A door assembly for a vehicle is provided. The door assembly includes an inner panel and an outer panel which are joined together to define a central space therebetween. The central space has a front which faces towards the front of a vehicle and a back which faces towards a back of the vehicle. Both the inner and outer panels are made of aluminum, and at least one reinforcement beam made of steel is disposed in the central space between the aluminum inner and outer panels to provide the door assembly with increased impact resistance.