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Transient Liquid phase Bonding of AL2024 Alloy and Evaluation of Joint Properties

Mahmoodi Ghaznavi, Majid | 2010

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 42536 (07)
  4. University: Sharif University of Technology
  5. Department: Materials science and Engineering
  6. Advisor(s): Kokabi, Amir Hossein; Ekrami, Ali Akbar
  7. Abstract:
  8. Transient liquid phase (TLP) bonding of Al2024-T6 alloy, using gallium interlayer, has been investigated. Bonding experiments were carried out using two basic methods; Conventional and Temperature gradient TLP bonding. Optimum values of isothermal solidification temperature and time, pressure, surface roughness, heating rate and homogenizing conditions were determined in conventional TLP bonding experiments. Aluminum oxide layer was not removed before TLP bonding to prevent gallium attack.this layer is crashed during bonding. Al2O3 particles are not soluble in the liquid and are pushed into liquid by solidification front progression. These particles are accumulated in grain boundaries and in the centerline of bonding zone. As a result, in this method, the boundary between two Al2024 specimens was not fully eliminated during bonding because of solidification with planar front presence of oxide particles. Depletion of bonding zone of copper is other important problem in conventional TLP bonding. Results showed that in the best cases, tensile strength of the bond is 220 MPa , which is less than 50% of base metal strength. These results illustrated that conventional TLP bonding of Al2024-T6 alloy using gallium interlayer is not a suitable process for joining this alloy, and basic changes must be carried out in the process. TLP bonding under temperature gradient offered very good results in bonding of Al2024. In this method, solidification mechanism changes from planar to dendritic and tensile and shear strength of joint increased to about 460 and 220 MPa respectively. Microstructure of bonding zone changed basically by changing of solidification mechanism. In This method, aluminum oxide particles distributed between dendrites and did not accumulate locally (special in grain boundaries) and a joint with microstructure and chemical analysis similar to base metal can be achieved. When the surface of solid Al was wet by liquid Ga, the Ga spontaneously penetrated and replaced the grain boundaries. Calculation of isothermal solidification time using Fick’s laws demonstrated that presence of grain boundaries result in this time to reduce from 600 hr to 4 hr. However, despite these attractive attributes of gallium, exposure of aluminum to excess liquid gallium can result in severe embrittlement of aluminum. Fatigue tests showed that formation of microscopic cracks in grain boundaries reduced fatigue life of the joints considerably compare to the base metal
  9. Keywords:
  10. Aluminum Alloy 2024 ; Temperature Gradient ; Fatigue ; Grain Boundary Diffusion ; Liquid Metal Embrittlement ; Gallium Arsenaide ; Transient Liquid Phase (TLP)

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