Effect of Process Parameters on the Microstructure of Semi-solid ZL101 Aluminum Alloy

Abstract

Aluminum alloy is the important alloy which has been widely used. To reduce waste, it is necessary to thixoform these alloys in near net shape. Semi-solid slurry is the key in process. In this work, the semi-solid slurry of ZL101 aluminum alloy was prepared by using serpentine channel. Effect of process parameters on the microstructure of semi-solid ZL101 aluminum alloy produced by serpentine channel was investigated. The results show that, the morphology of primary α-Al grains transformed from rosette to spheroid with decreasing pouring temperature. Satisfied semi-solid slurry of ZL101 aluminum alloy was prepared with pouring at 630°C to 670°C. Increasing curve number can improve the morphology of primary α-Al grain and decrease grain size with same pouring temperature. Desired slurry can be obtained with the lower pouring temperature when serpentine channel is preheated. It can make the primary nucleus gradually evolve into spherical and near-spherical grains with the effect of “self-stirring” in serpentine channel and chilling.

Key Words:
Semi-solid processing; Al-alloy; Microstructure; Slurry

1 Introduction

Many researchers have examined the benefits of semi-solid processing (SSP) over the last several decades. Semi-solid processing is a promising technology based on the thixotropic behavior of materials in semi-solid state, which takes place at temperatures between solidus and liquidus. The process depends on material behaving in a thixotropic way in semi-solid state ^1-4 . The required spheroidal microstructure can be obtained by a number of routes, such as mechanical stirring ^5,6, electromagnetic stirring ^7,8. However, the former readily pollutes metal and the latter has low electromagnetic stirring efficiency. Recently, a controlled nucleation method has been reported, which is simple, practical and less expensive because of no application of stirring. The preparation methods include new rheocasting ^9,10, vertical pipe^11,12, vibrating wavelike sloping plate process ^13-15, inverted cone pouring channel ^16,17 and cooling slope processes ^18-21. These techniques have similarities, such as, pouring the alloy melt through a plate or tube without stirring. Therefore, they greatly reduce energy consumption and save cost of production.

The serpentine channel process (SCP) was presented based on controlled nucleation method. During the period of preparing semi-solid slurry, the direction of alloy melt changes several times in field of gravity, so alloy melt has function of “self-stirring” ^21-25. With effect of chilling and stirring in serpentine channel, the primary nucleus gradually evolved¹into spherical and near-spherical grains. In this work, an innovative processing technique of semi-solid metal slurry, namely serpentine channel process, is introduced, and effect of process parameters on microstructure of semi-solid ZL101 aluminum alloy slurry were investigated.

2 Experimental procedure

2.1 Materials and apparatus

In the experiment, a commercial ZL101 aluminum alloy was used. Its chemical composition (mass fraction, %) is Si 6.6%, Mg 0.28%, Fe <0.16%, Mn <0.10%, Zn <0.10% and Al balanced. The liquidus and solidus are 617°C and 556°C, which were tested by differential scanning calorimetric (DSC) method, as shown in Fig. 1.

Fig.1
DSC cooling curves of ZL101 aluminum alloy

The schematic of SCP equipment and slurry preparation process is shown in Fig. 2. The serpentine channel is made of stainless steel and consists of two symmetrical blocks locked together by sleeve. The collective crucible, with size of $d$69 mm×150 mm, is made of stainless steel. The melting apparatus is a crucible resistance furnace. The temperatures of liquid aluminum alloy, semi-solid slurry and inner wall of the serpentine channel were measured by Ni−Cr/Ni−Si thermocouple. The temperature accuracy is ±1°C.

Fig. 2
Schematic diagram of preparing semi-solid ZL101 aluminum alloy slurry by serpentine channel: (a) Melt; (b) Preparation of semi-solid slurry; (c) Cooling. 1—K-type thermocouple; 2—Serpentine channel; 3—Pouring cup; 4—Serpentine bend; 5—Diversion pipe; 6—Melting crucible; 7—Collective crucible; 8—Slurry; 9—Cold water

2.2 Methods

Before serpentine channel process, as-cast ZL101 aluminum alloy were machined into feedstocks with mass of 1kg. The alloy was melted in a crucible resistance furnace at 720−730°C, and then incubated for 15 min. The alloy melt was cooled to chosen pouring temperature and casted into collective crucible via serpentine channel, and then rapidly quenched in cold water to obtain the high temperature solidification microstructure. The collective crucible was at room temperature. The temperature of serpentine channel were 100°C or 25°C before pouring. The processing parameters and characteristic parameter of semi-solid slurry are listed in Table 1.

Table 1
Processing parameters and characteristic parameter of semi-solid slurry

Sample No.	Curve number	Curve diameter/mm	Pouring temperature/°C	Serpentine channel temperature/°C	Pouring time/s	Slurry mass/kg	Equivalent particle diameter /μm	Average shape factor
A1	3	25	690	25	2	1.03	83	0.42
A2	3	25	670	25	2	0.99	75	0.47
A3	3	25	650	25	2	1.02	70	0.52
A4	3	25	630	25	2	1.04	62	0.59
B1	4	25	690	25	2	1.02	75	0.49
B2	4	25	670	25	2	0.98	68	0.56
B3	4	25	650	25	2	1.00	63	0.64
B4	4	25	630	25	2	0.99	59	0.68
C1	5	25	690	25	2	1.04	65	0.53
C2	5	25	670	25	2	1.06	63	0.58
C3	5	25	650	25	2	0.98	58	0.67
C4	5	25	630	25	2	0.98	55	0.72

Specimens for optical microscopy observation were cut from the center of quenched slurries and then roughly ground, finely ground, polished and etched with 0.5% HF aqueous solution finally. Microstructures of specimens were investigated by ZIESS optical microscope. The professional image analysis software (Image-Pro Plus) was adopted to analyze equivalent particle diameter (D) and average shape factor ($S_f$) of primary α-Al grains which were calculated by the following equations:

$D=\frac{{{\displaystyle \sum }}_{N=1}^N\sqrt{4A/\pi }}{N}$ (1)

$S_f=\frac{{{\displaystyle \sum }}_{N=1}^N\frac{4\pi A}{P^2}}{N}$ (2)

Where A and P are the area and perimeter of a particle, respectively. The $S_f$varies from 0 to 1, when the value of $S_f$ is close to 1, particle shape approaches to a circle.

3 Results and discussion

3.1 As-cast and SCP microstructures

ZL101 aluminum alloy semi-solid slurry is obtained with effect of chilling and stirring in serpentine channel. The microstructures of quenched slurry were prepared by serpentine channel and conventional casting, as shown in Fig. 3. The white phase is primary α-Al and the black matrix is the eutectic phase ^26,27.

Fig. 3
Microstructures of ZL101 aluminum alloy slurry prepared by two methods (a) conventional casting; (b) serpentine channel

Fig. 3(a) shows the microstructure of conventional casting. The particles morphology is typical dendrites and the length is more than 100μm. Fig. 3(b) shows the microstructure of semi-solid slurry prepared by SCP. It can be seen that non-dendritic primary α-Al particles are uniform throughout entire cast. The phenomenon indicates that collective crucible was filled completely and solidification of remaining liquid might have occurred throughout entire component ¹⁹. In addition, there are two main constituents in the microstructure: the relatively large α-Al globules (marked as α₁-Al) with particle diameter about 30-80μm, and the microstructure is composed of numerous small α-Al particles (marked as α₂-Al) of approximately 15μm diameter. There are two distinct solidification stages in the SCP. Solidification of slurry in serpentine channel is referred to as the first solidification, and solidification of slurry in collective crucible is referred to as the secondary solidification. The temperature declines rapidly in serpentine channel. A large number of nucleus are created in slurry, which grow larger α₁-Al particles in serpentine channel. The particles morphology is almost spherical and they are uniformly distributed throughout the microstructure. When alloy melt is poured into collective crucible, heterogeneous nucleation recurs in the remaining liquid. All the nucleus grow and solidify within a short time, forming the smaller α₂-Al particles, creating the ‘‘secondary solidification’’. Solidification time is very short because of quenching, and as a result, the primary α₂-Al particles are very small and less spherical in shape.

Equivalent particle diameter and average shape factor of primary α-Al grains are listed in Table 1. The equivalent particle diameters and the average shape factors of primary α-Al grains are 83, 75, 70, 62μm and 0.42, 0.47, 0.52, 0.59, which produced by 3 curves serpentine channel with pouring temperature at 690, 670, 650, 630°C, respectively. Fig. 4(a) ~ (d) show the microstructures of ZL101 aluminum alloy semi-solid slurry poured at 690, 670, 650, 630°C by serpentine channel with 4 curves, respectively. The equivalent particle diameters and the average shape factors of primary α-Al grains are 75, 68, 63, 59μm and 0.49, 0.56, 0.64, 68, respectively. The equivalent particle diameters and the average shape factors of primary α-Al grains are 65, 63, 58, 55μm and 0.53, 0.58, 0.67, 0.72, which produced by 5 curves serpentine channel with pouring temperature at 690, 670, 650, 630°C, respectively. Desired quality of slurry can be also achieved with lower pouring temperature when serpentine channel is preheated, as shown in Fig. 8.

Fig. 4
Microstructures of semi-solid ZL101 aluminum alloy slurry prepared by serpentine channel with 4curves at different temperatures (a) 690°C; (b) 670°C; (c) 650°C; (d) 630°C

Fig. 8
Microstructures of semi-solid ZL101 aluminum alloy slurry prepared by serpentine channel with different temperature: (a) 100°C; (b) 25°C

3.2 Microstructural evolution with increasing pouring temperature

Figs. 4 shows the microstructures of semi-solid ZL101 aluminum alloy slurry prepared by serpentine channel with 4 curves at different temperature .The characteristic parameter of semi-solid slurry is listed in Table 1. As shown in Figs. 4(a) ~ (d), primary α-Al grains are more and more spherical under the condition of decreasing pouring temperature using same serpentine channel. Fig. 4(a) shows that when pouring temperature is 690°C, the morphology of primary α-Al grains is irregular. A similar microstructure can be seen and more non-dendritic primary α-Al grains can be found in Fig. 4(b). Fig. 4(c) ~ (d) show that the primary α-Al grains are mainly near-spherical and spherical with pouring temperature decreased. Declining the pouring temperature leaded to equivalent particle diameter of the primary α-Al grains decreases and average shape factor increases, as shown in Fig. 5. The initial temperature of serpentine channel is low and it has chilling effect, which decreases the critical energy of nucleation and critical nucleus radius increases the nucleation ratio. Therefore, a large number of the primary α-Al nuclei will be generated. When pouring temperature is high, the inner wall of serpentine channel will be heated, resulting in chilling effect being weakened and nucleation ratio decreased. And grains have more time to grow. If pouring temperature is lower, the temperature of alloy melt in serpentine channel will decline rapidly below to liquidus. It is helpful for nucleation ²⁸. However, when pouring temperature is too low, the thickness of solidified shell in serpentine channel increases markedly, and the chilling effect of serpentine channel will be weakened. So semi-solid slurry can be prepared with satisfied quality when the pouring temperature is between 630 and 670°C.

Fig. 5
Effects of pouring temperature on D and S_f

3.3 Microstructural evolution with increasing curve number

Fig. 6(a) ~ (c) shows the microstructure produced by serpentine channel with different number of curves. Primary α-Al grains are smaller and more spherical made by 5 curves serpentine channel, as shown in Table 1. It can be seen from Fig. 6(a) that a small number of fine rosettes or dendrites primary α-Al grains apart from near-spherical or spherical. In addition, almost all of the primary α-Al grains are near-spherical or spherical which made by 5 curves, as shown Fig. 6(c). As the number of curves was increased, solid grains refining and the degree of spheroidization improving ²⁹. However, the hanging slurry increases with increasing curve number. The variation of equivalent particle diameters and average shape factors with different number of curves is shown in Table 1. Graphs about equivalent diameter and average shape factor of the primary α-Al grains prepared by three kinds of the serpentine channels with 3, 4, 5 curves respectively are shown in Fig. 7, which indicates that at the given pouring temperature of serpentine channel, equivalent particle diameter of the primary α-Al grains decreases and average shape factor of the primary α-Al grains increases with the curve number increasing. The inner wall of serpentine channel which acts as the concave nucleation substrate has a favorable effect on heterogeneous nucleation, so many primary α-Al nucleus form in a heterogeneous nucleation pattern. More crystal nucleus can be generated as the area of the heterogeneous nucleation substrate enlarges with increasing number of curves. Meanwhile, the self-stirring of alloy melt is strengthened with curve number increasing, which has a favorable effect on breaking of dendrites and refining primary α-Al grains. It can be seen from this group of experiments that better slurry can be obtained with 5 curves serpentine channel, but the hanging slurry is aggrandized. Therefore, the serpentine channel with 4 curves is appropriate.

Fig.6
Microstructures of semi-solid ZL101 aluminum alloy slurry prepared by serpentine channel with different curve numbers at 630°C (a) 3 curves; (b) 4 curves; (c) 5 curves

Fig. 7
Effects of curve number on D and S_f

3.4 Effect of serpentine channel temperature on the microstructure

Fig. 8 shows the microstructures of semi-solid slurry prepared by serpentine channel with different temperature. Primary α-Al are composed of vast spherical and a small number of fine rosettes grains. The serpentine channel was preheated at 100°C or 25°C. Comparing Fig. 8(a) with Fig. 8(b), equivalent particle diameter increases and average shape factors decreases at the same condition with the decreasing serpentine channel temperature. It can be seen in the experiment that efficiency is higher when the serpentine channel is preheated. Super-cooling is little when serpentine channel temperature is high, which is not good for refining grains. The number of crystal nuclei can be enlarged with decreasing pouring temperature, and getting favorable slurry.

The temperature of serpentine channel increases when slurry is produced abidingly. Cooling serpentine channel contributes to getting large super-cooling and receiving desired slurry. But it is difficult to control the serpentine channel temperature through cooling process which increased cost. Investigating the relationship between pouring temperature and serpentine channel temperature is more important. It is good for continuous production and saving cost. When the alloy melt flows through serpentine channel, its temperature continuously drops and viscosity increases. At the same time, the friction between alloy melt and inner wall varies with the viscosity, resulting in displacements in different points of the alloy melt, which will lead to shearing forces. What’s more, the shearing forces are diverse in different points of the alloy melt, so primary α-Al grains in the alloy melt will self-rotate. It can be seen in the experiment that lowing pouring temperature can solve the problem and receive good slurry.

4 Conclusions

The semi-solid ZL101 aluminum alloy slurry with desired quality can be generated by serpentine channel. Here the microstructural evolution of such materials in the semi-solid state has been analysed.

The equivalent particle diameter of primary α-Al grains decreases and the average shape factor of primary α-Al grains increases with the pouring temperature decreasing, and desired pouring temperature is in the range of 630 and 670°C.

Increasing curve number of serpentine channel can decrease the equivalent particle diameter of primary α-Al grains and increase the average shape factor of primary α-Al grains. Considering about quality of slurry and efficiency, 4 curves serpentine channel is appropriate.

Higher temperature of serpentine channel makes the grains coarse. Semi-solid slurry meets requirements of thixomolding. It ensures that continued access to slurry and reduce the preparation cost.

Acknowledgment

This work is supported by the National Natural Science Foundation of China (Nos.51204147, 51274175) and International Cooperation Project Supported by Ministry of Science and Technology of China (No.2014DFA50320), International Science and Technology Cooperation Project of Shanxi Province (Nos.2013081017, 2012081013).

References

• ¹
  Fan Z. Semisolid metal processing. International Materials Reviews. 2002;47(2): 1−37.
• ²
  Mao WM, Zhong XY. Semisolid metal forming technology. Beijing: China Machine; 2004.
• ³
  Flemings MC. Behavior of metal alloys in the semi-solid state. Metallurgical and Materials Transaction B. 1991;22(3):269-293.
• ⁴
  Zhao ZD, Mao WM. Preparation of semi-solid AlSi7Mg alloy slurry. Acta Metallurgica Sinica. 2008;21(2):139−145. doi:10.1016/S1006-7191(08)60031-9
  » https://doi.org/10.1016/S1006-7191(08)60031-9
• ⁵
  Alvani SM, Aashuri H, Kokabi A, Beygi R. Semisolid joining of aluminum A356 alloy by partial remelting and mechanical stirring. Transactions of Nonferrous Metals Society of China. 2010;20(9):1792−1798. doi:10.1016/S1003-6326(09)60376-9
  » https://doi.org/10.1016/S1003-6326(09)60376-9
• ⁶
  Ji S, Fan Z, Bevis MJ. Semisolid processing of engineering alloys by a twin-screw rheomoulding process. Materials Science and Engineering A. 2001;299(1-2):210–217. doi:10.1016/S0921-5093(00)01373-3
  » https://doi.org/10.1016/S0921-5093(00)01373-3
• ⁷
  Liu Z, Mao WM, Zhao ZD. Manufacture technique of semi-solid slurry of hypoeutectic Al-Si alloy by low superheat pouring and weak electromagnetic stirring. China Foundry. 2006;3(2):102–107.
• ⁸
  Zhang HT, Nagaumi H, Zuo YB, Cui JZ. Coupled modeling of electromagnetic field, fluid flow, heat transfer and solidification during low frequency electromagnetic casting of 7XXX aluminum alloys. Part 1: Development of a mathematical model and comparison with experimental results. Materials Science and Engineering A. 2007;448 (1–2):189-203. doi:10.1016/j.msea.2006.10.062
  » https://doi.org/10.1016/j.msea.2006.10.062
• ⁹
  Easton MA, Kaufmann H, Fragner W. The effect of chemical grain refinement and low superheat pouring on the structure of NRC castings of aluminum alloy Al-7Si-0.4Mg . Materials Science and Engineering A. 2006;420(1−2): 135−143. doi:10.1016/j.msea.2006.01.078
  » https://doi.org/10.1016/j.msea.2006.01.078
• ¹⁰
  Lukasson M, Apelian D, Das Gupta R. Alloy characterization for the new UBE rheocasting process. Transactions of American Foundry Society. 2002;110: 271−284.
• ¹¹
  Yang XR, Mao WM, Pei S. Preparation of semi-solid A356 alloy feedstock cast through vertical pipe. Materials Science and Technology. 2007;23(9): 1049−1053.
• ¹²
  Yang XR, Mao WM, Pei S. Influence of process parameters on microstructure of semisolid A356 alloy slug cast through vertical pipe. Transactions of Nonferrous Metals Society of China. 2008;18(1): 99−103. doi:10.1016/S1003-6326(08)60018-7
  » https://doi.org/10.1016/S1003-6326(08)60018-7
• ¹³
  Guan RG, Cao FR, Chen LQ, Li JP. Dynamical solidification behaviors and microstructural evolution during vibrating wavelike sloping plate process. Journal of Materials Processing Technology. 2009;209(5):2592−2601. doi:10.1016/j.jmatprotec.2008.06.007
  » https://doi.org/10.1016/j.jmatprotec.2008.06.007
• ¹⁴
  Guan RG, Li JP, Chen LQ, Wang C. Mechanism alloy microstructure formation during vibrating wavelike sloping plate process. Transactions of Material Research. 2008;22(4):363−368.
• ¹⁵
  Guo HM, Yang XJ, Hu B. Rheocasting of aluminum alloy A356 by low superheat pouring with a shear field. Acta Metallurgica Sinica. 2006;19(5): 328−334.
• ¹⁶
  Zhang XL, Xie SS, Li T, Yang HQ, Jin JZ. A356 aluminum alloy semisolid slurry prepared by damper cooling tube process. Rare Metal Material and Engineering. 2007;36(5):915−919.
• ¹⁷
  Yang B, Mao WM, Song XJ. Microstructure evolution of semi-solid 7075 Al alloy slurry during temperature homogenization treatment. Transactions of Nonferrous Metals Society of China. 2013;23(12):3592−3597.
• ¹⁸
  Cardoso LE, Atkinson HV, Jones H. Cooling slope casting to obtain thixotropic feedstock. I: Observations with a transparent analogue. Journal of Materials Science. 2008;43(16):5448−5455. DOI 10.1007/s10853-008-2828-2
  » https://doi.org/10.1007/s10853-008-2828-2
• ¹⁹
  Nourouzi S, Baseri H, Kolahdooz A, Ghavamodini S M. Optimization of semi-solid processing of A356 aluminum alloy. Journal of Mechanical Science and Technology. 2013;27(12): 3869-3874. DOI 10.1007/s12206-013-0931-z
  » https://doi.org/10.1007/s12206-013-0931-z
• ²⁰
  Birol Y. A357 thixoforming feedstock produced by cooling slope casting. Journal of Material Processing Technology. 2006;186(1-3):94-101. doi:10.1016/j.jmatprotec.2006.12.021
  » https://doi.org/10.1016/j.jmatprotec.2006.12.021
• ²¹
  Atkinson HV, Liu D. Microstructural coarsening of semi-solid aluminium alloys. Materials Science and Engineering A. 2008;496(1):439–446. doi:10.1016/j.msea.2008.06.013
  » https://doi.org/10.1016/j.msea.2008.06.013
• ²²
  Yang XR, Mao WM, Gao C. Semisolid A356 alloy feedstock poured through a serpentine channel. International Journal of Minerals, Metallurgy and Materials. 2009;16(5): 603−607. doi:10.1016/S1674-4799(09)60104-7
  » https://doi.org/10.1016/S1674-4799(09)60104-7
• ²³
  Dao V, Zhao S, Lin W, Zhang C. Effect of process parameters on microstructure and mechanical properties in AlSi9Mg connecting-rod fabricated by semi-solid squeeze casting. Materials Science & Engineering A. 2012;558: 95–102. doi:10.1016/j.msea.2012.07.084
  » https://doi.org/10.1016/j.msea.2012.07.084
• ²⁴
  Liu ZY, Mao WM, Wang WP, Zhneg ZK. Preparation of semi-solid A380 aluminum alloy slurry by serpentine channel. Transactions of Nonferrous Metals Society of China. 2015;25(5):1419−1426. doi:10.1016/S1003-6326(15)63741-4
  » https://doi.org/10.1016/S1003-6326(15)63741-4
• ²⁵
  Zhao Z, Chen Q, Chao H, Huang S. Microstructural evolution and tensile mechanical properties of thixoforged ZK60-Y magnesium alloys produced by two different routes. Materials and Design. 2010;31(4):1906–1916. doi:10.1016/j.matdes.2009.10.056
  » https://doi.org/10.1016/j.matdes.2009.10.056
• ²⁶
  Chen G, Chen Q, Wang B, Du ZM. Microstructure evolution and tensile mechanical properties of thixoformed high performance Al-Zn-Mg-Cu Alloy. Metals and Materials International. 2015;21(5):897-906. DOI 10.1007/s12540-015-5139-6
  » https://doi.org/10.1007/s12540-015-5139-6
• ²⁷
  Zhu WZ, Mao WM, Tu Q. Preparation of semi-solid 7075 aluminum alloy slurry by serpentine pouring channel. Transactions of Nonferrous Metals Society of China. 2014;24(4): 954−960. doi:10.1016/S1003-6326(14)63148-4
  » https://doi.org/10.1016/S1003-6326(14)63148-4
• ²⁸
  Chen Q Yuan B, Zhao G, Shu D, Hu C, Zhao Z, Zhao Z. Microstructural evolution during reheating and tensile mechanical properties of thixoforged AZ91D-RE magnesium alloy prepared by squeeze casting–solid extrusion. Materials Science and Engineering A. 2012;537: 25–38. doi:10.1016/j.msea.2012.01.002
  » https://doi.org/10.1016/j.msea.2012.01.002
• ²⁹
  Chen Q, Zhao Z, Zhao Z, Hu C, Shu D. Microstructure development and thixoextrusion of magnesium alloy prepared by repetitive upsetting-extrusion. Journal of Alloys and Compounds. 2011;509(26):7303–7315. doi:10.1016/j.jallcom.2011.04.113
  » https://doi.org/10.1016/j.jallcom.2011.04.113

Publication Dates

• Publication in this collection
  May-Jun 2016

History

• Received
  22 Sept 2015
• Reviewed
  05 Feb 2016
• Accepted
  07 Mar 2016