Study on Microstructure Distribution of Al-Cu-Mg Alloy in Squeeze Casting Process

The squeeze casting process is an advanced technology, which is suitable for a wide range of alloys including Al-Cu-Mg alloys. The high pressure in squeeze casting process both refines the microstructure and enhances the mechanical property of the alloys. The heat transfer and solidification process are complex with the chilling mould and pressure of machine punch. This results in the inhomogeneous microstructure and property in casting alloy. This study focuses on the distribution of microstructure, chemical composition, and hardness of Al-Cu-Mg alloy along the radial direction of casting. The composition and area fraction of eutectic phases are characterized. Moreover, the relationship between the microstructure and mechanical property was also analysed to show the characteristic of microstructure in squeeze casting process.


Introduction
Squeeze casting is an advanced forming technology in which the melt solidifies under high mechanical pressure [1][2]. During the process of squeeze casting, the melt contacts closely with the steel mould under pressure, which increases the heat transfer coefficient and cooling rate of the melt and thus refine the microstructure. The castings have high dimension accuracy, low surface roughness, low defects in microstructure and excellent mechanical properties [3][4][5].
Al-Cu-Mg alloy is a typical casting material with high strength, toughness and good corrosion resistance, so it is widely applied in aerospace field. However, many studies [6][7][8] show that the Al-Cu-Mg alloys have high sensitivity of hot tearing because the severe shrinkage of the alloy occurs during solidification, which impairs the castability of this alloy. The high mechanical pressure during squeeze casting process can effectively feed the shrinkage porosity and decrease the number of the defects in castings. Therefore, the squeeze casting process is applied to investigate the microstructure characteristic in Al-Cu-Mg alloys. The size of α-Al grain and eutectic phase are analysed along radial

Experimental methods
The nominal chemical composition of Al-4.5Cu-0.6Mg-0.4Mn-0.1Ti-0.15Zr was weighted using the pure Al, pure Mg, Al-60Cu, Al-5Mn, Al5TiB and Al5Zr master alloys. After loading the weighted ingots into the crucible, the resistance furnace was heated up to 730 ℃. The melt was then degassed by the rotating graphited rod blowing argon. The degassing time was preferred to 30 minutes. The chemical composition of the studied alloy was measured by spectrometer (SPECTROMAXx-LMX08, Germany) and the results were listed in Table 1. The pour temperature of the melt was 730 ℃, and the pre-heated temperature of the mould was 200 ℃. The squeeze casting process, which is shown in Fig.1, includes six procedures-cylinder tilt to pour melt, then back to straighten, cylinder rise, low-speed filling and high-pressure solidification, which is the most important stage to obtain castings with good quality, final open the mould to pick out castings. The schematic diagram of the casting was shown in Fig.2, and the cross section of A-A was chosen to observe the microstructure of Al-Cu-Mg alloy. Five equally spaced points (P1-P5) along the radial direction on A-A cross section were picked to display the microstructural distribution of Al-Cu-Mg alloys in the squeeze casting process.

Microstructure characterization
The SEM microstructure at five points was shown in Fig.3. It is seen that the microstructure of Al-Cu-Mg alloy consists of α-Al phases and eutectic phases. Due to the high content of Cu element, eutectic phases are shown in bright white colour. From the measurement results as shown in Fig. 4a, the size of α-Al grains is significantly decreased from P1 to P5, while the area fraction of eutectic phases is obviously increased. At P1, the average size of α-Al grain is 18.8 μm and the area fraction of eutectic phases is 2.2%. At P5, the average size of α-Al grain is 9.4 μm and the area fraction of eutectic phases is 8.8%. The refined α-Al grains and high volume fraction of eutectics result in high hardness of microstructure. The hardness increases from 56.7 HV at P1 to 93.0 HV at P5. Moreover, Fig. 4b is the line scan results showing the contents of Al/Cu/Mg/Mn elements along the radial direction. It can be found that the Cu content has an obvious increase from centre to edge position. The increase in Cu content results to the enhancement in hardness of Al-Cu-Mg alloys.

Microstructure-Hardness relationship
In pressure die casing process, the microstructure at the cross section of casting consists of three areas: refined grains at surface, eutectics band and coarse grains in centre [10]. The process of solidification is analysed as follow. Due to the chilling effect of the mould, the melt closed to mould has high cooling rate and then forms the solid shell. This "solid shell" impairs the chilling effect of mould, which results to the decrease of cooling rate in alloys. Thus, the temperature gradient of the melt at the cross section of casting is relatively uniform. As the solidification continues, the solidified particles (α-Al grains) with high volume fraction and liquid zone move forward under the push of the machine punch. The liquid zone has an obvious segregation and flows along the "solid shell", while the solid particles are located in the centre of castings. At the end of solidification, the eutectics are completely precipitated in liquid zone and the coarse α-Al grains form in microstructure. Finally, three areas included refined grains at surface, eutectic band and coarse grains in centre. Typical microstructure on the cross section of casting is displayed in Fig. 6, and schematic diagram showing the microstructure characteristic is also summarized in Fig. 6.

Conclusion
The microstructure and hardness along the radial direction of castings are characterized in Al-Cu-Mg alloys in squeeze casting process. The conclusions are summarized as follows: (1) From center to edge on the cross section of the casting, the size of α-Al grains in Al-Cu-Mg alloys has an obvious decrease from 18.8 μm to 9.4 μm, and the average area fraction of eutectic phase increases from 2.2% to 8.8%. (2) The refined α-Al grain structure and higher volume fraction of eutectic phase result in the increase of hardness in Al-Cu-Mg alloys. The hardness at edge position is 93.0 HV, which is nearly 64% higher than that at center position.
(3) Three regions including refined grains at surface, eutectic bands and coarse grains in center exist in microstructure. Moreover, the increase in Cu content and decrease in Al content are also observed along the radial direction. It is presumably that the eutectic phase (Al2Cu and Al2CuMg) provides hardness and strength of Al-Cu-Mg alloy in great extent.