Summary
Based on the Zhizhu Super Cloud - a professional CAE cloud platform in the field of die-casting, the impact of injection process parameters on the filling process of semi-solid rheological die-casting was simulated and analyzed using a handle aluminum alloy casting as the research object. An optimal uniform acceleration material barrel incubation semi-solid rheological die-casting process scheme was selected for actual product production testing. The results show that using the five stage low-speed injection mode B die-casting process scheme can make the alloy liquid flow smoothly in the pressure chamber, in a laminar state, without turbulence; Based on the simulation results, the semi-solid rheological die-casting process of the barrel was optimized, and the semi-solid rheological die-casting forming of the handle casting was successfully achieved, resulting in qualified die-casting parts. The primary α - Al phase in the microstructure is mainly in the form of nearly spherical or granular shapes, with no internal porosity defects, and can be processed by T6 treatment and welding.

Pressure casting, as a near net forming method with high productivity, low cost, and high dimensional accuracy, has been widely applied and rapidly developed in fields such as automotive, electronic communication, instrumentation, and hardware and electrical appliances. In the traditional die-casting process, the liquid melt will fill the mold cavity in a turbulent state, causing the gas inside the mold cavity to be unable to be discharged in time and rolled into the alloy, forming porosity defects. Pores reduce the effective load-bearing area of the casting, causing stress concentration and thus lowering the mechanical properties of the casting. Dong et al. believe that the issue of internal porosity in die-casting parts is currently the main problem limiting the further expansion of die-casting technology's application fields.
In order to eliminate internal porosity defects in die-casting parts, the new technology of semi-solid die casting has received increasing attention from researchers. In the semi-solid die casting process, due to the high solid fraction, viscosity, and round grain size of the metal slurry, under appropriate injection parameters, the metal slurry will flow in a laminar manner to fill the mold cavity. The filling process is smooth and uniform, and a die casting part without porosity defects can be obtained. The semi-solid forming technology has many unique advantages and is widely used in fields such as automotive, electronic communication, electrical appliances, aerospace, and medical. At present, rheological die-casting technologies such as stirring method, self inoculation method, GISS, RSF, shear low-temperature casting method, and serpentine channel method have been developed. Although semi-solid forming technology has been applied, it still needs further improvement and development in both theory and technology.
At present, most semi-solid rheological die-casting technologies involve the manufacturing process of semi-solid slurry, which inevitably leads to some phenomena such as gas entrapment and oxidation during the slurry making process, especially for small and medium-sized castings. Due to the small slurry amount and difficult temperature control, it affects product quality. For this purpose, this article will focus on small and medium-sized aluminum alloy castings. Based on the professional CAE cloud platform in the field of intelligent casting, a new technology of semi-solid rheological die casting with uniform acceleration material barrel incubation without slurry making process will be developed through numerical simulation optimization of the filling process in semi-solid rheological die casting. It has been practically applied in the production of related high-strength and high conductivity aluminum alloy die castings.

one
Die casting analysis and process design

This article takes a structural component handle with a simple shape and T6 processing requirement as the research object, exploring a new technology of semi-solid rheological die casting with uniform acceleration material barrel incubation. The three-dimensional shape of the handle casting is shown in Figure 1. The casting size is not large, but the wall thickness is uniform, with a thin wall of about 3mm and a maximum external size of about 136mm × 49mm × 23.5mm. The material is AlSi9Cu3 alloy, which is very suitable for pressure casting. The material's physical parameters are: density of 2.5 g/cm ³, liquidus temperature of 595 ℃, and solidus temperature of 540 ℃. According to the tonnage of the existing die-casting machine and the projected area of the casting, one the first mock examination and one cavity are selected. The thickness of the inner gate is 2.5 mm, the width is 125 mm, and the diameter of the injection punch is 50 mm. The detailed die-casting process scheme is shown in Figure 2.

two
Uniformly accelerated semi-solid rheological die-casting process and simulation

Apply Zhizhu Chaoyun - a professional CAE cloud platform in the field of die-casting for numerical simulation. Firstly, the 3D modeling software UG was used to create a 3D image and generate an STL file. Then, it was uploaded to the cloud computing platform of Zhizhu Super Cloud for pre-processing to achieve automatic mesh generation. The filling process of the casting casting was simulated. Four injection curves were set up as shown in Figure 3 to simulate the die-casting process of the handle casting, and the flow of alloy liquid in the pressure chamber during the injection process was analyzed.

The simulation results of the filling process of the single-stage low-speed injection mode die-casting process scheme are shown in Figure 4. From the figure, it can be seen that in the early stage of injection, due to the direct acceleration from zero to low speed, the acceleration is too large, resulting in the formation of vortices at the front end of the punch, as shown in Figure 4a-b. Moreover, when the alloy liquid reaches and fills the position of the pressure chamber diversion cone, it is not yet filled near the front end of the punch, causing the alloy liquid to reflux, as shown in Figure 4c. The eddy currents and backflow phenomena generated in the injection chamber during the injection process are prone to gas and oxide inclusions, and can be carried into any position of the casting during the high-speed injection process, thereby affecting the quality of the casting.

Figure 5 shows the simulation results of the filling process of the two-stage low-speed injection mode die casting process scheme. As shown in the figure, the filling process is basically consistent with the single-stage low-speed injection mode, and there are still eddy currents and backflow phenomena in the movement of the alloy liquid in the pressure chamber. The simulation results of the filling process of the five stage low-speed injection mode A die-casting process scheme are shown in Figure 6. From the figure, it can be seen that in the early stage of injection, the alloy liquid flows smoothly in the pressure chamber without turbulence, as shown in Figure 6a; When the current end alloy liquid approaches the position of the splitter cone, a certain vortex trapping phenomenon will gradually form, as shown in Figure 6b-c. The simulation results of the filling process of the five stage low-speed injection mode B die-casting process scheme are shown in Figure 7. From the figure, it can be seen that the entire injection process of the alloy liquid flows smoothly in the pressure chamber, in a laminar state, without turbulence, thus avoiding the inclusion of gas or oxidation slag in the pressure chamber. Based on numerical simulation results, a semi-solid rheological die-casting process with uniform acceleration material barrel incubation was proposed, which integrates semi-solid pulping and die-casting forming. By controlling the temperature of the alloy liquid discharged from the furnace and the temperature of the barrel, a certain solid phase will be formed after the alloy liquid is poured into the barrel. Then, the solid phase formed during the multi-stage low-speed injection process will further increase, and it will flow into the alloy liquid as heterogeneous nucleation nuclei. Finally, after entering the mold cavity, the presence of a large number of nuclei will inhibit the growth of dendrites, thus forming a non dendritic semi-solid structure.

three
Production validation

Based on the injection process curve of the five speed injection mode B, the semi-solid rheological die-casting process with uniform acceleration barrel was applied for the actual production test of the handle casting, and compared and analyzed with the ordinary die-casting products of the single-stage low-speed injection mode. The pouring temperature of the aluminum alloy liquid is 640 ℃, the maximum low-speed speed of the injection punch is 0.7 m/s, the high-speed speed is 2 m/s, and the preheating temperature of the mold is 200 ℃. The use of this die-casting process resulted in a button hand die-casting with clear contour, smooth surface, high dimensional accuracy, and no defects, as shown in Figure 8. By dissecting and analyzing the die-casting parts, it can be seen that there are a large number of porosity defects in the cross-section of castings using ordinary die-casting technology, as shown in Figure 9a; However, no porosity was found in the cross-section of the semi-solid die-casting part, as shown in Figure 9b. In addition, X-ray transmission testing of the entire casting showed that there were no obvious defects such as shrinkage porosity, shrinkage porosity, porosity, and oxide inclusions inside the handle casting. Figure 10 shows the microstructure of the as cast and T6 of the handle casting. From the figure, it can be seen that the primary α - Al phase is nearly spherical, with small and evenly distributed grains, and has obvious semi-solid structural characteristics. In addition, T6 treatment and welding tests were conducted on the castings. After solid solution treatment at 535 ℃ for 8 hours, it can be clearly seen that there are many bubbles on the surface of ordinary die-casting parts, while no bubbles were found on the surface of semi-solid castings, as shown in Figure 11; Similarly, there is bubbling near the weld seam of ordinary die castings after welding, while it does not exist in semi-solid castings, as shown in Figure 12. From this, it can be seen that the production of die-casting parts using the uniform acceleration barrel inoculation semi-solid rheological die-casting process can effectively eliminate the internal porosity problem of ordinary die-casting parts, and achieve T6 heat treatment and welding processing of die-casting parts.

four
conclusion

(1) By using the Intelligent Casting Super Cloud - a professional CAE cloud platform in the field of die-casting, the simulation analysis of the injection process of different injection process schemes was carried out. The results showed that when using the five stage low-speed injection mode B die-casting process scheme, the alloy liquid could flow smoothly in the pressure chamber, in a laminar state, without turbulence, thus avoiding the inclusion of gas or oxide slag in the pressure chamber.
(2) Based on the simulation results, the semi-solid rheological die-casting process of the barrel was optimized, and the five stage low-speed injection mode B die-casting process was successfully adopted to achieve the semi-solid rheological die-casting forming of the handle casting, obtaining qualified die-casting parts. The aluminum pouring temperature was 640 ℃, the maximum low-speed speed of the injection punch was 0.7 m/s, the high-speed speed was 2 m/s, and the mold preheating temperature was 200 ℃.
(3) The microstructure of ordinary die-casting parts with handles is mainly dendritic or rose shaped, and there are many porosity defects inside, which cannot be treated with T6 or welded; The semi-solid die-casting parts of the handle have no porosity defects inside, and the primary α - Al phase in the structure is mainly in the form of nearly spherical or granular particles, which can be processed by T6 treatment and welding.

Author:
Long Wenyuan
College of Aeronautical Manufacturing Engineering, Nanchang University of Aeronautics and Astronautics
Gong Jie, Song Guojin
Xiamen Greer Technology Co., Ltd
This article is from: Casting Magazine, Strategic Partner of Die Casting Weekly