Owing to their high specific strength aluminium metal–matrix composites (Al-MMC) are becoming popular for many critical engineering applications. These materials are prepared by controlled melting and solidification processing. In order to broaden their applications further, it is necessary to develop suitable joining techniques. Due to the metallurgical nature of these materials, conventional fusion welding techniques such as arc welding cannot be used. Diffusion bonding is one of the processes that has the potential for the joining of these materials without degrading their properties. At present, these techniques are used in vacuum, which is a time-consuming and expensive operation. It also puts a limit on the size of the component that can be bonded successfully. Therefore, a novel technique of vacuum-free diffusion bonding was investigated, where bonding was achieved in the open air with the aid of in situ surface treatment. The aim of this treatment was to disrupt and disperse the oxide layer and maintain a clean interface during diffusion bonding. The results of the investigation have shown that a joint efficiency of 90% and a percentage bonded area of over 75% can be achieved. This study involves the determination of the experimental set-up and the evaluation of the diffusion-bonding parameters. Metallographic studies and other evaluations show that this in situ surface treatment process is able to provide a clean joining surface despite the presence of a tenacious oxide layer.