When you weld a metal, the heat causes the grains within the metal to grow, known as grain growth. When a weld fails you will find it is not the actual weld that is failing but the heat efected area, that is the area right against the weld. As the grains grow with the efect of heat it makes that area brittle. It is that efect that causes the metal to fail. If you were to anneal the wheel after welding that will reduce the size of he metal grains, that will releive the stresses within the metal and make it more stable. When a new wheel is made it is manufactured from flat steel plate that is stamped and pressed to shape, this will produse work hardening within the metal but this is found to be acceptable and the usual practice. Even if the company producing the wheel is unknown the metal will have been produced in a metal foundry from a known metal. This will be a low carbon steel (ordinary metal to you and i). On a car there is many places you can weld but a wheel is not one i would risk, it is not the speed you go but the up and down pounding the wheels are subject to during the normal driving day, that is without even going off road and remember these are off road cars