Effects of Different Parameters on Porosity Defects Between the Horizontal and Vertical Shot Sleeve Processes
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The effects of different casting parameters on porosity defects between the horizontal and vertical shot sleeve processes are quantitatively studied based on experiments and simulations. The castings made using the two processes are characterized in terms of porosity defects by X-ray radiographic inspection and blistering formation by T4 heat treatment process. Experimental results indicate that much fewer porosities and blisters are formed in the castings made using the vertical process than those made using the horizontal one. Meanwhile, numerical simulations have been performed to quantitatively investigate the flow dynamics behavior of molten metal in the horizontal and vertical shot sleeves. The simulation results demonstrate that compared with the conventional horizontal process, the vertical shot sleeve process is more efficient in elimination of waves and reduction of air entrapment and oxide formation. Moreover, the vertical process needs a lower pouring temperature which can decrease hydrogen solubility in the molten metal; it has a smaller temperature gradient of molten melt which can reduce the formation of cold flakes and pre-solidified materials. Therefore, the vertical shot sleeve process is more competitive in improving the internal integrity of die castings.
Keywordshigh-pressure die casting numerical simulation air entrapment oxide cold flake porosity
The high-pressure die casting (HPDC) process is currently the most common method used to produce near net shape components of aluminum alloy with good surface quality and high productivity in the automotive industry.1 However, porosity is one typical defect which impairs internal integrity and mechanical properties, as well as restricts the application of heat treatment and welding processes.2,3 There are two principal sources of porosity: gas and shrinkage porosities.4 Gas porosity is mainly caused by air entrapment, steam, and burning products of organic lubricants in the shot sleeve; shrinkage porosity is related to thermal conditions and solidification process of dies and castings.5
Some studies have been conducted on porosity defects in the HPDC process. Tian et al.6 studied the effect of melt cleanliness on porosity formation and identified the types of inclusion: amorphous oxides, oxide films, and sludge particles. Tsoukalas et al.7 evaluated the effect of die casting machine parameters on porosity, and the results indicated that multiplied pressure in the third phase has the most significant effect; plunger velocity in the second phase, die cavity filling time, and fast shot set point also have a significant effect. Lee et al.8 investigated the effect of process parameters on porosity distributions, and the results showed that a decrease in both gate velocity and melt temperature can decrease the total amount of porosity. Dispinar et al.9 studied degassing, hydrogen, and porosity phenomena in the A356 alloy, and they pointed out that the decrease of hydrogen solubility in liquid aluminum with temperature is believed to be a major source of gas porosity.
Correspondingly, some studies have been carried out on flow mechanisms of molten metal and optimum of shot profile for the horizontal HPDC process to reduce gas porosity. One research group, Lopez,10,11 Faura,12 Hernandez,13 and Zamora et al.14,15 investigated the wave dynamics and air entrapment mechanisms of molten metal. They developed an analytical model to predict the critical plunger speed, optimum acceleration, and other process parameters and then verified the optimum parameters for reducing air entrapment and minimizing gas porosity. Korti et al.16–18 studied the influence of process parameters on the evolution of interface aluminum liquid–air profile, and the parameters were optimized to minimize air entrapment. Fiorese et al.19,20 determined the plunger kinematic parameters, estimated their correlation with the casting quality, and then improved the quality by optimal plunger motion planning.
Little work is available in the literature on the internal integrity of castings produced by the vertical HPDC process. Although the HPDC machine equipped with a vertical shot sleeve has not been widely used in the workshop, the advantages demonstrate that the unique process has potential competition in reducing air entrapment and oxide formation effectively in the vertical shot sleeve.
The aim of this work is to study the quality of engine cover castings produced by the horizontal and vertical HPDC processes. First, some castings are produced using the two processes. Then, castings are selected randomly and checked by X-ray radiographic method to identify the porosity positions. Meanwhile, other castings are performed by T4 heat treatment process to evaluate blistering formation on the casting surface. Moreover, numerical simulations are implemented for the two processes to quantitatively evaluate the different effects of flow dynamics in the shot sleeve on porosity defects.
Experiments and Results
Parameters of the Two Processes
Pouring temperature (°C)
Dies temperature (°C)
Slow shot velocity (m/s)
Fast shot velocity (m/s)
Cycle time (s)
Diameter of plunger (mm)
Length of shot sleeve (mm)
In-gate area (mm2)
In-gate thickness (mm)
In-gate fast velocity (m/s)
Vent area (mm2)
Overflow volume (mm3)
X-ray Radiographic Inspection
T4 Heat Treatment Process
Simulations and Results
Numerical modeling was established using FLOW3D CAST software to quantitatively compare the different effects of flow dynamics and solidification process on porosity defects between the horizontal and vertical processes. The detailed parameters listed in Table 1 were used as initial and boundary conditions in the modeling. The transition from slow shot to fast shot was assumed to be at the point when the liquid metal reaches the in-gate for the horizontal process, and was at the point when the liquid metal passes the in-gate by 10 mm for the vertical process. Since the geometries of the castings and the gating systems are very similar to the two processes, the simulation is focused on the flow dynamics of molten metal in the shot sleeve.
Entrapped Air Volume
Meanwhile, the filling time of each process can be known from Figure 7; for the horizontal process, filling time is 1.654 s; for the vertical process, filling time is 0.441 s.
Height of Wave
Free Surface Area of Molten Metal
For the horizontal process, with the plunger moving forward, the molten metal is pushed toward the die and eventually fills the cavity. In this period, the free surface area of molten metal varies with time, as shown in Figure 8. For the vertical process, when the plunger pushes molten metal upward to the die cavity, the free surface area is basically equal to the section area of the shot sleeve or runner due to gravity.
When the molten metal is initially poured into the sleeve, the simulated free surface area in a horizontal sleeve is about 0.0611 m2; the corresponding theoretical value is about 0.0591 m2. Meanwhile, in a vertical sleeve, it is only about 0.0102 m2. So, the free surface area of molten metal in a horizontal sleeve is about 6 times higher than that in a vertical one.
Heat Modulus of the Molten Metal
The larger the heat modulus, the smaller the temperature gradient or cooling rate of molten metal. It means the heat of molten metal releases slowly, and it is beneficial to hold the metal above liquidus temperature of the alloy.
Analysis and Discussion
Based on above experimental and simulation results, the different effects of casting parameters on gas porosity defects between the horizontal and vertical processes can be evaluated comprehensively.
Gas porosity mainly results from three factors: (1) air entrapment, (2) oxidized formation, and (3) premature solidification or cold flakes in the shot sleeve.21–23 The possibility of shrinkage porosity distribution is very similar to the horizontal and vertical processes according to the simulation results shown in Figure 14. Meanwhile, combined with the experimental results shown in Figures 5 and 6, it can be inferred that gas porosity is more vulnerable to occur in the horizontal process than in the vertical one.
On the one hand, the modification of overflows, gating, and venting is beneficial to avoid air entrapment; on the other hand, the vertical shot sleeve can effectively reduce oxide formation with the decreasing of turbulent flow, wave formation, and wave reflect.
The influence of these three factors on gas porosity can be quantitative interpreted based on simulation results.
First, as for air entrapment, it can be known from Figure 7 which indicates entrapped air volume of the horizontal process is about 1.2 times than that of the vertical one.
Second, as for oxidized formation, it can be analyzed comprehensively from Figures 8, 9, 10, 11, and 12 which demonstrate free surface oxide mass of horizontal process is about 17.2 times than that of the vertical one.
For the horizontal shot sleeve, flow turbulence brings in wave formation, as illustrated in Figure 8. When the wave moves forward and reaches the entrance to the runner, it will be reflected and jump up and down. Then, severely unstable waves occur, as shown in Figures 9 and 10. Meanwhile, the free surface area of molten metal in a horizontal shot sleeve is about 6 times larger than that in the vertical one when the molten metal is initially poured into the shot sleeve, as shown in Figure 11. Thus, in a horizontal shot sleeve, as the plunger starts to push liquid metal forward, the free surface area of molten metal creases linearly, indicating that the oxide film on the molten metal has to be broken up. As a result, new oxides are formed instantaneously at the positions where old oxides are fractured. The fragments of the oxide can be entrapped into molten metal. While oxide entrapment is unlikely to occur in a vertical shot sleeve because there is almost no obvious wave formation. Once oxide film is formed, it is unlikely to be broken up. In fact, the oxide film, undisturbed, should be useful in preventing further oxidation of the molten metal and in protecting the molten metal from absorbing hydrogen from moisture or gases in the shot sleeve. Therefore, much less oxide is formed in a vertical shot sleeve than that in a horizontal one, as shown in Figure 12.
Finally, as for premature solidification or cold flakes, it can be seen from Figure 13 that the unique feature of the vertical process is a smaller amount of heat loss of molten metal than that of the horizontal one because of the larger heat modulus. The benefits of having a larger modulus for the vertical process include (1) lower pouring temperature, which is beneficial for not only the service life of shot tooling and dies but also the decrease of hydrogen solubility in molten metal24, 25 and (2) fewer cold flakes and less pre-solidified metals in the shot sleeve. Cold flakes and pre-solidified solids will bring in porosity and then deteriorate the mechanical properties because of larger dendritic length scares and oxide formation on the surfaces of cold flakes.
The experimental results of X-ray radiographic inspection and T4 heat treatment process indicate that castings made using a vertical shot sleeve process contains fewer porosities and blisters.
Numerical simulation results suggest that the free surface area of molten metal in a shot sleeve is much smaller for the vertical process than that of the horizontal one. Waves are formed in the horizontal shot sleeve because of turbulent flow. Wave formation and wave reflect both cause entrapment of gases and oxides that increases the porosity level of the castings made using the horizontal process. While for the vertical process, the shot sleeve is always completely filled and the flow in the molten metal is less turbulent. As a result, less oxide and mold gases are entrapped in the vertical shot sleeve.
Numerical simulation results also demonstrate that the heat modulus of molten metal is higher in the vertical sleeve than in the horizontal one. Consequently, liquid metal can be poured into the shot sleeve at a lower temperature for the vertical process, which means that less hydrogen, pre-solidified material, and cold flakes are formed in the vertical shot sleeve.
The authors gratefully acknowledge financial support from Fiat Chrysler Automobiles LLC under the R&D project Agreement No. 00059689.
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