EVOLUTION AND METHODS OF RESIDUAL STRESSES MEASUREMENT IN AL-SI-CU-MG CASTINGS: ROLE OF HEAT TREATMENT

The first part of this work was to explore the development of residual stresses in I-4 and V-6 engine blocks made of B319.1 alloy using the sectioning technique. The results suggest that there is refinement in the microstructure due to an increase in cooling rate along the cylinder. The developed residual stresses were found to be tensile for both I-4 and V-6 engine blocks. Furthermore, the variation in the developed residual stresses was found to be insignificant. The results also indicate that air cooling produced the highest residual stresses compared to warm water and cold water quenching. Solution heat treatment and freezing (-30 degrees C) led to the maximum amount of residual stress relaxation where 50% of the residual stresses were reduced after the solution heat treatment step. Aging time and aging temperature are directly proportional to the residual stress relaxation. Correlations between alloy hardness and the evolved residual stress have been established. In the second part, residual stresses were obtained from the milled surface of a rectangular workpiece (30 x 172 x 302 mm, five blocks/alloy for five studied alloys). Thirteen layers of material were removed from each block, where each layer consisted of ten paths, and the depth of cut was 1.35 mm, using fresh and dull inserts. In this case, measurement of residual stresses was carried out on the blocks at the end of the milling process, using the X-ray technique. All stresses were of tension-type compared to compression type in the un-machined shoulders. Due to the high applied forces required when using dull inserts, the resulting residual stresses were almost twice that generated by new inserts in spite of the large difference in the machining distance.