The right main landing gear collapsed during a landing roll. The certified flight instructor (CFI) stated the accident occurred during the seventh landing while he and the student were practicing touch and go landings. The CFI stated that they were approximately 3/4 the way down the runway and at 20 miles per hour, when the right main gear collapsed. The airplane then slid off the right side of the runway and came to rest in the grass. The student pilot reported that during the landing roll when approximately 1/2 way down the runway, the landing gear warning horn sounded, and the gear down and locked light flickered on and off. He said they kept the airplane straight on the runway, and as they slowed down the right main gear collapsed. Photographs of the right main gear down lock link were examined by a National Transportation Safety Board metallurgist. The metallurgist reported that the fracture contained the characteristics of an overload failure.