During implementing a composite metal part by compaction of an insert having reinforcing fibers in a metal body or container, gas used for compaction may enter the cavity formed in the container for receiving the insert between a lid covering the insert and the container, which can prevent or degrade compaction and diffusion welding of fiber sheaths of the insert therebetween and/or with walls of the cavity. To solve this problem, the present method includes initiating isostatic compaction by a phase including raising and maintaining temperature, followed by a phase including hot-feeding pressurized gas, and machining an assembly to obtain the part. The temperature raising phase includes a diffusion pre-welding of material rigidly connecting the pressure-adjusted walls of the lid and the container. The method can be used for designing parts having a tensile and compression resistance, such as parts for aircraft landing gear.