Hot Tearing analysis and process optimisation of the fire face of Al-Cu alloy cylinder head based on MAGMA numerical simulation

ABSTRACT This paper proposes a casting improvement process to enable the casting of Al-Cu alloy cylinder heads to achieve the required quality. The hot tearing defects generated in the fire face of the cylinder head during casting were simulated using MAGMA software. The simulation results show that the large wall thickness transition difference in the fire face of the cylinder head is the reason for the large temperature and stress gradient during solidification. Therefore, the first solution was optimised by forming the fire face as a whole into a flat plate, but the fire face centre area stress is still too high. The second optimisation option optimises the cold iron structure of the fire face, the probability of cracking in the fire face is reduced to 17%. These results have important reference significance for the optimisation of the Al-Cu alloy cylinder head hot tearing problem.


Introduction
The cylinder head is one of the key components of the vehicle engine, its working environment is harsh, the working process is constantly subjected to high temperature, high burst pressure and complex alternating load.The fire surface of the cylinder head is one of the core areas of the cylinder head, consisting of intake valve holes, exhaust valve holes, spark plug holes, etc.The main functions of the cylinder head are realised in this part [1,2], so the cylinder head castings need to have excellent mechanical properties and no defects [3,4].
In the actual development process, on the one hand, the internal structure of the cylinder head fire surface area is complex and the wall thickness difference is large, easy to produce casting defects [5,6].On the other hand, casting using Al-Cu alloy crystallisation range is wide, solidification shrinkage and paste solidification characteristics, so that the casting properties of the alloy, such as fluidity, hot tearing resistance and airtightness is poor, and wall thickness sensitivity, easy to produce macro-crater, shrinkage, hot tearing and macro-crossing and other casting defects [7][8][9][10].Therefore, the design of the cylinder head casting process is to consider not only the casting structure but also the casting material characteristics.
Solidification hot tearing defects are one of the most serious defects that occur in Al-Cu alloys during casting.It occurs above the solid phase temperature of the alloy [11][12][13][14].Hot tearing occurs when the maximum principal stress exceeds the strength of some of the solidified metal due to the lack of effective compensatory shrinkage [15][16][17].Casting stresses are generally generated by solidification shrinkage and thermal shrinkage restraint, and are generally concentrated in the region of thermal joints or cross-sectional abruptness where solidification of the casting continues [18,19].Therefore, the casting process of the cylinder head should be designed to consider hot tearing defects during solidification of the casting, but also to control the temperature gradient and the maximum stress distribution during solidification.
MAGMA software, which can simulate the temperature field and stress field of the casting alloy filling and solidification process, can effectively predict the formation and distribution of defects in the casting process [20][21][22][23].S. Aravind et al.Simulating the casting process for solidification of centrifugal pump crankcase and modifying the process design based on the simulation results to obtain zero defect castings [24].Kiełbus A et al. used EV31A magnesium alloy for the manufacture of engine substrates and used simulation software to simulate the casting process and control the quality of the castings [25].Huang M et al. were simulating defects in die-casting process castings and adjusting parameters to obtain excellent castings [26].Aneesh T et al. simulated the process of gravity casting of engine substrates to get rid of the large number of casting defects that cannot be identified using conventional methods and to improve economic efficiency [27].
Al-Cu alloys are increasingly used in engine components, especially cylinder heads, which are subjected to high temperatures, high burst pressures and complex alternating loads due to their corrosion and heat resistance as well as their ability to withstand large loads.However, the fluidity, thermal crack resistance and airtightness of these materials are poor, and the wall thickness is sensitive and prone to defects such as hot tearing.Therefore, this paper uses MAGMA software to simulate the cylinder head casting process, and for areas prone to hot tearing, such as the cylinder head fire face area, the temperature and stress fields are used to simulate and analyse the process and thus improve it.Finally, casting trials were carried out and X-ray inspection was carried out to obtain an engine cylinder head free of casting defects.

Materials
The material used in this study was AlCu4TiMg aluminium alloy.Table 1 shows the chemical composition of this alloy.It is an alloy used in the sand casting form mainly in the Engine industry.

Casting process design
In this paper, a cylinder head with three cylinders and one cap is the object of research and verification.The internal structure of the cylinder head is very complex, the overall 660 mm×300 mm×165 mm, the internal structure is composed of upper water chamber, lower water chamber, intake channel and exhaust channel, the minimum wall thickness at the gas channel wall is only 4 mm, the maximum wall thickness ratio is about 1:23 at the fire surface position.
The casting process system of the casting is composed of runner, riser, filter and chill, which is poured by sand mould bottom injection.The casting process is shown in Figure 1.
The sprue adopts a flat design to avoid eddy current in the aluminium liquid during pouring and reduce the risk of air entrapment.The riser is used to feed the casting to prevent shrinkage porosity and macrosegregation.Finally, in order to ensure the high performance of the cylinder head fire surface, a circular thick cold iron is arranged on the fire surface.

Simulation
The simulation used the MAGMASoft software, whose mathematical model closely reflects the processes occurring during the mould cavity filling.Table 2 shows the material's thermophysical properties used in the simulation.

XRD Examinations
The X-ray testing used X5000 industrial CT X-ray inspection system.Figure 2 shows the casting X-ray test position.From the highest cracking areas shown in the simulation, samples were cut for studies.

Initial programme
Figure 3 shows the results of the initial programme casting.Casting of cylinder head castings using the casting process system.A large number of visually visible cracks appear at the intake and exhaust valve holes of the cast cylinder head fire face and extend to the edge of the fire face.As these cracks could not be repaired, they eventually led to the scrapping of the cylinder head.Systematic analysis of the casting using MAGMASoft software for the cracking of the fire face of the cylinder head.Figure 4 shows the results of the initial scenario casting hot tearing simulation.Figure 4(a) shows hot tearing at the intake and exhaust valve holes of the cylinder head fire face at a solidification time of 54s. Figure 4(b) shows the simulation results for a solidification time of 64s with a risk of hot tearing at the middle and edge of a single cylinder on the fire face of the cylinder head, with a probability of up to 50%.
Hot cracking was the most common solidification defect in Al-Cu alloy castings.It was a defect formed at the grain boundaries of the alloy during solidification due to blocked shrinkage.After the alloy had been poured, the solidification temperature decreases and nucleation begins within the casting and dendrites were produced and bridged to each other to form a skeleton.As the solidification process continues, when the end of the dendritic lap, the initial formation of the skeleton, the casting of solid shrinkage began to produce, if the shrinkage of the casting is hindered, the dendritic skeleton is subject to shrinkage caused by the tensile stress, tensile stress is greater than the strength of the liquid film, will produce cracking at the liquid film; dendritic skeleton after cracking, if there is sufficient liquid metal near the crack to compensate for the crack, the hot tearing defect will not appear; if the crack location could not be complementary shrinkage,   the casting will produce hot crack defects.Therefore, the hot tearing was related to the temperature and stress changes.
Figure 5 shows the results of the initial solution casting temperature simulation.Figure 5(a) for the solidification time 46s when the cylinder head fire surface temperature distribution, from the figure can be seen, the cylinder head fire surface into the valve hole and exhaust valve hole at the thin wall area first cooled, while the single cylinder middle and edge position is still high temperature area.Figure 5 (b) for solidification time 62s when the cylinder head fire surface temperature distribution, by the figure can be seen at this time the cylinder head fire surface temperature down to 580°C and fire surface edge position temperature is still in high temperature state.Comprehensive Figure 5 can be seen, the cylinder head in solidification there is a significant temperature change, thin-walled area heat dissipation fast, thick and large hot joints part of the slow heat dissipation, making the cylinder head fire surface and the edge of the region there is a significant macro thermal gradient.
Figure 6 shows the results of the initial scenario casting stress simulation.Uneven stresses on the fire face of the cylinder head are due to different cooling rates in various parts of the fire face of the cylinder head.Figure 6(a) shows the stress distribution on the fire face of the cylinder head when the solidification time is 56s.The figure shows that the maximum stress is concentrated in the middle part of the fire face of the cylinder head, and the inlet and exhaust ports of the fire face of the cylinder head are subject to tensile stress caused by shrinkage, which increases the risk of hot tearing.Figure 6(b) shows the stress distribution of the cylinder head fire face when the solidification time is 69s, the maximum main stress at this time is concentrated in the cylinder head fire face area, the cylinder head fire face single cylinder middle and edge position is subjected to increased stress, the probability of hot tearing increases.
Combined with the cylinder head structure analysis can be seen, the cylinder head fire surface is divided into thin-walled areas and thick hot section parts.Due to the thin wall area preferential solidification, making the casting thick part of the thin wall area to form a complementary shrinkage, thick hot joint at the liquid phase reduction.The cylinder head using aluminiumcopper alloy, hot tearing tendency is larger, intergranular shrinkage capacity is poor, making the thick hot joints in the late solidification risk of hot tearing increased.Secondly, the cylinder head in the solidification, thin wall area priority solidification, shrinkage, and the thick large hot joint parts solidification slow.This leads to an obvious macroscopic thermal gradient in the fire face and edge area, increased stress in the casting and increased risk of hot tearing.
For the casting fire surface structure wall thickness difference, from thin wall parts to thick hot section parts of the wall thickness of the sudden change in size, adjust the cylinder head fire surface wall thickness transition difference and solidification when the temperature, stress gradient is to eliminate hot tearing defects of the effective method.

Cylinder Head Body Optimisation Solution
Figure 7 is the optimisation solution of cylinder head body.From the above analysis, it can be seen that the crack of the fire surface of the cylinder head is caused by the large wall thickness difference of the fire surface structure, especially at the inlet valve hole and the exhaust valve hole of the fire surface.For this case, the structure of the cylinder head body is optimised and improved.The positions of the intake valve hole and the exhaust valve hole of the fire surface are optimised.The intake port and the exhaust port of the fire surface are sealed, so that the fire surface forms a flat plate as a whole to reduce the wall thickness transition difference in the hot spot area.
Figure 8 shows the hot tearing simulation results of the optimised solution for the cylinder head body.Figure 8(a) shows the hot tearing tendency at the edge of the firing surface when the solidification time is 54s.The simulation result in Figure 8(b) shows the hot tearing at the solidification time of 64s.The risk of hot tearing at the edge of the fire surface increases, and the probability of occurrence is close to 35%.Through the comparison of Figure 4, it can be seen that reducing the wall thickness transition difference in the hot spot area can reduce the probability of hot cracking of the cylinder head fire surface to a certain extent, but the cylinder head fire surface still has the risk of cracking.In order to further study the causes of hot cracking in the cylinder head during solidification, the parts where cracks may occur are deeply explored.
Figure 9 shows the simulation results of the casting temperature of the optimised solution for the cylinder head body.Figure 9(a) is the temperature distribution of the cylinder head fire surface at the solidification time of 46s.It can be seen from the diagram that the thin-walled area at the intake valve hole and exhaust valve hole of the cylinder head fire surface is cooled first, while the middle and edge positions of the single cylinder are still high-temperature areas.Figure 9(b) for the solidification time of 62s cylinder head fire surface temperature distribution, cylinder head fire   surface temperature drops while the edge of the temperature is still in the high state.Comparison with Figure 5 cylinder head fire surface in solidification temperature gradient change similar.
Figure 10 shows the results of the casting stress simulation for the optimised solution for the cylinder head body.Figure 10(a) shows the stress distribution on the fire surface of the cylinder head at a solidification time of 56s.The graph shows that the maximum stress at this point is concentrated in the centre of the cylinder head fire surface.Figure 10(b) shows the stress distribution on the fire surface of the cylinder head at a solidification time of 69s.The maximum principal stresses at this point are distributed from the central area in all directions to a cut-off at the edge of the fire face.In contrast to Figure 6, sealing the fire face inlet and exhaust tracts so that the fire face as a whole forms a flat plate helps to expand the force area in the area around the fire face and diversify the direction of force.At the same time reducing the transition difference in wall thickness in the hot joint area can reduce the maximum principal stress in the fire face of the cylinder head to some extent.
The body optimisation solution for cylinder heads reduces the probability of cracking in the fire face area of the cylinder head to some extent, but there is still the possibility of cracking at the edge of the fire face.A comprehensive analysis of the thermal crack simulation results and the temperature and stress fields from the ontology optimisation solution shows that the thermal crack occurs at the edge of the cylinder head fire surface, at the intersection of the hightemperature region and the low-temperature region, and at the edge of the maximum principal stress.Therefore, the cylinder head solidification to expand the intersection of high temperature region and low temperature region, so that the maximum main stress in the edge region of the force area to expand is a solution to the cylinder head fire surface of hot tearing.In order to obtain better performance in the fire face area of the cylinder head, the cold iron is arranged in the fire face area of the cylinder head to give priority to the solidification of the fire face area, but due to the uneven wall thickness in the fire face area of the cylinder head resulting in different solidification rates.The optimised solution for the cylinder head body shows that hot tearing occurs at the intersection of the high-temperature region and the lowtemperature region.At the edge of the maximum principal stress.Therefore, this solution chooses to eliminate the cold iron in the inter-fireface region, adjusts the solidification sequence in the cylinder head fireface region, expands the intersection of the high-temperature region and the low-temperature region, so that the maximum main stress in the edge region of the force area is expanded.

Optimised cold iron solutions for cylinder head
The cylinder head casting process optimises the structure of the cooling device on the fire face, from a circular cold iron structure with exhaust slots and triangular-shaped cooling devices on both sides, as in   Figure 12 shows the simulation results of the optimised cold iron solution for the cylinder head temperature field.Figure 13 shows the simulation results of the optimised cold iron solution for the cylinder head stress field.It can be seen from the figure that the optimisation of the cold iron structure can adjust the solidification sequence of the cylinder head fire face area, improve the distribution of the maximum stress and peak value of the cylinder head fire face central area.From the data in Figure 14(a,b), it can be seen that the probability of hot tearing at the edge of the fire face during the solidification stage is less than 0.17.According to the actual casting experience, the probability of cracking on the fire face of the cylinder head under this scheme is small, and this process can be adopted for trial production.
Figure 15 shows a trial diagram of an optimised cold iron solution.Figure 15(a) shows the cylinder head core making drawing, and Figure 15(b) shows the cylinder head casting.From the figure optimised cold iron solution trial casting surface without cracks can be seen.Figure 16 is the cylinder head castings cut

Conclusion
This paper proposes a structural improvement scheme for Al-Cu alloy multi-cylinder integrated cylinder head that can eliminate hot tearing, and by changing the cold iron structure on the fire face of the cylinder head casting, to adjust the solidification rate of the fire face part of the cylinder head solidification process, so as to eliminate hot tearing.The results achieved are summarised below: 1.The generation of thermal cracks on the fire surface of the cylinder head is related to the temperature gradient and stress distribution when the cylinder head is solidified.
2. The location of the thermal cracks calculated from the simulation of the cylinder head fire surface using Magma simulation software is consistent with the actual casting verification results.
3. The use of the cylinder head fire surface into the valve hole and exhaust valve hole closed is an effective way to eliminate the cylinder head fire surface hot tearing defects.
4. Improve the structure of the cylinder head fire face cooling device, reduce the cylinder head between each fire face in the solidification process of stress, is to reduce the Al-Cu alloy cylinder head this kind of casting hot tearing more effective method.

Figure 1 .
Figure 1.Diagram of the casting process.

Figure 9 .
Figure 9. Temperature field simulation results for cylinder head body optimisation solution (a) Solidification time 46s (b) Solidification time 62s.

Figure 11 (
Figure 11(a) shows the cold iron distribution on the fire face of the cylinder head for the original solution.In order to obtain better performance in the fire face area of the cylinder head, the cold iron is arranged in the fire face area of the cylinder head to give priority to the solidification of the fire face area, but due to the uneven wall thickness in the fire face area of the cylinder head resulting in different solidification rates.The optimised solution for the cylinder head body shows that hot tearing occurs at the intersection of the high-temperature region and the lowtemperature region.At the edge of the maximum principal stress.Therefore, this solution chooses to eliminate the cold iron in the inter-fireface region, adjusts the solidification sequence in the cylinder head fireface region, expands the intersection of the high-temperature region and the low-temperature region, so that the maximum main stress in the edge region of the force area is expanded.The cylinder head casting process optimises the structure of the cooling device on the fire face, from a circular cold iron structure with exhaust slots and triangular-shaped cooling devices on both sides, as in

Figure 10 .
Figure 10.Stress field simulation results for cylinder head body optimisation solution (a) Solidification time 56s (b) Solidification time 69s.

Figure 11 .
Figure 11.Cold iron distribution on the fire face of the cylinder head (a) Original scheme (b) Optimised cold iron scheme.

Figure 11 (
Figure 11(a), to a semi-circular cold iron structure with exhaust slots and both sides of the cooling device with the tip of the sharp corner removed, as in Figure 11(b).Figure12shows the simulation results of the optimised cold iron solution for the cylinder head temperature field.Figure13shows the simulation results of the optimised cold iron solution for the cylinder head stress field.It can be seen from the figure that the optimisation of the cold iron structure can adjust the solidification sequence of the cylinder head fire face area, improve the distribution of the maximum stress and peak value of the cylinder head fire face central

Figure 12 .
Figure 12.Temperature field simulation results for optimised cold iron solution for cylinder head (a) Solidification time 46s (b) Solidification time 62s.

Figure 13 .
Figure 13.Stress field simulation results for optimised cold iron solution for cylinder head (a) Solidification time 56s (b) Solidification time 69s.

Figure 14 .
Figure 14.Hot tearing simulation results for optimised cold iron solution for cylinder heads (a) Solidification time 54s (b) Solidification time 64s

Figure 15 .
Figure 15.Optimising cold iron solutions for physical casting.

Figure 16 .
Figure 16.Optimising X-ray inspection of cold iron solution castings.