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Aluminum Alloy with High Magnesium Content: Casting Studies for Microstructural Evolution and Bifi lm Index Studies with Diff erent Alloying Elements

Kamil Armağan Gül¹, Özen Gürsoy², Eyüp Sabri Kayali³, Eray Erzi², Derya Dışpınar²

¹OYAK-Renault Automobile Factories A.S., RnD Center, Bursa, Turkey²Istanbul University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Istanbul, Turkey

³Istanbul Technical University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Istanbul, Turkey

Abstract

The use of lighter, more robust and highly recyclable materials are now widespread strategy when compared to existing materials. In the present study, casting of Aluminum Alloy with High Magnesium Content has been performed with addition of Boron, Strontium and Titanium elements. Castability and Bi-film index evolution have been studied.Traditional gravity casting have been selected as the casting method. Effect of temperature and constituents into spiral mold casting has been studied to assess flow behavior Solidification under vacuum conditions have been performed to evaluate bifilm index and metal oxide layers which are entrapped within the structure during casting and soldification. Tensile tests have been performed to evaluate mechanical strenght of the manufactured alloys.

1. Introduction

In order to study high content magnesium alloy and alloying elements effect, bifilm index studies and microstructural changes must be evaluated jointly.

According to Campbell, pore formation is due to oxide films within the metal melt. They are the source of pore formation and property degradation eventually failure of the structure [1]

Characterization of melt is important to assess bifilm index and subsequent material performance.Because melt quality and pore formation strongly influence subsequent property evolution of the material [2].

Reduced Pressure Test (RPT) for Bifilm Assessment is a technique which provide assesment for the numbers and size of the bifilms in the melt.[3] Therefore mechanical

property decrease can be explained if it is due to metal melt, bifilms or due to other factors

Therefore both microstructural studies and RPT experiments have been done for this study. Tensile tests have also been performed to asses mechanical properties.

2. Experimental Procedure

The alloys used in experiments are AlMg1 and AlMg5 base alloys. The addition of strontium , Boron , Titanium and Zirconium have been done after the melting of base alloys.

The mold patterns used in the research are gravitational casting type molds. First mold is cyclinder type with u-shape pouring tip. In order to study different soldification rates and microstructural evolution, 3-step soldification mold has been used. The dimensions are given in figX.

The alloy used in each experiment was prepared in an electrical resistance melting furnace with a casting temperature of 750 C. A3 type casting laddle has been used.

Degassing and upgassing have not been performed. Although degassing procedure clearly improves melty quality and decrease pore formation and bifilm formation [2] , at this stage of investigation, degassing has not applied.

After a stage of melting , alloying and holding; the alloy is poured into mold cavities after surface has been cleared of oxide layers. Steel molds have been pre-heated to 50degC to prevent fast solidification.

2.1. BiFilm ndex Studies

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For bifilm study and counts cupcake geometry RPT mold has been used to perform casting under vacuum condition.

Specimens have been poured into the RPT machine mold and left to solidify under vacumed conditions for 5 minutes. Bifilm specimens were cast in RPT machine under 2atm pressure drop level.

Figure 1. Molds that has been used for tensile tests and microstructural evaluation.

2.2. Microstructural Studies

Microstructural analysis specimens were cut from the middle and fragments have been molded with cold resins for each cross section of the step geometry. These specimens have been prepared with Metkon rotary grinder/ polisher machine with 400,800,1200, grade SiC grinding papers and polishing paper with silica solution.

2.3. Mechanical Tests

Tensile tests have been done in Instron testing machine with 10-2 strain rate.

3. Results and Discussions

In Table 1 , major alloying scheme has been given for present studies.For AlMg5 alloy derivation TitaniumBoronStrontium addition has been found effective for microstructural refinement, evolution and bi-film index counts. Zirconium addition did not yield that outcome.

Table 1. List of Alloy Complexity of the Castings

Alloys Composition AlMg5 Standart Composition AlMg5 5083 H111 grade AlMgXY X: Boron and Y:

Strontium AlMgX AlMgXT T:Titanium AlMgXZ Z: Zirconium

AlMgXYZTW W: Antimony Table 2. Spiral Mold Fludity Values

Distance Reached

Preheat T 0C

AlMg-BorSr 23 100 AlMg-BorSr 24,5 150 AlMg-BorSr 28,5 200

AlMg - TiBorSr 24 100 AlMg TiBorSrZr 24 100

From spiral mold fluidity studies, addition of Titanium increase fluidity of AlMg4 alloys for same preheat condition when compared with Boron Strontium alloyed counterpart.Zirconium addition did not give better results in terms of fluidity.It must be taken into account that fluidity study has been conducted with a permanent steel mold.

As given in table 3, only Boron and Boron Strontium addition has been ruled out due high bi-film index and formation of metal oxide layers and porosities during solidification

Table 3. Bi-Film Index Counts and Numbers

AlMg5 + Bi-Film Index Number

Bor 416 496

Bor 322 349

BorSr 355 172

BorSr 205 252

BSrTi 44 69

BSrTi 162 244

BSrTi 65 84

BSrTi 107 150

Figure 2.AlMgBor RPT specimen Cross Section

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Figure 3. AlMgBSr RPT specimen Cross Section

Figure 4. AlMgBorSrTi RPT specimen Cross Section

Refinement of solidification structure can be seen through Figure 2-4. It can be seen from the Figure 5 and Figure 6 that Titanium addition significantly change microstructrue and provides grain refinement.

Figure5. Effect of Boron-Strontium in spiral mold cross section

Figure 6 :Effect of Titanium-boron-Strontium in spiral mold cross section

Effect of Ti and Zr on AlMgBoron microstructure has been given in Figure 7 to 9

Alloying elements had no significant effect for Yield strength and tensile strength values for AlMg alloy.

Elongation and Ductility has been improved compared to pure AlMg and AlMgBoron alloy.

It has been also seen upon inspected fracture surfaces that tensile test specimens have failed abruptly at porosity and stress intensity locations.

Figure 7. AlMgB Microstructure

Figure 8. AlMgBTiZr Microstructure

Figure 9.AlMgBorTiSr

YS, Tensile % El. at break

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AlMg4B 82 151 6,7

AlMg4BSr 84,5 140,5 6.35

AlMg4BZr 75 149,5 10,5

AlMg4BSrTi 81,3 157,6 13,45

AlMg4BSrZr 80 147 8,25

In order to observe melt quality effect with higher alloying effect, in the second step, industrial grade 5083 H111 alloy has been used as base AlMg5 source. Titanium and Zirconium amount have been increased up to 0,75% weight percentage. To protect Magnesium Antimony has been added at %0,4 [x].

Figure 10.BSrTiSn RPT specimen Cross Section

Figure 11.BSrZrSn RPT specimen Cross Section

Figure 12.BSrTiZrSn RPT specimen Cross Section

Figure 10 shows only Ti addtion, Figure 11 shows only Zr addition and Fig 12shows both Ti and Zr addition to AlMg alloy.

Table 5. Bi-Film Index Counts and Numbers

AlMg5 + Bi-Film Index Number

BSrSnTi 230,9 159

BSrSnZr 268 182

BSrSnTiZr 182,8 183

According to Figure 7-9 and Table 5, alloying a high magnesium content alloy with high Titanium and Zirconium content yielded similar Bi-film index counts and numbers when compared alloyed specimens in table 3. This is attributed to the importance of melt quality of base alloy.

Another important point is to apply degassing and non-traditional casting which reduces turbulent flow of melt.

4. Conclusion

Bi-film count is decreased with alloying elements to Al-Mg5 as shown in table 2. Titanium and Zirconium is successful alloying options.Table 5 shows similar Bi-film index values despite higher Ti and Zr content addition.

Melt quality is important factor when high content master alloying is desired.

Tensile tests shows lower experimental values than prediction. As specimens have been fabricated via conventional gravity casting into the steel molds, these low values have been attributed to faster cooling rates , porosity formation and casting defects which cannot be prevented thus causing failure.

Higher Magnesium content for Alluminum alloys requires degassing and more uniform/ slow cooling rates. Otherwise porosities and metal films will stay in the microstructure causing abrupt failure.

For high magnesium and hgh master alloy content casting studies, sand Casting, degassing, sustaining non turbulent metal flow, low pressure casting must be taken into account if good casting quality is desired.

Acknowledgment A. Gul is supported by TUBITAK- BIDEB. 2211-/A national fellowship program for PhD studies. Doc.Dr Murat Baydo an from ITU Metallurgy and Material Departement is highly acknowledged for his support in mechanical tests. Seykoc Aluminum A. . is aknowledged for their support in raw material supply.

References

[1] Campbell, J. Entrainment defects. Mater. Sci. Technol. 2006, 22, 127–145 [2]Uludag,M.;Çetin,R.,Di pinar,D.,Tiryakioglu,M.Characterization of the Effect of Melt Treatments on Melt Quality in Al-7wt %Si-Mg Alloys. Metals 2017,(5),157. [3]Dispinar, D.; Campbell, J. Use of bifilm index as an assessment of liquid metal quality. Int. J. Cast Met. Res. 2006,19, 5–17.