The results of EDAX analysis indicate that they contain higher RE and Si content, less Fe and Mg. The gray particles without RE or with less RE contain higher Fe and Si. The RE additions don’t make Mg aggregate and form intermetallic constituents containing Mg, resulting in Mg₂Si content decreasing in alloys. This is because RE and Si interact strongly, forming Al-Si-RE intermetallic constituents. However, RE and Mg interact weakly. So the addition of RE doesn’t make Mg₂Si content decrease and thus result in strength of alloys decreasing. White particles containing RE and Si and gray particles containing Fe and Si usually grow together(Fig.4(b)). Solution treatment time in T5 is shorter than that in T6, making particles containing RE and Si under-nodularizing. The interface between white particles containing RE and Si and gray particles containing Fe and Si generates stress concentration, becoming crack origin and resulting in plasticity decreasing.

Table 1 Parameters of α(Al) in A356 alloys

Table 2 Parameters of silicon particles in A356 alloys

Fig.4 SEM images of alloys: (a) T5 treatment, 0.04%Sr+0.1%Ti+0.3%RE; (b) T5 treatment, 0.1%Ti+0.45%RE; (c) T6 treatment, 0.04%Sr+0.1%Ti+0.3%RE

Table 3 Results of EDAX analysis of precipitated phases(mass fraction, %)

Grain refining with 0.01%Ti from GRAI plus 0.03% B makes the dendrite α(Al) grain structure transform into equiaxed α(Al) grain structure(Table 1). Because B reacts with Ti to form TiB₂, the excess B in melt undergoes eutectic reaction at 659.7 ℃ (L→α(Al)+AlB₂). When the temperature reaches the liquidus of A356 alloys, the solid grows from α(Al) obtained from eutectic reaction[14]. AlB₂ preferentially grows on TiB₂, TiB₂ covered with AlB₂ furthers grain refining. So grain refining with 0.01%Ti plus 0.03%B makes the elongation values increase obviously.

The mean diameters of silicon particles in T5 treatment condition are smaller than that in T6 treatment condition. The roundness of silicon particles in T5 treatment condition is higher than that in T6 treatment condition(Fig.3 and Table 2). These are of advantage to plasticity. The disintegration and spheroidization of well-modified eutectic silicon is finished within minutes of exposure to temperatures above 500 ℃[15]. A longer exposure time makes the silicon particles grow up, resulting in fracture elongation decreasing. The small difference between 3.9% and 16.7% of T5 and T6 strength indicates that the diffusion of Si and Mg in the α(Al) matrix is almost finished at 535 ℃ for 3 h.

Modification and refining with the Ti, B, Sr and RE in A356 alloys with T6 treatment make the α(Al) grain structure equiaxed, the silicon particles fine and matrix purifying, which result in the best mechanical properties (σ_b=307.6 MPa, σ_0.2=238.2 MPa, δ=9.6%). Modifying and refining with Ti, B and Sr in A356 alloys with T5 treatment make the α(Al) grain structure equiaxed, the silicon particles fine, which result in the best mechanical properties (σ_b=285.2 MPa, σ_0.2=208.5 MPa, δ=13.3%). In general, adding RE to alloys with T5 treatment makes the α(Al) matrix purifying, but shorter solidification time makes spheroidization of RE- containing particles partly, resulting in stress concentration, which makes plasticity decrease.

4 Conclusions

1) The addition of RE to A356 alloys containing Ti and/or B and Sr makes strength and elongation increase in A356 alloys in T6 treatment condition, but makes elongation decrease in alloys in T5 treatment condition, at the same time, the long axis of α(Al) grain structure decreases and the mean diameters of silicon increase particles in T6 and T5 treatment condition.

2) Grain refining with 0.01%Ti from GRAI plus 0.03% B makes the dendrite α(Al) grain structure transform into equiaxed α(Al) grain structure, resulting in obvious increasing of elongation of A356 alloys in T5 and T6 treatment condition.

3) Prolonging the solution time from 3 to 5 h at 535 ℃ the strength increases between 3.9% and 16.7%. The mean diameters of silicon particles in T5 treatment condition are smaller than that in T6 treatment condition. The roundness of silicon particles in T5 treatment condition is higher than that in T6 treatment condition. These are of advantage to plasticity.

4) Modifying and refining with the Ti, B and Sr in A356 aluminum alloys with T5 treatment make the α(Al) grain structure equiaxed, the silicon particles fine, which result in the best mechanical properties (σ_b=285.2 MPa, σ_0.2=208.5 MPa, δ=13.3%). In general, adding RE to alloys with T5 treatment makes α(Al) matrix purifying, but the shorter solidification time make spheroidization of RE-containing particles partly, resulting in stress concentration, which makes the plasticity decrease.

5) Modifying and refining with the Ti, B, Sr and RE in A356 aluminum alloys with T6 treatment make the α(Al) grain structure equiaxed, the silicon particles fine and α(Al) matrix purifying, which results in the best mechanical properties (σ_b=307.6 MPa, σ_0.2=238.2 MPa, δ=9.6%).

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(Edited by LONG Huai-zhong)

Corresponding author: ZUO Xiu-rong; Tel: +86-371-67767776; E-mail: zuoxiurong@sohu.com