The analysis of forming defects in combination extrusion

• Main Authors: LI, WEN-SHUN, 李文舜
• Other Authors: LIN, HENG-SHENG
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/snx528

碩士 === 國立高雄應用科技大學 === 模具工程系 === 106 === Extrusion process has been broadly used in forming fastener products. There are forward and backward extrusions in its application. The so-called combination extrusion is when the two schemes are used concurrently. The metal flow tends to be complex and beyond prediction in the combination extrusion. Surface cracks and fracturing damage are phenomenal and to be discussed in this work. The die outlet angle and die radius are two essential design guidelines in achieving successful extrusion. However, proper selection of the extrusion ratios becomes essential when the product geometry contradicts the design guidelines. Surface cracks or even fracturing damage would occur when the selection of extrusion ratios violates the range for safe forming. In this study, annealed ASTM A36 structural steel bar with a diameter of 25 mm and a height of 25 mm was used to investigate the forming defects of combination extrusion. Finite element software DEFORM-2D axisymmetric was used for the analysis. The normalized Cockcroft and Latham, Ayada, McClintock and Rice & Tracy damage criteria were used to predict the occurrence of forming defects. The forward extrusion ratio ranged from 1.33 to 4.00 and the backward extrusion ratio ranged from 1.20 to 4.00. The results of simulation were subsequently verified with experiments. The distribution of mean stress was also used to help explain the discrepancies between the damage prediction and experimental verification. The result shows that the damage criteria of Ayada and McClintock can roughly predict the piercing defects of the combination extrusion, except for a small forward extrusion ratio of 1.33. However, the distribution of mean stress can well predict the piercing defect. The piercing mechanism is similar to that of hole punching in sheet stamping. The four damage criteria and mean stress distribution cannot predict well enough for the occurrence of surface cracks. However, Ayada and McClintock damage criteria predict for somehow better than the rest of the two other criteria. Finally, a forming limit chart was constructed from the results of the combination-extrusion experiment. This chart contains three zones namely “safe forming”, “surface cracks” and fracturing damage. The trend shows that safe forming appears when both the forward extrusion ratio (FER) and backward extrusion ratio (BER) are large. Surface cracks occur when both FER and BER are reduced. The cracks would occur at the inner surface of the backward extruded cavity when the FER is less than the BER during the late stage of combination-extrusion. However, the cracks would occur at the outer surface of the forward extruded shaft when the FER is larger than the BER during the early stage of combination-extrusion. Fracturing damage occurs when both FER and BER are further reduced. It starts from the inner corner of the backward extruded cavity to the outer corner of the forward extruded shaft when the FER is less than the BER. Moreover, fracturing damage would start reversely when the FER is larger than the BER. Forming loads tend to be low when fracturing damage occurs at small FER and BER. Crack propagates easily when the pressure of extrusion chamber is insufficient. Therefore, Cracks or fracturing damage would occur at the backward extruded part which corresponding to higher deformation when FER is less than BER. On the contrary, cracks or fracturing damage would occur at the forward extruded part which corresponding to higher deformation when FER is larger than BER. Forming loads would increase when both FER and BER are high. The safe forming would result because the tendency of crack propagates would be suppressed when the pressure of extrusion chamber increases.