Investigation of fast diffusion bonding of aluminium alloys for extrusion-based additive manufacturing
File(s)
Author(s)
Altiparmak, Sadettin Cem
Type
Thesis
Abstract
The increasing demands on engineering components, which must endure challenging conditions without generating defects, require the use of highly reliable joining techniques. In this regard, solid-state joining techniques have the advantage that they allow joining of metals at temperatures below their melting points and thereby avoid melting-based metallurgical defects such as hot cracking. Diffusion bonding, which is a solid-state joining technique, is widely applied for joining aluminium (Al) and Al alloy components, particularly those with intricate geometries, such as heat exchangers including complex multiple internal channels. Diffusion bonding is a slow process characterised by long holding times, typically varying between 30 minutes and 2 hours. As an alternative to the production of highly complex components using diffusion bonding, extrusion-based additive manufacturing (AM) of Al and Al alloy components can be used to shorten production times, thanks to the design freedom offered by AM technology. From the material perspective, Al and Al alloys are highly susceptible to oxidation because of the tendency of these metals to reaction with oxygen leading to chemically stable thin aluminium oxide (Al2O3) scale on their surfaces, particularly at high processing temperatures. In diffusion bonding and extrusion-based AM processes, the Al2O3 scale generated on the surfaces to be joined restricts the degree of metal-to-metal contact, which reduces the bonding quality of the components. Techniques developed to mitigate or eliminate the formation of Al2O3 scale in the diffusion bonding and extrusion-based AM of Al and Al alloys are detailed in the thesis; however, this problem has not yet been fully eliminated.
The current research therefore proposes a different technique entitled the ‘‘fast diffusion bonding technique’’ to mitigate or eliminate the Al2O3 scale forming in the diffusion bonding of Al alloys and improve the bonding quality of Al alloy joints. The proposed technique facilitates diffusion bonding in short holding times. The proposed technique could be combined with an extrusion-based AM process using a specific design after conducting a fully extrusion-based AM-focused research to hybridise the AM technique. A test rig was designed and built on a hydraulic press with an associate furnace to investigate the varying diffusion bonding conditions of similar Al alloys using an abrasive file to mechanically scrape the surfaces to be joined. Individual and combined temperature-pressure-time bonding curves of AA7075-T6 and AA6061 were generated from results obtained from slightly over 100 fast diffusion bonding experiments. It was found that the proposed technique improved the bonding strength of similar AA7075-T6 and AA6061 joints by 11.75% and 11.38%, respectively, with respect to those of samples bonded under otherwise the same conditions in which the proposed technique was not applied. A mathematical model was then proposed to predict the bonding time required in the diffusion bonding of similar AA7075-T6; this includes a treatment of the presence and deformation mechanisms of the Al2O3 scale to achieve more accurate prediction of bonding time. The conclusions of the current work were that: (i) the proposed fast diffusion bonding technique is beneficial for improving the bonding strength of the Al alloy joint, (ii) the new technique is successful in eliminating residual microvoids in the bonding interfaces, resulting in higher-quality bonding, as evidenced by less visible bonding lines, and (iii) the model is capable of predicting the bonding time required in the diffusion bonding of similar Al alloys.
The current research therefore proposes a different technique entitled the ‘‘fast diffusion bonding technique’’ to mitigate or eliminate the Al2O3 scale forming in the diffusion bonding of Al alloys and improve the bonding quality of Al alloy joints. The proposed technique facilitates diffusion bonding in short holding times. The proposed technique could be combined with an extrusion-based AM process using a specific design after conducting a fully extrusion-based AM-focused research to hybridise the AM technique. A test rig was designed and built on a hydraulic press with an associate furnace to investigate the varying diffusion bonding conditions of similar Al alloys using an abrasive file to mechanically scrape the surfaces to be joined. Individual and combined temperature-pressure-time bonding curves of AA7075-T6 and AA6061 were generated from results obtained from slightly over 100 fast diffusion bonding experiments. It was found that the proposed technique improved the bonding strength of similar AA7075-T6 and AA6061 joints by 11.75% and 11.38%, respectively, with respect to those of samples bonded under otherwise the same conditions in which the proposed technique was not applied. A mathematical model was then proposed to predict the bonding time required in the diffusion bonding of similar AA7075-T6; this includes a treatment of the presence and deformation mechanisms of the Al2O3 scale to achieve more accurate prediction of bonding time. The conclusions of the current work were that: (i) the proposed fast diffusion bonding technique is beneficial for improving the bonding strength of the Al alloy joint, (ii) the new technique is successful in eliminating residual microvoids in the bonding interfaces, resulting in higher-quality bonding, as evidenced by less visible bonding lines, and (iii) the model is capable of predicting the bonding time required in the diffusion bonding of similar Al alloys.
Version
Open Access
Date Issued
2023-05-18
Date Awarded
01/08/2023
License URL
Advisor
Shi, Zhusheng
Lin, Jianguo
Sponsor
Turkey. Millî Eğitim Bakanlığı
Imperial College London
Publisher Department
Department of Mechanical Engineering
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)