Good link
"The first AK-47s were stamped receivers. They were strong and easy to build. But in an attempt to improve their strength the Russians turned to a milled receiver for many years. The labor and material required to make a milled receiver were too much and the stamped receiver was again adopted. There is some debate over which is stronger or better, and both have advantages and weaknesses.
Milled Receivers are strong, will not twist, should last for many years of shooting. They also cost more, and are heavy. They have fewer stock options and few parts kits are available for the milled receivers.
Stamped Receivers are much more common. Are easy to build on and make yourself. Can be very strong. They can twist and warp with stress. Rivets, welds and screws can work loose."