Effects of Process Parameters on Microstructure of A2017 Alloy Strip Produced by a Novel Semisolid Rolling

Abstract

In order to produce A2017 alloy strip, a novel semisolid rolling process was studied. The effects of process parameters on microstructure were investigated. The experimental results show that the grain size and roundness of A2017 alloy strip increase with the increment of casting temperature. The grain size of strip decreases firstly and then increases with the increment of vibration frequency. When the sloping plate length was between 500 mm and 600 mm, the casting temperature was between 650 ^∘C and 680 ^∘C, the roll speed was 0.087 $\text{m}·{\text{s}}^{-1}$ , and the vibration frequency was 50 Hz, A2017 alloy strip with a cross-sectional size of 4 × 160 mm was produced. The microstructure of strip is mainly composed of fine spherical or rosette grains with a certain elongation along rolling direction, and the average grain size is 36.4 μm.

1. Introduction

Semisolid metal forming is one of the advanced metal forming techniques, which has advantages of short forming process, easy deformation, and good microstructure and properties of product, so it has become a hot research topic in the recent years [1]. Flemings proposed firstly semisolid metal processing (SSP) [2, 3]. As an advanced forming technology, roll casting was developed quickly [4]. During semisolid rolling process, how to prepare good-quality semisolid slurry with low cost and high efficiency becomes an important subject for developing rheo-rolling process [5]. Up to now, many semisolid processing techniques have been developed; sloping plate process has the advantages of high efficiency and low cost and attracts much attention all over the world. The melt was poured onto the surface of the vibrating sloping plate, and the melt was cooled and stirred by sloping plate and melt flow. Therefore, semisolid slurry with fine spherical primary grains and remnant liquids can be prepared. Several researches about the sloping plate process have been reported. Motegi has successfully manufactured semisolid billet of Al-Si-Mg alloy [6]. Kapranos et al. reported the thixoforming of an automotive component using an A390 alloy [7]. Kang et al. studied the deformation mechanism of steel strip fabricated by semisolid rolling [8]. Cai et al. obtained the fine microstructure of alloy ZL101 by the method of cooling slope tube [9]. Dai et al. studied the effects of the length and angle of inclined cooling plate on the stability of preparing A356 alloy slurry [10]. Kang and Lee studied the rheology forming process with a vertical-type sleeve with electromagnetic stirring [11]. Haga et al. developed the rheo-rolling process of aluminum alloys [12]. However, the problems of adhesion of the slurry on the plate surface and the liquid segregation during rheo-rolling process need to be resolved. In this paper, a novel rolling process with a vibrating sloping plate device was developed by combining the vibrating sloping plate and the shape rolling mill. In this process, the vibration has two main advantages: the first one is that the trouble of slurry adhesion on the plate surface was solved, and the other is that the microstructure of semisolid slurry was improved. The effects of process parameters on microstructure of A2017 alloy strip were investigated.

2. Experimental

The experimental material was A2017 alloy. The chemical compositions of A2017 aluminum alloy are Cu 4, Mn 0.6, Mg 0.6, and Al balanced (mass %). The experimental equipment is the self-designed machine of semisolid rolling. The rolls were cooled by circulating water. The diameter of the rolls is 400 mm, the roll gap is 4 mm, and the maximum rolling speed is 22 m·min⁻¹. Figure 1 is a schematic illustration of the experimental apparatus.

Figure 1:

Schematic diagram of continuous semisolid rolling process.

When the melt temperature approximately reached 700°C, the melt was homogenized by mechanical stirring and refined. After complete mixing, the melt was held at 700°C for 30 min and then carried to the testing machine, and then the melt was cast on the vibrating sloping plate under protection by argon. Under the present conditions, the casting temperature varied from 640°C to 700°C, the vibration frequency varied from 40 Hz to 100 Hz, and the roll speed varied from 0.040 m·s⁻¹ to 0.100 m·s⁻¹; the casting speed from tundish is consistent with the roll speed, the sloping plate length was between 300 mm and 600 mm, and the width is 160 mm. Once the melt was cast onto the vibrating sloping plate surface, the melt nucleated rapidly under strong cooling provided by the cooling plate. The primary grains grew into fine spherical structures under stirrings of vibration and melt flow, so semisolid alloy with fine nondendrites and remnant liquids was prepared. Subsequently, the slurry was filled into the roll gap. The two broadsides of roll gap were constrained by the convex and concave rolls, so the semisolid slurry was rolled directly by the shape rolling mill. The rolling speed of semisolid alloy could be much higher than that of conventional roll casting. In order to study the process parameter influence on microstructure of A2017 alloy strip, different specimens were taken from the strips produced under different process parameters. The specimens were etched by the solution of 2 mL HF + 3 mL HCl + 5 mL HNO₃ + 150 mL H₂O; microstructures were observed under an OLYMPUS PMG51 metallographic microscope. The average grain size was calculated by

$d=\frac{L_T}{N\times n},$

(1)

where d is the average grain size, L _T is the length of measure line, N is the grain number that is covered by the measure line, and n is magnification value. The average roundness of grain shape was calculated by

$S_F=\frac{L_p^{\mathrm{2}}}{\mathrm{4}\pi A},$

(2)

where S _F is the average grain roundness, L _p is total circumference of the measured grains, and A is total grain areas. The average grain size and the average roundness of grain shape were measured by OLYCIA m3 image analysis software.

3. Results and Discussion

3.1. Effect of Sloping Plate Length on Microstructure of A2017 Alloy Strip

Figure 2 shows the microstructures of A2017 alloy strips produced under different sloping plate lengths. It is found that the microstructures of strips were mainly composed of fine spherical or rosette grains. As shown in Figure 3, the grain size and roundness of A2017 alloy strip decrease with the increment of sloping plate length. It is commonly believed that heterogeneous nucleus forms on the plate surface and dendrite breakage causes the microstructure refinement during the sloping plate process [13, 14]. Actually, recent studies revealed that the thickness of velocity boundary layer is much smaller than that of temperature boundary layer during sloping plate process [15]. Therefore, temperature distribution in the melt is homogenous. Simultaneously, the cooling rate of melt on water-cooling sloping plate can reach 100–1000 k/s and is much larger than that of conventional casting which is usually less than 100 k/s. Under this situation, eruptive nucleation can happen in the whole melt [16]. Heterogeneous nucleation and eruptive nucleation caused formations of fine spherical grains. The nucleation rate of heterogeneous nucleation and eruptive nucleation are all affected by the cooling and stirring time of melt on the cooling sloping plate. The cooling and stirring time increases with the increment of sloping plate length, so the nucleation rate increases with the increment of sloping plate length correspondingly. Therefore, the strip grain size decreases with the increment of sloping plate length. However, the plate length is limited by the alloy flow ability. Slurry adhesion usually appeared when the plate length was longer than 600 mm. The sloping plate length between 500 mm and 600 mm for A2017 alloy is suggested.

Figure 2:

The center microstructures of A2017 alloy strips on the cross-section under different sloping plate lengths (casting temperature is 670°C, roll speed is 0.087 m·s⁻¹, and vibration frequency is 50 Hz): (a) 300 mm; (b) 600 mm.

Figure 3:

Effects of sloping plate length on average grain diameter and roundness of A2017 Al alloy (casting temperature is 670°C, roll speed is 0.087 m·s⁻¹, and vibration frequency is 50 Hz).

3.2. Effect of Casting Temperature on Microstructure of A2017 Alloy Strip

Figure 4 shows the microstructures of A2017 alloy strips on cross-section under different casting temperatures. Figure 5 shows the relationships of casting temperature and grain size and roundness. It is found that the grain size and roundness of A2017 alloy strip increase with the increment of casting temperature. Heterogeneous nucleation and eruptive nucleation determine the nucleation rate and affect microstructure of the strip finally. Heterogeneous nucleation and eruptive nucleation are tightly related to casting temperature. When the temperature was high, the solid fraction of the slurry was low and there was much remnant liquid in the slurry. In this case, secondary crystallization took place in roll gap, and the nucleation rate was low, so primary grains ripened and coarsened, and dendrites remained. When the casting temperature reached 680°C, little primary nucleus formed on the plate because of high temperature; simultaneously, little nucleus formed in the roll gap, so large dendrites formed in the strip. However, when the casting temperature was lower than 650°C, the flow ability of melt was not good because of high solid fraction, and the process usually failed. So in order to produce A2017 alloy strip with good microstructures, the casting temperature range from 650°C to 680°C is suggested.

Figure 4:

The center microstructures of A2017 alloy strips on the cross-section under different casting temperatures (sloping plate length is 500 mm, roll speed is 0.087 m·s⁻¹, and vibration frequency is 50 Hz): (a) 650°C; (b) 660°C; (c) 670°C; (d) 680°C.

Figure 5:

The relationships of casting temperature and grain size and roundness (sloping plate length is 500 mm, roll speed is 0.087 m·s⁻¹, and vibration frequency is 50 Hz).

3.3. Effect of Roll Speed on Microstructure of A2017 Alloy Strip

Figure 6 shows the microstructures of A2017 alloy strips on cross-section under different roll speeds. Table 1 shows grain size comparison of strips produced under different roll speeds. When the roll speed was 0.087 m·s⁻¹, the filling speed of semisolid slurry was also high, so the liquid fraction of slurry in roll gap was improved. Simultaneously, the cooling ability of water-cooling rolls was very strong. Particularly, the cooling rate of the slurry under high roll speed was improved. Under this situation, secondary crystallization took place in the roll gap rapidly, so the strip size became smaller. On the contrary, when the roll speed was 0.052 m·s⁻¹, the secondary crystallization was not obvious because of high solid fraction of slurry in the roll gap, and small deformation of solid particles took place. Therefore, grain size of A2017 alloy strip was relatively big under low roll speed, and small deformation occurred in microstructure. The roll speed of 0.087 m·s⁻¹ is suggested.

Table 1:

Grain size comparison of strips produced under different roll speeds (sloping plate length is 500 mm, vibration frequency is 50 Hz).

Pouring temperature	Roll speed
	0.052 m·s−1	0.087 m·s−1
660°C	48.83 μm	42.24 μm
680°C	53.18 μm	45.14 μm

Figure 6:

The center microstructures of A2017 alloy strips on cross-section under different roll speeds (sloping plate length is 500 mm, casting temperature is 660°C, and vibration frequency is 50 Hz): (a) 0.052 m·s⁻¹; (b) 0.087 m·s⁻¹.

3.4. Effect of Vibration Frequency on Microstructure of A2017 Alloy Strip

Slurry adhesion on stationary plate surface usually appeared. Therefore, a vibrating sloping plate was adopted by present work to avoid slurry adhesion. In addition, vibration can promote stirring of melt and is helpful to establish homogenous solute and temperature fields that are the ideal conditions for eruptive nucleation. Moreover, vibration can accelerate the dendrite breakage. Figure 7 shows the center microstructures of A2017 alloy strip on cross-section produced under different vibration frequencies. Figure 8 shows the relationships of vibration frequency and grain size and roundness. It is found that the grain size and roundness decrease firstly and then increase with the increment of vibration frequency. Stirring strength of vibration affected the alloy grain size. The actual stirring strength of vibration depends on the dominant action of vibration frequency or amplitude. The vibration frequency and vibration amplitude have a tight relationship, as shown in Table 2. The vibration amplitude decreases with the increment of vibration frequency. When the vibration frequency is lower than 50 Hz, the frequency dominates the stirring strength, so the grain size decreases with the increment of frequency. Once the vibration frequency is higher than 50 Hz, the amplitude becomes the dominant factor that determines the stirring strength, so the grain size increases with the increment of frequency and the decrease of amplitude. The proper vibration frequency of 50 Hz is suggested.

Table 2:

Relationship of vibration frequency and amplitude of experimental device.

Vibration frequency, Hz	20	40	60	80	100
Vibration amplitude, mm	1.4	1.1	0.9	0.55	0.35

Figure 7:

The center microstructures of A2017 alloy strip on cross-section produced under different vibration frequencies (sloping plate length is 500 mm, casting temperature is 660°C, and roll speed is 0.087 m·s⁻¹): (a) 40 Hz; (b) 50 Hz; (c) 60 Hz; (d) 80 Hz.

Figure 8:

The relationships of vibration frequency and grain size and roundness.

3.5. Optimized Process Parameters and the Microstructure of Product

Through the experiment, the optimized process parameters were obtained, as shown in Table 3. Under the optimized process parameters, A2017 alloy strip with a cross-sectional size of 4 × 160 mm was prepared, and the surface quality of strip is good, as shown in Figure 9(a). The microstructures of the products are shown in Figures 9(b) and 9(c), respectively. The microstructure is mainly composed of fine spherical or rosette grains, and obvious elongation occurs along rolling direction. The average grain size of A2017 alloy strip is 36.4 μm.

Table 3:

Optimized process parameters for producing A2017 alloy strip.

Parameters	Sloping plate length	Casting temperature	Roll speed	Vibration frequency
Values	500~600 mm	650°C~680°C	0.087 m·s−1	50 Hz

Figure 9:

A2017 alloy strip and its microstructures obtained at the casting temperature of 650°C: (a) A2017 alloy strip; (b) microstructure of product on the cross-section; (c) microstructure of product on the longitudinal section.

4. Conclusions

(1) The grain size and roundness of A2017 alloy strip decrease with the increment of the sloping plate length. The grain size and roundness of A2017 alloy strip increase with the increment of casting temperature. Because of combining actions of vibration frequency and amplitude, the grain size of A2017 alloy strip decreases firstly and then increases with the increment of vibration frequency.

(2) When the sloping plate length was between 500 mm and 600 mm, the casting temperature was between 650°C and 680°C, the roll speed was 0.087 m·s⁻¹, and the vibration frequency was 50 Hz, A2017 alloy strip with a cross-sectional size of 4 × 160 mm was produced. The microstructure of strip is mainly composed of fine spherical or rosette grains with a certain elongation along rolling direction, and the average grain size is 36.4 μm.