In this study, it is reported that the diffusion welding of α-titanium and (α + β) titanium alloys with aluminium has been carried out by applying sufficient compression to the welded samples to give a high level of plastic deformation in the Al layers sandwiched between layers of titanium alloy with a moderate surface finish. Both metals are active in the air; such diffusion processes can take place under vacuum. However, we report, that high-quality welds were achieved at temperatures between 610 and 630 °C under ambient air atmosphere, without the use of vacuum or reducing atmospheres. The resulting microstructures and intermetallic phases were characterised using scanning and transmission electron microscopy and EDX microanalysis. This showed that under the welding parameters selected, Ti diffused across the interface into Al and Al into Ti, since the protective Al2O3 thin film on the surface of aluminium layer has been disrupted due to the Al layer plastic deformation. Impact tests of welds gave relatively high energy impact values.