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1040 IMMC 2016 | 18th International Metallurgy & Materials Congress

Th e Eff ects of High Pressure Die Casting in Th in Sections of Heat Treated 7075 Aluminum Alloy

Tayfun Durmaz, Ali Kalkanlı

Middle East Technical University - Türkiye

Abstract

High strength 7075 aluminum alloy is mostly used in automotive, defense and aerospace industries due to their superior and promising physical and mechanical features. In this study, the effects of high pressure die casting method in thin sections were investigated for as-cast and heat treated 7075 aluminum alloy. The aim of this study is mainly research microstructures of thin sections in order to be able to see the effects of high pressure die casting (HPDC) method in terms of grain size and hardness. Moreover, porosity percentage was also determined with the help of special software. Thus, work specimens were prepared and characterized. After that, obtained information was evaluated for different heat treatment conditions for thin sections.

1. Introduction

According to the literature, high pressure die casting can be seen as fast casting method in which liquid metal is injected to the mold cavity [1]. This means that high kinetic energy of liquid metal is used during this casting process. HPDC can be also seen as a kind of permanent mold casting [2]. During this process, high pressure is applied with high speed thanks to some special casting machines. Moreover, pressure is also applied during solidification processes. This method is used to create large surface areas with good surface properties for intricate casting parts [2]. Nevertheless, there are some disadvantage of HPDC. The first one is that both of high pressure die casting machine and its dies are pretty expensive. That is why, they are generally preferred for mass production. The other disadvantage is that this process brings about turbulent filling in the mold [2]. Therefore, turbulent flow causes gas inclusions and porosity in the casting parts. This situation can be also problematic during heat treatment processes since gas inclusions and porosities turn to blisters when different heat treatments are applied. In order to prevent this problem, some vacuum equipment are used together with this machine. This means that if

there is no vacuum process, it is expected to see porosity problem.

Aluminum alloys are generally preferred for high pressure die casting method. In this study, aluminum 7075 alloy was used. This allow is a good candidate for defense and aerospace applications due to its high mechanical properties. There are some different heat treatments for this alloy in order to improve its mechanical properties. During this study, T6 heat treatment was applied to aluminum alloy after HPDC process. T6 heat treatment includes solutionizing, quenching and aging stages respectively.

2. Experimental Studies

In this study, horizontal high pressure die casting machine was used in the Department of Metallurgical and Materials Engineering of Middle East Technical University without vacuum unit as it can be seen in Figure 1. This machine can apply 15 tons injection whereas, it can also work with 200 tons capacity of mold closure.

Figure 1. High Pressure Die Casting Machine

During this process, a special metal mold was used in order to produce casting parts with thin sections. During casting, metal mold was preheated up to 250oC. 6 different specimens were produced like in Figure 2. One of them are used to determine microstructure of as-cast part. For other 5 specimens, T6 heat treatment was applied with different solutionizing temperatures.

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465, 470, 475, 480 and 485oC temperatures were chosen as solutionizing temperatures and it applied during 90 minutes respectively. After that, these specimens were quenched in the water and all of these 5 specimens were aged in the the furnace. Aging step was applied at 120oC during 24 hours. By this way, T6 heat treatment was completed.

Figure 2. Produced Specimens

Figure 3. Specimen Preparation

After all of production and heat treatment processes, each specimens were cut as in Figure 3 in order to be able to obtain the thinnest sections. The thickness of these sections is approximately 3.5mm for each of specimens. These specimens were prepared for metallographic examination and their microstructures were determined by the help of optic microscope. Grain size determination were done with a special software according to ASTM standard. With the same program, porosity percentage was also measured. After that, micro hardness values were measured. Even if determination of micro hardness values is not so suitable for this casting parts, it was also used to get some information about heat treated aluminum alloys.

3. Results and Discussion

After T6 heat treatment with different solutionizing temperatures, these microstructures were obtained as Figures 4, 5, 6, 7, 8 and 9. As it can be seen in these figures, cellular microstructures exist as it was expected because of low thermal gradient of HPDC

method. After those microstructures was taken, their grain sizes were measured. For each microstructures, 20 different grain size values were taken with software in order to get better results statistically.

Figure 4. Microstructure of As Cast 7075 Aluminum

Figure 5. Microstructure of T6 Heat Treated 7075 Aluminum Alloy with 465oC Solutionizing Temperature

Figure 6. Microstructure of T6 Heat Treated 7075 Aluminum Alloy with 470oC Solutionizing Temperature

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Figure 7. Microstructure of T6 Heat Treated 7075 Aluminum Alloy with 475oC Solutionizing Temperature

Figure 8. Microstructure of T6 Heat Treated 7075 Aluminum Alloy with 480oC Solutionizing Temperature

Figure 9. Microstructure of T6 Heat Treated 7075 Aluminum Alloy with 485oC Solutionizing Temperature

Table 1. Results of Grain Size Measurement for T6 Heat Treatment with Different Solutionizing Temperatures

Figure 10. Average Grain Size according to T6 Heat Treatment Solutionizing Temperatures

When the results of grain size measurement were evaluated, it was seen that the lowest grain size values were determined in T6 heat treatment with 475oC solutionizing temperature. Therefore, if it is taken into consideration that lowest grain size gives better mechanical properties, it can be said that 475oC is the better T6 heat treatment condition for 7075 aluminum alloy.

More importantly, the average grain size values for all specimens are really low in thin section due to the effects of HPDC method. In the literature, while the average grain size of heat treated 7075 aluminum alloy is determined in between 120 to 150 micron; these values were determined in between 30 to 50 micron in this study. Therefore, it is able to be stated that HPDC decrease grain size in thin sections drastically.

1 26.85 73.65 43.80 36.8 36.8 52.102 30.95 52.10 30.95 26.85 43.8 52.103 26.85 73.65 43.80 43.8 36.8 36.804 30.95 43.80 30.95 36.8 43.8 52.105 36.80 36.80 52.10 30.95 30.95 43.806 26.85 36.80 43.80 36.8 36.8 43.807 30.95 43.80 30.95 30.95 52.1 52.108 30.95 43.80 36.80 30.95 30.95 61.959 30.95 21.90 36.80 26.85 52.1 52.10

10 21.90 43.80 43.80 30.95 36.8 52.1011 30.95 30.95 26.85 26.85 52.1 43.8012 26.85 43.80 30.95 30.95 30.95 43.8013 21.90 43.80 36.80 30.95 30.95 30.9514 26.85 43.80 36.80 36.8 30.95 61.9515 30.95 36.80 30.95 36.8 43.8 52.1016 26.85 36.80 43.80 36.8 36.8 36.8017 26.85 30.95 36.80 36.8 36.8 52.1018 26.85 36.80 43.80 30.95 30.95 52.1019 30.95 36.80 43.80 30.95 30.95 30.9520 26.85 30.95 36.80 30.95 43.8 30.95

Aveg.* 28.49 42.08 38.06 33.03 38.45 46.72*All values were measured as "micron", HT (Heat Treatment)

As Cast T6 HT with 465°C Soln T.

T6 HT with 470°C Soln T.

T6 HT with 475°C Soln T.

T6 HT with 480°C Soln T.

T6 HT with 485°C Soln T.

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After grain size measurement, micro hardness values were also determined. Even if it was thought that micro hardness values are not going to be so accurate for this alloy because of high precipitation concentration in the microstructure, all of micro hardness values were also checked. According to this results, hardness values were consistent. Although it was expected to see the highest average hardness value for T6 heat treatment with 475oC solutionizing temperature due to the lowest grain size, average hardness values of T6 heat treatment for 465 and 480oC solutionizing temperatures were high as well. The reason of this situation can be related to the regions in which hardness values were taken because, precipitates can be delusive if hardness values were taken from their surface.

Table 2. Results of Hardness Measurement for T6 Heat Treatmen with Different Solutionizing Temperatures

Figure 11. Average Hardness Values according to T6 Heat Treatment Solutionizing Temperatures

After all of these examinations, porosity percentage was also determined by phase analysis ability of a software. As it is known, porosity and gas inclusions were expected in HPDC method. During this examinations, porosities were received as a different phase. By this way, their percentages were determined. There Phase analysis can be seen in figures 12, 13, 14, 15, 16 and 17. Moreover, their porosity percentages were measured as Table 3. There images were taken from polished surfaces without etching.

Figure 12. Phase Analysis of As Cast 7075 Aluminum Alloy

Figure 13. Phase Analysis of T6 Heat Treated 7075 Aluminum Alloy with 465oC Solutionizing Temperature

Figure 14. Phase Analysis of T6 Heat Treated 7075 Aluminum Alloy with 470oC Solutionizing Temperature

Figure 15. Phase Analysis of T6 Heat Treated 7075 Aluminum Alloy with 475oC Solutionizing Temperature

Figure 16. Phase Analysis of T6 Heat Treated 7075 Aluminum Alloy with 480oC Solutionizing Temperature

1 73 167 138 167 171 1492 72 161 141 165 169 1563 89 159 149 171 166 1574 74 176 141 165 167 1615 83 174 146 169 169 165

Aveg.* 78.2 167.4 143 167.4 168.4 157.6*All values were determined in Brinell scale, HT (Heat Treatment)

As Cast T6 HT with 465°C Soln T.

T6 HT with 470°C Soln T.

T6 HT with 475°C Soln T.

T6 HT with 480°C Soln T.

T6 HT with 485°C Soln T.

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Figure 17. Phase Analysis of T6 Heat Treated 7075 Aluminum Alloy with 485oC Solutionizing Temperature

Table 3. Phase Percentages for T6 Heat Treated Specimens with Different Solutionizing Temperatures

When these values are investigated, percentage of porosity was determined about 4% for each specimens. This value can be considered as normal for high pressure die casting method because of turbulent flow in the mold.

4. Conclusion

In this study, the effects of high pressure die casting in thin sections of heat treated 7075 aluminum alloy was investigated. During this researches, not only the effects of HPDC in microstructures was determined, but also the effects of heat treatment in hardness values were also investigated. At the end of this study, the results were evaluated. According to these results:

HPDC causes low grain size in thin sections due to low thermal gradient and high undercooling value. When thermal gradient is low and undercooling becomes high, cellular structures are expected to see in the microstructures. As it was seen in the all microstructures, average grain sizes were measured between approximately 30 and 50 micron. It can be considered very low grain size for aluminum alloys.

Due to the turbulent flow in the mold during the movement of molten metal, it is seen some gas inclusions and porosities. Even if there was no blister problem in this study, this situation can also cause blisters in big cast parts during heat treatments.

For T6 heat treatment of 7075 aluminum alloy, 475oC was determined as the most suitable solutionizing temperature in order to obtain the lowest grain size. Even if it cannot

be said the same about hardness values for this study, it was seen that this solutionizing temperature is also good condition for hardness values as well.

5. References

[1] G.E. Totten, D. S. MacKenzie (ed.), Handbook of Aluminum, Vol1, Physical Metallurgy and Processes, Taylor & Francis, New York (2003).

[2] D.G. Altenpohl, Aluminum: Technology, Applications and Environment, A Profile of Modern Metal, The Aluminum Association, Inc. (1999).

[3] V.S. Zolotorevsky, N.A. Below, M.V. Glazoff, Casting Aluminum Allooys, Elsevier: Linacre House, Jordan Hill, Oxford (2007).

[4] N. Chawla, K.K. Chawla, Metal Matrix Composites, Springer Science and Media, Inc (2006).

[5] T.P.D. Rajan, R.M. Pillai, B.C. Pai, Reinforcement Coating and Interfaces in Aluminum Metal Matrix Composites, Journal of MaterialScience 33 (1998) 3491-3503.

[6] ASM Special Handbook, Aluminum and Aluminum Alloys, (1999).

[7] K. Doherty, R. Squillacioti, B. Cheeseman, B. Placzankis, D. Gallardy, Expanding the Availability of Light weight Aluminum Alloy Armor Plate Produced from Detailed Military Specifications, 13th International Conference on AluminumAlloys (ICAA13), pp. 541-546, Pittsburgh, PA (2012).

[8] Y.H. Zhao, X.Z. Liao, Z. Jin, R.Z. Valiev, Y.T. Zhu, Microstructures and Mechanical Properties of Ultrafine Grained 7075 Al Alloy Processed by ECAP and Their Evolutions During Annealing, Acta Materialia 52 pp 4589-4599 (2004).

[9] L. Sang-Yong, L. Hung-Hwan, L. Yong-Seon, Characterization of Al 7075 Alloys after Cold Working and Heating in the Semi-Solid Temperature Range, Journal of Materials Processing Technology 111 pp 42-47 (2001).

[10] T. Durmaz, A. Kalkanl , S k t rma ve Kokil Döküm Yöntemleri ile Üretilmi Alüminyum 7075 ve 7085 Ala mlar n n Termo-Mekanik lemleri, 7. Alüminyum Sempozyumu, stanbul (2015).

Phase 1 95.826 96.415 95.876 95.51 95.171 95.169Porosity 4.174 3.585 4.124 4.49 4.829 4.831

T6 HT with 485°C Soln T.

*These values were determined as %Area, HT (Heat Treatment)

As Cast T6 HT with 465°C Soln T.

T6 HT with 470°C Soln T.

T6 HT with 475°C Soln T.

T6 HT with 480°C Soln T.