Dr. Thaddeus C. Radzilowski, of Farmington Hills, president of the Piast Institute, a national research center devoted to Polish and Polish-American affairs, died Friday, July 20 at age 80.Visitation will be from 3 p.m. to 9 p.m. Tuesday, July 24, at Heeney Sundquist Funeral Home, 23720 Farmington Road in Farmington.