Effect Of Heat Treatment On Toughness And Crack Growth Of Al-12.7Si-0.7Mg Alloy

The eutectic Al-Si alloy had good casting and was the largest amount of cast aluminum. Morphology of eutectic Si was a major factor in the mechanical properties of Al-Si alloy. Although Al-Si eutectic alloy by DC casting with appropriate heat treatment and plastic processing could get fine organization and had excellent deformation capacity and became deformed aluminum alloy, less research on toughness and crack growth for this new wrought aluminium alloy that contains more Si particles. By using optical microscope (OM), laser scan confocal microscope (LSCM) and scanning electron microscope (SEM), measuring compact tension K 1c and crack growth to study the influence on Si morphology in semi-continuous casting and foundry crucibles slow cooling of A1-12.7Si-0.7Mg alloy, and to analyze the impact of Si particles and the matrix on toughness and crack growth of DC casting Al-12.7Si-0.7Mg alloy, and to reveal toughness characteristics and mechanism of crack growth for DC casting Al-12.7Si-0.7Mg alloy with different heat treatment. The main results were as follows:1. Optical micrographs of the eutectic Si in the ingot of DC casting Al-12.7Si-0.7Mg alloy was the mixture of spot and strip, but its 3D morphology was coral like structure.2. The coral-like Si coarsened when the semi-continuous casting ingot was in the early stage of the reheating and holding treatment, furthermore, some of the long rod-like eutectic Si changed its morphology into the continued gourd-like shape which then broke up into granules getting coarsening and spheroidization afterwards, when the holding time of the heat treatment was prolonged.3. The diameter of the eutectic Si particle and the holding time were in accordance with a power of 2.29-2.67 relationship during heating and holding the ingot at 485 ℃.4. The optical microstructure of the eutectic Si in the Al-12.7Si-0.7Mg alloy produced by slow cooling casting was mixed with big block and long lamellar and the morphology did not change significantly after heating at 475 ℃ or 515℃ for 24h. When the ingot was 40% hot-rolled at 480℃ or 530℃, the long lamellar eutectic Si was broken and big block eutectic Si had no obvious change. Then the alloy was heat treated at 53O℃ for 4h. The size of Si phase didn’t change so much but the edges showed roundness.5. As for the 30mm rolled plate of eutectic Al-12.7Si-0.7Mg alloy produced by semi-continuous casting, after solution treatment and then aging treatment at 170℃, the Kq values gradually reduced with increasing the aging time. The Kq values of the under aging, peak aging and over aging were 23.0MPa·m1/2、16.7MPa·m1/2、15.4MPa·m1/2.6. For the 30mm rolled plate of eutectic Al-12.7Si-0.7Mg alloy produced by semi-continuous casting, after strengthening heat treatment, the fracture surface was identified as ductile fracture. The density of dimples on the fracture corresponding to the density of coarse Si particles, and the Si particle was the main phase in the dimple, which proved that tearing crack was caused by Si particles.7. In the plate of eutectic Al-12.7Si-0.7Mg alloy produced by semi-continuous casting, the matrix of annealed state was relatively soft and Si particles were broken toughly, so the cracks could easily bypass the Si particles; The matrix of peak-aged state was hard which meaned Si particles were broken easily and the cracks tended to pass through Si particles and the fracture mode contained transgranular and intergranular fracture. For the matrix of the under aged and over aged states, the cracks contained both bypassing and passing through the Si particles.