Study on Rheological Squeeze Casting Process of Ultra-high Strength Al-11Zn-1.8Mg-0.9Cu-0.15Zr-0.15Sc Alloy

7xxx series Al alloys with excellent mechanical properties are widely applied in aerospace, automotive, and defense industries. However, the casting performance of the alloy is relatively poor, and defects such as shrinkage porosity and thermal cracking are easy to occur during the solidification process. In this study, Al-11Zn-1.8Mg-0.9Cu-0.15Zr-0.15Sc alloy with excellent casting and strength performance developed by our research group, was studied by combining numerical simulation with experimental research. The simulation results show that hot joints appear in the thick wall of the wheel parts as the melt temperature increases. The increase of mold temperature and the decrease of squeeze pressure lead to the increase of the existence time of hot joints, which will aggravate the shrinkage and micro-segregation of castings. The experimental results show that electromagnetic stirring melt treatment can significantly improve melt uniformity, refine grain, and a wheel-shaped part with high performance can be obtained when the melt temperature, the mold temperature, the squeeze pressure are chosen as 660 °C, 200 °C, 50MPa, respectively.


Introduction
Al-Zn-Mg-Cu alloys with many advantages such as high strength and tensile, excellent corrosion resistance and good weldability, and heat treatment properties, excellent stability and reliability, have been widely applied in aerospace, automotive,and defense industries [1-5] .However, their casting performance is relatively poor, and the defects such as shrinkage porosity and thermal cracking are unavoidable during the solidification process, meanwhile, easily leading to coarse, uneven structure and component segregation .At present, the structural parts of such alloys are mainly produced by deformation processing, with low material utilization, long processing time and high cost.Therefore, the development of short-route and low-cost casting technology has important practical value [6] .
In order to solve the above problem, lots of efforts have been made in the design of alloy composition and process modification.A new Al-11Zn-1.8Mg-0.9Cu-0.15Zr-0.15Scalloy was developed, which has good casting and mechanical properties, and it is easier to cast in comparison with the traditional 7xxx wrought alloy [7] .Meanwhile, a new rheological squeeze casting method was developed [8] , where the melt treatment by annular electromagnetic stirring (A-EMS) is used to homogenize the alloy melt before the melt is poured into the mold for squeeze casting, and the temperature field, component field and solidification structure can be homogenized greatly, which also benefits for further improving its casting performance.However, little work has been done to explore the optimal technological parameters of the rheological squeeze casting process of the Al-11Zn-1.8Mg-0.9Cu-0.15Zr-0.15Scalloy.
In this study, a wheel-shaped part with practical engineering application background was taken as the research object, and the influence of the process parameters of rheological squeeze casting such as melt temperature, mold temperature and squeeze pressure on the casting quality was studied by combining numerical simulation with experimental research.This work is expected to provide technical support for the engineering application of rheological squeeze casting technology of the alloy.

Numerical simulation and experimental method
In the process of rheological squeeze casting, melt temperature, mold temperature and squeeze pressure are important parameters that affect the performance of castings and the occurrence of defects.The numerical simulation of the casting filling and solidification process can help the staff to effectively predict the location, size and occurrence time of various defects that may occur in the casting process by optimizing the process design, which ensures good casting quality, short experiment period and low production cost.

Model establishment and parameter selection
2.1.1.Model building.Rheological squeeze casting process are simulated by ProCAST software with its advanced numerical algorithms and models to simulate flow, solidification and other processes during the casting process [9] .The profile diagram of the target wheel is shown in Fig. 1, where the outer diameter of the wheel part is 200mm, and the height of the wheel part is 120mm.In the casting process, the wall thickness in the middle center part of the wheel is the largest, and the defects such as shrinkage and porosity are most likely to occur.Through simulating the influence of melt temperature, squeeze pressure, mold temperature and other process parameters on the casting defects.the shrinkage and micro-segregation of castings can be determined by thermal node analysis, and the appropriate process parameters can be selected.The simulation process chart is shown in Fig. 2.

Squeeze casting simulation process chart
The simulation process is as follows: firstly, according to the shape characteristics of the casting, the upper and lower molds are designed and assembled in UG, and the assembly body is imported into the mesh module of ProCAST software.And then the model is meshed and simulation parameters are set.Finally, loading calculations are performed.After the results are observed and analyzed in the Viewer module, the process parameter settings are adjusted based on the simulation results, and simulation calculations are performed again to select the best process parameters.
2.1.2.Parameter setting.The Al-11Zn-1.8Mg-0.9Cu-0.15Zr-0.15Scalloy is used in the study, and the mold material is set to H13 steel.Based on their respective properties, the alloy melt is set to the ALLOY type, the mold is set to the MOLD type, and the mold is approximately treated as a rigid body.Alloy components are input into ProCAST software to automatically generate parameters such as material thermal conductivity and density.
The interface heat transfer coefficients are set as follows.In dividing the mesh, some nodes on the interface between the molten metal and the mold will overlap, so the contact type is set to COINC.Since the same material is chosen in the upper and lower parts of the mold and has a continuous temperature curve, the contact interface is set as EQUIV.The interface between the mold and air, where the nodes on the boundary do not overlap, is set as NCOINC.
The interface between the mold and the air is set as NCOINC interface with a heat transfer coefficient of 10W/(m 2 .K). Referring to literature [10] , the heat transfer coefficient h of the COINC interface has the following relationship with the specific pressure P within a certain range: h=1090.5+94.85PAccording to the above equation, the heat transfer coefficient of the COINC interface can be calculated between the casting and the mold under a certain pressure.When conducting numerical simulation calculations, the heat transfer coefficient of the COINC interface is set and manually input after calculation.
The boundary of the temperature field in numerical simulation is the contact surface between molds and the air, and between molds and the alloy melt.The mold surface in contact with the air is selected as air-cooling.with a temperature of 20℃.Load conditions is set on the boundary between the mold and the alloy melt, which is selected by the practical pressure during squeeze casting.
Based on the previous casting experiment experience, the process parameters are examined as follows: melt temperature: 660-700℃, squeeze pressure: 25-75MPa, and mold temperature: 150-250℃.In the course of simulation, the end condition is set as 100% solid phase ratio.

Rheological squeeze casting and test methods
The high-purity Al and Al-50Cu intermediate alloys are added into the well-resistance melting furnace and heated to 750℃ for full melting, and the temperature was lowered to 700℃ for adding Zn and Mg and stirring until completely melting.The melt temperature is raised to 720℃ for holding 30min, and the Al-10Zr alloys and Al-2Sc alloys are added and stirred until completely melting, holding for 10min.After refining, degassing and slagging, melt temperature is controlled at 720℃ for pouring.The casting test is divided into two groups,in the case with A-EMS, melt is first poured into the stainless steel crucible preheated to 500℃, and then put the melt in the crucible into the melt treatment device by A-EMS, as shown in Fig. 3, and the stirring current is at 10 A, stirring frequency is at 15 Hz.A thermocouple is inserted into the melt to measure the temperature.When the melt temperature in the crucible reaches the pouring temperature, it is poured into the die for squeeze casting.In the case without A-EMS, the melt in the crucible at the pouring temperature is directly poured into the die for squeeze casting.
The castings obtained in the experiment are cut along the central position, and sampling position for metallographic observation and hardness test is shown in the Fig. 4. The size of the sample block is 15*15*15mm, and the sample block is numbered 1-3 from top to bottom.After T6 heat treatment, the sample block is smoothed and polished with sandpaper of different particle sizes, and is mechanically polished to the mirror surface with diamond grinding paste to completely eliminate scratches.The sample surface is anodized with 2.5% HBF4 solution, the voltage is 30V, and the current is within 1A.Coating time 50s-60s, metallographic microscope was used to observe the optical structure, and color images were   6 shows the cross section of velocity field distribution when the melt is filled to different states at the squeeze speed of 20mm/s.In the process of squeeze casting, the filling process of the alloy will be affected by the squeeze speed.In Fig. 6(a), the contact area between the upper die and the alloy melt is small at the initial filling stage.In this state, the melt velocity in the contact part with the upper die increases, while the velocity on the side of the upper die changes significantly,and reaches the maximum as the molt surface rises gradually.As the upper die continues to move down, as shown in Fig. 6(b, c), the melt on the side wall is directly filled up.The velocity direction changes from axial to radial, and the wheel shape is gradually formed.Fig. 7 and Fig. 8 show velocity components in axial and radial directions, respectively.It is noted from Fig. 7 that, there is a radial velocity of melt in the beginning of the filling, as the upper die continues to move down, the radial velocity goes down.It is noticed from Fig. 8 that, .thevelocity below the upper die moves down axially, as the upper die gradually goes down, the filling speed of melt on the side of the upper die gradually increases..In addition, as the upper die goes down, the flow state of the melt remains stable, and no defects such as enfranchisement and slag inclusion will occur in this process.Influence of melt temperature on rheo-squeeze casting parts.Fig. 9 shows the simulation results of casting hot spots conditions at different melt temperatures, and the specific parameters are shown in Table 1.As shown in Fig. 9, the Y-Z direction is intercepted to observe the simulation results, and the hot spot always exists in the middle of the wheel, that is, the part with the thickest wall thickness.With the rise of melt temperature, the existence time of the hot spot shows a non-linear increasing trend.According to the simulation results, hot spot is used to judge the zone and time of hot spot.In squeeze casting, the shorter time of hot spot, the faster the solidification speed in the isolated liquid phase of melt, which is more conducive to the refinement of grains and the reduction of microscopic segregation.Fig. 10 shows solidification time at different melt temperatures.As shown in Fig. 10, under different melt temperatures, the solidification time of the edge part of the casting is basically unchanged.For the part with the thickest wall thickness, the solidification time of the middle part of the wheel increases with the rise of melt temperature, and the trend of micro-relaxation increases.Considering grain refinement and defects occurrence, the optimal melt temperature can be selected as 660℃.2. As shown in Fig. 11, the greater the pressure, the shorter the existence time of the hot spot, and the hot spot always exists in the middle of the wheel.The squeeze pressure affects the heat transfer coefficient between the mold and the melt, and further affects the existence time of the hot spot.Fig. 12 shows solidification time at different squeeze pressure.As shown in Figure 12, with the increase of pressure, the solidification time in the middle of the wheel is shortened, and the gap of the solidification time between the center and the side is reduced, which is more conducive to reducing the generation of micro-porosity.According to the simulation results, the optimal squeeze pressure can be selected as 75MPa.3.As shown in Fig. 13, when other conditions remain unchanged, with the increase of mold temperature, there is no obvious change in the area and size of the hot spot, and the existence time of the hot spot is non-linear increased.Fig. 14 shows solidification time at different mold temperature.As shown in Fig. 14, with the increase of mold temperature, the solidification time is obviously increased.When the mold temperature is at 150℃, the solidification time of the middle part of the wheel is the shortest.But the low mold temperature will also lead to the direct solidification of the casting edge, and the shrinkage and loosening tendency will increase in the middle part, and also the mold temperature is too low, probably leading to cold isolation.Therefore, the optimal mold temperature still needs to be further verified by experiment.

Experimental verification of rheo-squeeze casting parts
According to the above simulation results, the rheological squeeze casting experiment was carried out, and the grain size and hardness of the castings were analyzed by means of metallographic and tensile samples.

3.2.1
Casting appearance quality.Fig. 15 shows the appearance of rheo-squeeze casting wheel-shaped parts.The castings have good formability and no defects such as cracks and cold insulation on the surface.Fig. 16 shows the longitudinal section of the rheo-squeeze casting wheel-shaped parts.No macroscopic shrinkage holes were observed in the section, which is well consistent with the simulation results.As shown in Fig. 17, with the increase of melt temperature, the grain size increases, and also grain uniformity becomes bad.The average grain size of the upper samples at 660℃, 680℃, 700℃ are 28μm, 29μm,45μm, respectively, and the average grain size of the lower samples at 660℃, 680℃, 700℃ are 47μm, 35μm, 45μm, respectively, as shown in Fig. 18(a).The reason may be that the melt with high temperature comes into contact with the mold surface at a low temperature, the mold surface temperature rises, and the heat transfer between the melt and mold and the cooling rate of the melt slow down, and the nucleating crystal nuclei have sufficient time to grow and coarsen, resulting in relatively coarse grains.
As shown in Fig. 18(b) , the melt temperature has a little effect on the hardness of the wheel, the average hardness is about 170HBW.The hardness of the middle part of the casting is slightly lower than that of the top and bottom, and there is a hardness difference.The reasons may be that the cooling rate in the top and bottom of the wheel is not uniform, resulting in the formation of a harder structure at the edge and softer structure in the inner part.The uneven heat treatment process of solution aging may lead to differences in hardness between the surface and the interior; The squeeze pressure is not uniform, the top and bottom pressure in the squeeze casting process is greater than the middle, resulting in greater hardness.
Interestingly, when the melt temperature is at 700℃, the hardness of the wheel is higher.With the melt temperature increased, the hardness of the casting has an upward trend.The increase of melt temperature leads to the coarse grain, but the reason for the increase of hardness may be that the increase of temperature leads to the increase of the solubility of the main strengthening phase η phase and T phase of Al-Zn-Mg-Cu alloy, resulting in the increase of hardness.
Fig. 19 shows the microstructure of the rheo-squeeze casting wheel-shaped parts at different melt temperatures, and Fig. 20 shows statistical results of grain size and hardness at different melt temperatures.As shown in Fig. 19, after the electromagnetic stirring melt treatment, the microstructure uniformity of the casting is apparently improved, and not only the grains are smaller, but also the difference in grain size among the different parts of the casting is reduced.As shown in Fig. 20(a), the average grain size of the upper samples at 660℃, 680℃, 700℃ are 28μm, 27μm, 37μm, and the average grain size of the lower samples at 660℃, 680℃, 700℃ are 29μm, 39μm, 38μm.As shown in Fig. 20(b), the hardness reaches about 180HBW.After electromagnetic stirring, the hardness of castings has been increased, and the hardness gap in different parts is reduced.The reason may be that the electromagnetic stirring process leads to the eruption of melt nucleation, and the grain is smaller and the size gap is reduced, resulting in more uniform hardness.To sum up, the electromagnetic stirring melt treatment can significantly improve the melt uniformity and refine the grains.In order to obtain castings with fine grain size, good uniformity and high hardness, the melt temperature should be selected as 660℃.

3.2.3
Influence of mold temperature on microstructure and hardness.Fig. 21 shows the effects of mold temperature on the grain size and hardness of the rheo-squeeze casting wheel-shaped parts at a melt temperature of 660℃ and a squeeze pressure of 75MPa.With the decrease of the mold temperature, the grain size decreases first and then increases.When the mold temperature is low, the temperature difference on the boundary between melt and t mold is larger, since the cooling rate is faster, the melt casting process begins to solidize rapidly, therefore the solidification is completed before the cavity is filled, and the storage energy is larger.After heat treatment, the recrystallization is more adequate and the grain size is larger.When the mold temperature is too high at 250℃, the grain will become coarser.Besides, if the mold temperature is too high, mold surface contact more closely under the action of pressure melt, and active atoms are easier to migrate with the atoms in the melt, resulting in improved wettability between the melt and the mold surface and sticky mold.With demoulding force increases,casting and mold are seriously damaged in the process of demoulding.In summary, the optimal mold temperature should be selected as 200℃.

3.2.4
Influence of squeeze pressure on microstructure and hardness.Fig. 22 shows the effects of squeeze pressure on the grain size and hardness of the rheo-squeeze casting wheel-shaped parts at a melt temperature of 660℃ and a mold temperature of 200℃.As shown in Fig. 22(a), the average grain size of the upper samples at 25MPa, 50MPa, 75MPa are 35μm, 27μm, 28μm, respectively, and the average grain size of the lower samples at 25MPa, 50MPa, 75MPa are 50μm, 38μm, 40μm, respectively.As shown in Fig. 22(b), when the pressure is 25MPa, the hardness is about 171HBW, and when the pressure is 75MPa, the hardness is about 180HBW.As the squeeze pressure is higher, due to the finer grains, the hardness is higher.
It is indicated that the grain size decreases at the beginning, and tends to be stable with the increase of the squeeze pressure.When the pressure is increased to 75MPa, the grain size does not decrease significantly.At this time, the pressure increase will no longer improve the performance of the casting.The reason for the grain refinement under pressure is that high pressure will increase the supercooling degree of solidification, thereby improving the nucleation rate, and finally the grain refinement.It is known that the high pressure makes the heat exchange coefficient between the shell and the mold wall increase, so that the effective contact area between the shell and the mold wall increases, resulting in an increase of the solidification rate of the alloy, thus refining the grain.Considering long mold life, the optimal pressure should be selected as 50MPa.

Conclusion
(1) The hot spot appears in the thick part of the middle wall of the wheel-shaped part during the filling process of squeeze castings, and high melt temperature, high mold temperature, and low squeeze pressure will lead to an increase in the duration of the hot spot, exacerbating the shrinkage porosity of the casting.
(2) Electromagnetic stirring melt treatment significantly improve melt uniformity, refine grain, and a wheel-shaped part with high performance can be obtained when the melt temperature ,the mold temperature, the squeeze pressure are chosen as 660 ℃, 200 ℃, 50MPa, respectively.

Fig. 5
Fig. 5 Distribution of melt temperature field and solidification time at different times Fig.6shows the cross section of velocity field distribution when the melt is filled to different states at the squeeze speed of 20mm/s.In the process of squeeze casting, the filling process of the alloy will be affected by the squeeze speed.In Fig.6(a), the contact area between the upper die and the alloy melt is small at the initial filling stage.In this state, the melt velocity in the contact part with the upper die increases, while the velocity on the side of the upper die changes significantly,and reaches the maximum as the molt surface rises gradually.As the upper die continues to move down, as shown in Fig.6(b, c), the melt on the side wall is directly filled up.The velocity direction changes from axial to radial, and the wheel shape is gradually formed.

Fig. 21
Fig. 21 Statistical results of (a) grain size, and (b) hardness at different mold temperatures

Fig. 22
Fig. 22 Statistical results of (a) grain size, and (b) hardness at different squeeze pressures