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Wear Behavior of Al-Fe-Cu Composite Produced by Friction Stir Processing

Karimi, Mehran | 2017

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 50108 (07)
  4. University: Sharif University of Technology
  5. Department: Materials Science and Engineering
  6. Advisor(s): Movahedi, Mojtaba
  7. Abstract:
  8. Composite production is an important method for increasing abrasion resistance in materials. Friction stir processing (FSP) is a solid composite production method which has eliminated the problems of liquid composite production. This research used FSP to study the possibility of in situ production of 1050 aluminum composite with a mixture of copper and iron powder with micrometer sizes of 25-14 and 10 μm, respectively, as reinforcing particles. The following weight percentages of reinforcing powder was added to the sample: 75% iron-25% copper, 50% iron-50% copper and 25% iron-75% copper. In addition, 3 and 5 process passes were considered as another variable in this study. Afterward, the composite with the highest abrasion resistance at 400 °C was subjected to heat treatment in the furnace for 30, 90, and 180 minutes. After producing the composites, their microstructural changes and mechanical properties (abrasion resistance and hardness) were investigated. The results showed that after adding copper and iron reinforcing particles to the aluminum matrix, the abrasion resistance of the sample was almost 5 times that of the processed sample without the reinforcing particles, and its friction coefficient decreased by 40%. On the other hand, composites produced with a powder ratio of 75% to 25% (copper to iron and iron to copper) did not exhibit high abrasion resistance due to the agglomeration of reinforcing particles and their non-uniform distribution. Composites produced with 3 passes (in all powder weight ratios) did not have a good abrasion resistance since the powder particles were agglomerated. Therefore, the best composite in terms of abrasion resistance was the composite with 50% iron-50% copper ratio processed in 5 passes. It was also found that the 50% iron-50% copper sample had the highest hardness in non-heat-treated samples. With increased duration of heat treatment in this sample, the hardness of the stir zone increased as a result of the atomic diffusion and the growth of intermetallic compounds (IMC). Study of worn surfaces showed that the mechanically mixed layer plays a significant role in the abrasion resistance of the samples, and reinforcing particles remain in the matrix during wear provided that they have a good bond with it and significantly promote the abrasion resistance of the samples by protecting the mechanically mixed layer. Defects were observed in the cross-section between the powder and the aluminum matrix which formed due to the lack of time for reaction of the reinforcing particles with the matrix. Afterward, by applying heat treatment for 30 minutes, these defects disappeared as a result of atomic penetration and its abrasion resistance almost doubled and its friction coefficient decreased by about 10% compared to the 50% iron-50% copper sample processed in 5 passes. However, with increased duration of heat treatment to 90 and 180 minutes, due to the large difference in the thermal expansion coefficient of IMCs and the matrix, cracks developed in the reinforcing particles and the abrasion resistance was reduced proportionally. In the course of wear, these cracks prevent the reinforcing particles from bonding with the matrix and these particles were placed between two abrasion surfaces. Therefore, when these particles which had high hardness were between two abrasion surfaces, three-body wear was created which significantly decreased abrasion resistance. On the other hand, with the removal of the particles, the aluminum matrix lacks reinforcing particles making it impossible to protect the mechanically mixed layer. In fact, reinforcing particles have a dual behavior; if they have a good bond with the matrix, they improve abrasion resistance; however, in case of weak bond with the matrix, the abrasion strength of the sample decreases
  9. Keywords:
  10. Microstructure ; Intermetallic Compounds ; In-Situ Composite ; Aluminum ; Heat Treatment ; Friction Stir Welding ; Iron Powder ; Copper Powder ; Wear Behavior

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