Stresstech Bulletin 8
Text: Murat Deveci, figures: Stresstech
Machining is the general term for turning, milling, drilling, planing and grinding. The common point of these applications is the cutting process that heats the surface of the material due to the friction and forms chips. The figure below shows the process steps in machining operations that form residual stresses.
The existing residual stress state of the component may be changed due to mechanical, thermal, or chemical factors. The mechanical factor is the material removal process itself, the thermal factor is the work done by friction, and the chemical factor is the possible reactions caused by cutting fluids.
Mechanical effects of the machining processes change the surface finish and microstructure. Plastic deformations are observed as well.
Thermal effects are created by the process causing a change in the dislocation density and distribution as well as surface integrity. In addition, pressure and the temperature of the cooling are important effects which contribute to formation of residual stresses.
The combination of these effects together with other parameters of the machining may form cracks on the surface of the component. The cracks are the consequence of residual stresses which are formed because of plastic deformation and possible phase changes. Phase changes are caused by local heating. Local heat increases due to friction and other abrasive effects of the machining. Tensile residual stresses are formed partially because of local heating. Machining residual stresses affect the reliability and lifetime of the component.
Machining residual stresses are affected by thermal and mechanical factors. Usually, surface and near-surface layers of the component are affected by both thermal, and mechanical factors, and subsurface layers are affected by mechanical factors only.
Milling is another common chip-forming machining process which induces residual stresses on the workpiece. It’s known that when a plain carbon steel milled in different ways, the resultant residual stress depth profiles could be different, depending on the way of milling.
It is also known that the changes in cutting speeds would change the residual stress depth profiles. Usually, the lowest cutting speed also produces the lowest residual stress and the highest cutting speed which causes a temperature increase between the tool and workpiece produces the highest residual stress on workpiece.
Residual stresses induced during the machining operations can lead to lower fatigue life hence they should be analyzed and controlled. It should be also noted that in practice, various machining operations could be used together. Even though different processes will induce their own residual stress profiles on the workpieces, it is common that the very last machining operation’s residual stress profile will be dominant.
Have questions about residual stress measurement?
Advances in surface treatments: technology, applications, effects. Vol. 4, Residual stresses, A Niku-Lari, Oxford: Pergamon, 1987.
Residual stress modeling in machining processes, Jiann-Cherng Su, Atlanta, Ga. : Georgia Institute of Technology, 2006.
Machining-Induced Residual Stress: Experimentation and Modeling, Kurt Jacobus; R E DeVor; S G Kapoor, Journal of Manufacturing Science and Engineering, v122 n1 (2000): 20
Milling induced residual stresses in structural parts out of forged aluminium alloys, B Denkena; L de Leon, International Journal of Machining and Machinability of Materials, 4, no. 4, (2008): 335-344