Phase Evolution in A356 Alloy

Phase Evolution in A356 Alloy

M.N. Mazlee

School of Materials Engineering, Universiti Malaysia Perlis (UNIMAP)
P.O Box 77, Pejabat Pos Besar, 10007 Kangar, Perlis
mazlee@unimap.edu.my

Introduction

Solution treatments have became prime important in determining the successful for the entire heat treatment of precipitation hardening of aluminium alloys. In process, the alloy is quenched to room temperature just after solution treatment at a rate sufficient to inhibit the formation of precipitates which resulting in non-equilibrium supersaturated solution (SSS) [1]. The purpose of solution treatment is to dissolve maximum practical amount of hardening solutes such as Mg, Cu and Zn into the solid solution in an aluminium matrix [2].
The precipitation hardening in Al-Si-Mg alloys has been extensively studied [3-6]. The precipitation sequence in Al-Si-Mg alloys can be presented as follows [1]:

α (SSS) --- GP zone --- β'' --- β' --- β phase

A356 alloy is one of the most widely applied Al-Si-Mg alloys in the commercial industries due to its good castability and can be strengthened by precipitation hardening [1,3,5-6].
This study has focused on the differential scanning calorimetry (DSC) thermograms to trace the process of formation and dissolution of GP zones, metastable phases and precipitated equilibrium phase.

Experimental Methods

Materials
The materials used in this present work were cast ingot A356 (Al-7%Si-0.3%Mg) alloy. Four different treatments of specimens were studied namely untreated as-received A356 alloy (AR) and three solution treated and as-quenched A356 alloys at 540ºC (ASQ540), 550ºC (ASQ550) and 560ºC (ASQ560) respectively.

Apparatus and Procedures
DSC analysis was carried out using SDT Q600 DSC Instrument. Solution treatments were carried out at heating rate of 10ºC/minute for 7 hours at 540ºC, 550ºC and 560ºC respectively in a normal atmosphere followed by quenching in iced water. A scanning rate of 10ºC/min was used for all temperature ranges except for the all as-quenched between 400ºC to 600ºC which used a scanning rate of 5ºC/min.

Results and Discussion

DSC thermograms in Figure 1 can be described as follows:

1) A ‘low temperature’ section ranging from room temperature to 293ºC corresponds to the formation (A1) and dissolution (A2) of GP zones as shown in Figure 1a. Peak temperature at formation and dissolution of GP zones was observed to be faster when solution treatment temperatures decreased as in Table 1.

2) Figure 1a also illustrates an ‘intermediate temperature’ section between 293ºC and 362ºC which corresponds to the formation and dissolution of metastable phases where 3 exhothermal peaks (B1) and 1 endothermal peak present in solution treated and as-quenched specimens. ASQ550 was found to be the fastest in reaching peak temperature at formation phase as shown in Table 2.

3) A ‘high temperature’ section where an exhothermal peaks C1 and an endothermal peak C2 (Figures 1b and 1bi) corresponding to the formation and dissolution of equilibrium phases as shown in Table 3. In general, the peak temperatures at both formation and dissolution of equilibrium phases were almost similar with a small variation.

In general, a faster formation of GP zone and metastable phases were observed in as-quenched Al-10%Si-0.4Mg which had undergone solution treatment at 525ºC for 12 hours [3] comparatively. This faster trend is may be due to higher both Si and Mg contents which contribute to the formation of Mg2Si precipitates towards the acceleration of age hardening process and a longer solutionising time where the solute atoms dissolved more to form a single phase solid solution.

Conclusions

1) ASQ550 has achieved peak temperature in a shorter time compared to other specimen believed due to the enhanced formation of metastable phase.
2) A faster formation of metastable phases in ASQ550 has contributed to the faster formation of equilibrium phases comparatively.
3) A faster formation of GP zone and metastable phases in A356 alloys is believed can be achieved by prolong the solution treatment time.

References

1. W.D. Callister, Materials Science and Engineering; An Introduction, (1994), Wiley, 783.
2. Hatch J.E., Aluminium: Structure and Physical Metallurgy, Metal Park, OH: ASM International, (1998), 150-160.
3. R.X. Li , R.D. Li, Y.H. Zhao, L.Z. He, C.X. Li, H.R. Guan & Z.Q. Hu, Age-Hardening Behavior of Cast Al-Si Base Alloy, Mater. Lett., 58, (2004), 2096-2101.
4. C.M. Estey, S.L. Cockcroft, D.M. Maijer & C. Hermesmann, Constitutive Behavior of A356 During The Quenching Operation, Mater. Sci. Eng., A 383, (2004), 245-251.
5. Y.J. Li, S. Brusethaug & A. Olsen, Influence of Cu on the Mechanical Properties and Precipitation Behavior of AlSi7Mg).5 Alloy During Aging Treatment, Scripta Mater., 54, (2006), 99-103.
6. Choong Do Lee, Damping Properties on Age Hardening of Al-7Si-0.3Mg Alloy During T6 Treatment, Mater. Sci. Eng., A 394, (2005), 112-116.