A forced convection stirring process for Rheo-HPDC aluminum and magnesium alloys

Mingfan Qi a,∗, Yonglin Kanga, Bing Zhoua, Wanneng Liaoa, Guoming Zhua, Yangde Li b,
Weirong Li b
a School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China b Dongguan EONTEC Corporation, Ltd., Dongguan 523662, China

Abstract

A simplified process, namely forced convection stirring (FCS), was proposed to prepare four different
semisolid slurries ofAl andMg alloys.Asimple rheological high pressure die-casting (Rheo-HPDC) process
was developed by combining the FCS device with a HPDC machine. The temperature and composition
fields of the melt in the FCS process were simulated. The microstructures and mechanical properties
of the alloys formed by the FCS Rheo-HPDC were studied and compared with those formed by HPDC
and other Rheo-HPDC processes. The results showed that the FCS process can prepare slurries in which
the primary particles are fine, spherical and uniformly distributed in the matrix and the process can
effectively suppress macrosegregation. The strong convection rapidly promotes the uniformity of the
temperature and composition of the melt. Compared to HPDC and other Rheo-HPDC alloys, the alloys
formed by FCS Rheo-HPDC have an improved ultimate tensile strength, yield strength, elongation and
hardness, due to the refinement of microstructure and the decrease of porosity. Heat treatments can be
utilized to further enhance the mechanical properties of FCS Rheo-HPDC alloys.

Keywords

Forced convection stirring process, Semisolid slurries, Rheo-HPDC, Microstructure, Mechanical properties

Introduction

For the past few years, in order to provide solutions to environmental and energy issues, the importance of Al and Mg alloys as light-weight metals has been widely recognized, especially in the fields of 3C (computer, communication and consumer electronics) and automotive (Sui et al., 2015). Nowadays, high pressure die-casting (HPDC) is the process that is primarily used for manufacturing the majority of Al and Mg alloys applications, due to its low cost and high efficiency.

However, Jin et al. (2015) demonstrated that many porosities, caused by gas entrapment during the high speed mold filling, exist in HPDC parts. Cáceres and Selling (1996) showed that porosity can seriously decrease mechanical performance by serving as local strain concentration.

Porosity also causes issues during heat treatment, where heat results in the expansion of air within the pores, creating bubbling in HPDC parts. Improvement of the HPDC process to form parts with high quality and increased performance is thus of highly worthwhile. Flemings (1991) demonstrated that the Rheo-HPDC technique can be used to prepare high quality parts. Compared to traditional HPDC, Rheo-HPDC provides parts with many advantages such as low porosity, heat treatability and high performance.

In the RheoHPDC process, the liquid metal is initially turned into semisolid slurry and then the slurry is shaped directly using a HPDC machine. Various preparation processes have been developed to prepare the semisolid slurry. The twin-screw stirring process was studied by Fan et al. (2005), the gas induced semisolid (GISS) process was reported by Wannasin et al. (2006) and the serpentine channel pouring (SCP) process was used by Liu et al. (2015).

These preparation techniques have been widely studied and used in labs, and are able to prepare semisolid slurries with equiaxed microstructures. However, many drawbacks still exist, such as difficulties in cleaning up the devices and complicated disassembly and installation of the devices, which limit their extensive use for industrial applications.

In order to deal with the issues that exist in the slurry preparation, a self-developed forced convection stirring (FCS) device was designed for the preparation of semisolid slurry by Zhou et al. (2014a,b). It is an environmentally friendly device with a simple operational process and low cost, and it can run continuously without cleanup. It is convenient to combine a FCS device with HPDC machines to develop a Rheo-HPDC process and such procedure is termed as, FCS Rheo-HPDC.

In this work, the semisolid slurries of A356, A380 and 7075 Al-alloys, as such of AZ91D Mg-alloy were prepared by the FCS process and then formed by HPDC machines to four different parts or quenched by water to ingots. The effect of the FCS process on the microstructure and macrosegregation of ingots was studied.

The microstructure, porosity and mechanical properties of the FCS Rheo-HPDC alloys were studied and compared to those formed by HPDC and other Rheo-HPDC processes. Based on heat transfer and hydrodynamics, the effect of the rotation speed on temperature, composition and solid fraction were simulated by the commercial computational fluid dynamic software Flow-3D.

Experimental procedure

2.1. FCS process
The FCS device is presented in Fig. 1. It consists of a stirring system, a temperature control system and a discharge system. The stirring system contains a stirring rod equipped with a helicoid blade, and its major function is to turn the melt into high-quality semisolid slurry under the FCS action. There are huge amounts of changing interfacial areas among the solidifying melt, helicoid blade and barrel during slurry preparation. This makes the preparation process of slurry significantly efficient for fast heat dissipation. Heating cables are twined around the stainless steel barrel wall. The discharge operation is controlled by a discharge handle and a graphite blockage, which are linked using a center pin. It should be noted that a graphite lining is embedded in the stirring chamber for solving sticking problem.

2.2. Raw materials

The alloys, including commercially available cast Al-alloys A356
and A380, a commercially available wrought Al-alloy 7075, and a
commercially available cast Mg-alloy AZ91D, are used in this work.
Table 1 lists the chemical compositions and freezing ranges of the
four alloys.

2.3. Slurry preparation and FCS rheo-HPDC

The alloys were melted in top-loading resistance furnaces at the temperature of 70 ◦C above their liquidus. The melt was cooled to a pouring temperature (0–30 ◦C above their liquidus) after drossing and degassing. Then the melt was poured into the pre-heated FCS device. The detailed melt mass and FCS process parameters are listed in Table 2. After the melt was stirred to a preset time, the semisolid slurry with a certain solid fraction was prepared and was sent into the shot sleeve of a HPDC machine to form parts.

The detailed HPDC machine models and HPDC process parameters are also listed in Table 2 and the photographs of forming parts are presented in Fig. 2. For comparison, traditional HPDC parts were also prepared without FCS treatment under the same pouring temperature and HPDC parameters.

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Fig. 1. The schematic diagram (a) and actual image (b) of the integrated FCS device.
Fig. 1. The schematic diagram (a) and actual image (b) of the integrated FCS device.
Detailed parameters and initial conditions for simulation
Detailed parameters and initial conditions for simulation
Fig. 2. Photographs of the four different parts formed by FCS Rheo-HPDC technology
Fig. 2. Photographs of the four different parts formed by FCS Rheo-HPDC technology
Fig. 3. The simplified 3D model (a) and generation mesh (b) of the FCS device.
Fig. 3. The simplified 3D model (a) and generation mesh (b) of the FCS device.
Fig. 4. Temperature field simulation of the melt in a FCS device at different stirring time
Fig. 4. Temperature field simulation of the melt in a FCS device at different stirring time
Fig. 5. Distribution simulation of the particles in a FCS device at different stirring time.
Fig. 5. Distribution simulation of the particles in a FCS device at different stirring time.
Fig. 6. Microstructures of the four different alloy ingots prepared by NC and FCSC: (a) A356-NC, (b) A380-NC, (c) 7075-NC, (d) AZ91D-NC, (e) A356-FCSC, (f) A380-FCSC, (g) 7075-FCSC, (h) AZ91D-FCSC.
Fig. 6. Microstructures of the four different alloy ingots prepared by NC and FCSC: (a) A356-NC, (b) A380-NC, (c) 7075-NC, (d) AZ91D-NC, (e) A356-FCSC, (f) A380-FCSC, (g) 7075-FCSC, (h) AZ91D-FCSC.
Fig. 7. The radial element concentration evolution of the NC and FCSC 7075 Al-alloy ingots (a) and a sketch of the six tested locations (b)
Fig. 7. The radial element concentration evolution of the NC and FCSC 7075 Al-alloy ingots (a) and a sketch of the six tested locations (b)
Fig. 8. Temperature and solid fraction simulations as well as the microstructures of the 7075 Al-alloy melt at different rotation speeds
Fig. 8. Temperature and solid fraction simulations as well as the microstructures of the 7075 Al-alloy melt at different rotation speeds
Fig. 9. Temperature difference variation of the melt over stirring time at different rotation speeds.
Fig. 9. Temperature difference variation of the melt over stirring time at different rotation speeds.
Fig. 10. Effect of the rotation speed on the average size and shape factor of 1-Al particles
Fig. 10. Effect of the rotation speed on the average size and shape factor of 1-Al particles
Fig. 11. Pore defects of control arm parts at different locations by HPDC and FCS Rheo-HPDC.
Fig. 11. Pore defects of control arm parts at different locations by HPDC and FCS Rheo-HPDC.
Fig. 12. Number of pores at different locations in Fig. 11
Fig. 12. Number of pores at different locations in Fig. 11
Fig. 13. Microstructures of HPDC (a-c) and FCS Rheo-HPDC (d-f) A380 alloy shock absorber parts at different regions: (a), (d) region A; (b), (e) region B; (c), (f) region C.
Fig. 13. Microstructures of HPDC (a-c) and FCS Rheo-HPDC (d-f) A380 alloy shock absorber parts at different regions: (a), (d) region A; (b), (e) region B; (c), (f) region C.
Fig. 15. Microstructures of AZ91D alloy steering wheel parts formed by HPDC and FCS Rheo-HPDC as well as EDX line-scans: (a) HPDC AZ91D; (b) FCS Rheo-HPDC AZ91D; (c) EDX line-scan for HPDC AZ91D in (a); (d) EDX line-scan for Rheo-HPDC AZ91D in (b).
Fig. 15. Microstructures of AZ91D alloy steering wheel parts formed by HPDC and FCS Rheo-HPDC as well as EDX line-scans: (a) HPDC AZ91D; (b) FCS Rheo-HPDC AZ91D; (c) EDX line-scan for HPDC AZ91D in (a); (d) EDX line-scan for Rheo-HPDC AZ91D in (b).
Fig. 16. SEM images of FCS Rheo-HPDC parts of AZ91D alloy after T4 heat treatment (a) and T6 heat treatment (b).
Fig. 16. SEM images of FCS Rheo-HPDC parts of AZ91D alloy after T4 heat treatment (a) and T6 heat treatment (b).
Fig. 17. SEM fractographs of the AZ91D Mg-alloy tensile parts: (a) low magnification, HPDC; (b) high magnification, HPDC; (c) low magnification, FCS Rheo-HPDC; (d) high magnification, FCS Rheo-HPDC; (e) low magnification, FCS Rheo-HPDC + T4 treatment; (f) high magnification, FCS Rheo-HPDC + T4 treatment; (g) low magnification, FCS Rheo-HPDC + T6 treatment and (h) high magnification, FCS Rheo-HPDC + T6 treatment.
Fig. 17. SEM fractographs of the AZ91D Mg-alloy tensile parts: (a) low magnification, HPDC; (b) high magnification, HPDC; (c) low magnification, FCS Rheo-HPDC; (d) high magnification, FCS Rheo-HPDC; (e) low magnification, FCS Rheo-HPDC + T4 treatment; (f) high magnification, FCS Rheo-HPDC + T4 treatment; (g) low magnification, FCS Rheo-HPDC + T6 treatment and (h) high magnification, FCS Rheo-HPDC + T6 treatment.
Fig. 18. Schematic diagrams of traditional crystal dissociating (a) and crystal dissociating in the FCS device (b).
Fig. 18. Schematic diagrams of traditional crystal dissociating (a) and crystal dissociating in the FCS device (b).
Fig. 19. The solidification process of the melt in HPDC and FCS Rheo-HPDC techniques.
Fig. 19. The solidification process of the melt in HPDC and FCS Rheo-HPDC techniques.

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Conclusions

(1) A simplified process, namely forced convection stirring (FCS), was developed for the preparation of semisolid slurries. The FCS process can prepare slurries in which primary particles are fine, spherical and uniformly dispersed in the liquid matrix.
(2) The strong convection rapidly promotes the uniformity of the temperature and composition of the melt, which destroys the formation environment of dendrites and improves primary particles in the quantity and morphology.
(3) A simple Rheo-HPDC process is developed by combining the FCS device with a HPDC machine. The microstructures of FCS Rheo-HPDC parts at different regions are all composed of fine spherical particles and low porosity levels (0.3-0.5%). For the control arms of A356 alloy, the total pore areas of the FCS RheoHPDC parts are decreased by 3 times in comparison to that of the HPDC parts.
(4) FCS Rheo-HPDC parts show improved mechanical properties to those formed by HPDC and other Rheo-HPDC techniques, and the increasing rates of UTS, YS and elongation are over 10% in comparison to HPDC.

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