Amir A. Shirzadi
King’s College
Cambridge
Cambridge
Abstract
Development of a suitable joining technique for advanced aluminium alloys and composites will enable them to be more widely used. The aim of this Ph.D. research was to develop new joining methods for these materials for which conventional welding methods have been unsuccessful. The research led to six new bonding methods and also to an analytical model which may be applicable to all transient liquid phase (TLP) bonding processes. In the early stage of the research, two new methods for TLP diffusion bonding of aluminium-based composites (aluminium alloys with silicon carbide particles as reinforcement) were developed. The methods were based on applying isostatic pressure (rather than conventional uniaxial compression), and bonds were fabricated with shear strengths as high as 242 MPa which is 92% of the shear strength of the parent material.
This value is far greater than the highest bond strength reported to date for these aluminium-based composites. Based on simple finite element analysis modelling, a third method was developed which allows the joining of superplastic alloys/composites with minimal deformation. This method is based on a combination of conventional TLP diffusion bonding and hot isostatic pressing without encapsulation. It allows the fabrication of intricate parts with virtually no deformation during the bonding process so that dimensional tolerances are
preserved. A fourth method, which is based on a new approach to TLP diffusion bonding by introducing a temperature gradient, is capable of producing reliable bonds with shear strengths as high as those of the parent alloys. The use of this method led to the formation of non-planar interfaces, compared to planar interfaces associated with conventional diffusion bonding methods. Therefore the strength and reliability of the bonds, made using this method, were improved considerably. This fourth method has already been patented in the United Kingdom (UK 9709167.2). A comprehensive analytical model was developed to predict the bonding time and the microstructure of the bond line when using temperature gradient TLP diffusion bonding. The model has been experimentally verified. The model also may be applicable to all TLP diffusion bonding approaches. The fifth method, developed in the current research, is capable of fabricating reliable bonds in air with shear strengths as high as 90% of those of the parent material. This is the highest bond strength, reported to date, for diffusion-bonded aluminium joints made in air. The approach overcomes the limitations associated with bonding aluminiumbased materials in vacuum; this has been a major restriction in exploitation of the process. Patent protection has been sought (UK 9811860.7). Based on a combination of the fourth and fifth methods, a sixth method is proposed to improve the reliability of bonds made in air (temperature gradient TLP diffusion bonding in air). Preliminary results are very promising and some suggestions for further work on this method are proposed.
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Development of a suitable joining technique for advanced aluminium alloys and composites will enable them to be more widely used. The aim of this Ph.D. research was to develop new joining methods for these materials for which conventional welding methods have been unsuccessful. The research led to six new bonding methods and also to an analytical model which may be applicable to all transient liquid phase (TLP) bonding processes. In the early stage of the research, two new methods for TLP diffusion bonding of aluminium-based composites (aluminium alloys with silicon carbide particles as reinforcement) were developed. The methods were based on applying isostatic pressure (rather than conventional uniaxial compression), and bonds were fabricated with shear strengths as high as 242 MPa which is 92% of the shear strength of the parent material.
This value is far greater than the highest bond strength reported to date for these aluminium-based composites. Based on simple finite element analysis modelling, a third method was developed which allows the joining of superplastic alloys/composites with minimal deformation. This method is based on a combination of conventional TLP diffusion bonding and hot isostatic pressing without encapsulation. It allows the fabrication of intricate parts with virtually no deformation during the bonding process so that dimensional tolerances are
preserved. A fourth method, which is based on a new approach to TLP diffusion bonding by introducing a temperature gradient, is capable of producing reliable bonds with shear strengths as high as those of the parent alloys. The use of this method led to the formation of non-planar interfaces, compared to planar interfaces associated with conventional diffusion bonding methods. Therefore the strength and reliability of the bonds, made using this method, were improved considerably. This fourth method has already been patented in the United Kingdom (UK 9709167.2). A comprehensive analytical model was developed to predict the bonding time and the microstructure of the bond line when using temperature gradient TLP diffusion bonding. The model has been experimentally verified. The model also may be applicable to all TLP diffusion bonding approaches. The fifth method, developed in the current research, is capable of fabricating reliable bonds in air with shear strengths as high as 90% of those of the parent material. This is the highest bond strength, reported to date, for diffusion-bonded aluminium joints made in air. The approach overcomes the limitations associated with bonding aluminiumbased materials in vacuum; this has been a major restriction in exploitation of the process. Patent protection has been sought (UK 9811860.7). Based on a combination of the fourth and fifth methods, a sixth method is proposed to improve the reliability of bonds made in air (temperature gradient TLP diffusion bonding in air). Preliminary results are very promising and some suggestions for further work on this method are proposed.
Download PDF File of Chapter
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