Correcting the Stress-Strain Curve in Hot Compression Process to High Strain Level
- PDF / 873,404 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 2 Downloads / 177 Views
ODUCTION
BULK deformation processes that involve large strains and a high strain rate such as rolling, forging, and extrusion are usually conducted at temperatures higher than approximately two-thirds the melting point of the material. A fundamental method in investigating the working behavior of materials is analyzing the true stress–true strain curves combined with the microstructure observation, which reflects their intrinsic mechanical properties of materials. However, friction between the material and the tools during the working process have to be considered before further investigation,[1–5] because practical measurements of the deformation curve depart from the real mechanical response of the materials to a certain degree. In practice, for many metalworking processes friction is the predominant factor. The friction between a working piece and the jig can lead to heat generation, wear, pickup and galling of the tool surface, which contributes to the premature failure of the tools.[3–5] Friction increases nonhomogeneous deformation, leading to defects in the finished products. In addition, friction affects the evaluation of the deformation behavior of materials because the flow stress of the sample is strongly influenced by friction; this is especially true in hot compression processes,[6] where the friction is hard to eliminate completely even when the lubricant is added between the sample and anvil surfaces.[7] Therefore, reducing the friction coefficient during the metalworking process and correcting the deformation curve for real behavior are very important topics for both researchers and engineers in hot working. Y.P. LI, Researcher, E. ONODERA, Researcher, H. MATSUMOTO, Assistant Professor, and A. CHIBA, Professor, are with the Institute of Materials Research, Tohoku University, Sendai, Japan. Contact e-mail: [email protected] Manuscript submitted May 22, 2008. Article published online February 10, 2009 982—VOLUME 40A, APRIL 2009
Usually, lubricants are added to reduce the friction coefficient. However, this is not always effective especially in large strain deformation process, because the lubricants are only effective up to a limited strain level. At higher strain levels such as those for the cylindrical hot compression process shown in Figure 1, the lateral side of the sample without lubricant came closer to the anvil surface in compression as the strain level increased due to barreling, and it subsequently came into contact with the anvil surface. This lateral side later formed the outer area of the contacting surface when the strain attained a certain level, leading to sticking in the outer area of the sample or a great increase in the friction coefficient based upon some preliminary observations. In practice, directly predicting the friction coefficient at a random strain level is difficult because of the difficulty in measuring related parameters of specimens in the working process; researchers often simply assume that the friction coefficient is a constant or independent of the strain level.[5,8–11] The detai
Data Loading...
