분무성형법으로 제조된 과공정 Al-25Si계 합금의 고온 변형 거동 및 전산모사

다국어 초록 (Multilingual Abstract)

The high temperature deformation behavior of spray-formed and subsequently extruded hypereutectic Al-25Si based alloy containing fine Si and ultra-fine intermetallic phases was examined by compressive tests and the results were compared with the developed processing maps. this approach was coupled to the fracture criteria generated from finite element simulation. In the compressive tests, cylindrical compressive specimens were compressed in the temperature range of 523~743K and strain rate range of 1.0x10-3~1.0x100/s. The true stress-true strain curves obtained from the compressive tests revealed a peak stress at the initial stage of deformation. The peak stress decreased with increasing temperature or decreasing strain rate. A close relationship was observed between the peak stress and the constitutive equation for high temperature deformation. In the deformed specimens, fine equiaxed grains were observed with a mean grain size of 330~590 nm, which was much finer than that measured prior to deformation (1.4 µm). A dislocation structure within the grains was also observed in the deformed specimens, indicating the occurrence of dynamic recrystallization during high temperature deformation of the present alloy. The occurrence of dynamic recrystallization was also supported by the existence of a peak stress in the flow curve.
Based on true stress-true strain curves obtained from the compressive tests, processing maps were developed to evaluate the power dissipation efficiency and instability parameter of the flow behavior. It was found that the deformed specimens tended to be fractured with increasing strain rate or decreasing temperature. In the processing maps, cracks were observed in the instability domains and did not occur where the power dissipation efficiency was high. Optimum processing parameter of the studied alloy was 743K and 1.0 x 10-3/s having efficiency of 86%. As a result of the finite element simulation with four fracture criteria, the fracture criterion expressed by the Cockcroft & Latham criterion showed good agreements with the experimental results.

목차 (Table of Contents)

• 1. Introduction 1
• 2. Background 3
• 2.1. Spray forming process 3
• 2.2. Processing map 4
• 2.3. Ductile fracture criterion 6
• 3. Experimental Procedures 7
• 4. Results and discussion 9
• 4.1. Microstructure of as-received material 9
• 4.2. Compressive deformation behavior 12
• 4.3. Microstructure of the deformed materials 18
• 4.4. Relationship between shape of deformed specimens and processing map 25
• 4.5. Criterion for cracking 35
• 4.6. Finite element simulation 37
• 4.6.1. Computational conditions 37
• 4.6.2. Maximum principal 38
• 4.6.3. Cockcroft & Latham criterion 43
• 4.6.4. Freudenthal criterion 46
• 4.6.5. Modified Cockcroft & Latham criterion 51
• 5. Conclusion 54
• 6. References 56