铝铸轧辊套材料的表面离散合金强化技术及热疲劳性能研究

In Twin-Roll Casting of Aluminum Alloys,the surface of the roller-sleeve was contacted periodically with the melted aluminium alloy, and was acted by thermal shock along the circumference and the radial.The surface of the roller-sleeve will appear some cracks by the thermal stress and mechanical stress.The damage degree of the roller-sleeve surface decides to its life and the quality of the aluminuim alloy sheets.Therefore, the cracking mechanism and the strengthening method of the roller-sleeve surface need to be studied. In this paper, we prepare to strengthen the roller-sleeve material 32Cr3Mo1V with alloy powder by laser-arc composited processing.The disperse alloy strengthened areas have the higher strength and the better resistance to oxidation in high temperature than the base material and good toughness aim to decrease the possibility of the crack initiation and to limit the length and depth of the crack. Moreover,the thermal fatigue property of disperse alloy strengthened surface will be studied by the method of continuous YAG laser heating. With the comprasion of the thermal fatigue property between the strengthened area and the base material,the crack initiation of the strengthened area and the retardant behavior of its in the crack extension will be found, the thermal fatigue mechanism of disperse strengthened surface will be revealed,the technology of laser-arc composited processing will be optimized.

铝铸轧辊套表面与熔融的铝液接触，承受着周期性的热冲击。辊套表面在热应力和机械应力的共同作用下，发生疲劳开裂。辊套表面的开裂程度直接影响到了辊套使用寿命和铝板带的质量，因此研究铝铸轧辊套表面热疲劳开裂机制和强化措施具有重要意义。在本申请中，拟采用光电复合表面合金强化技术对铝铸轧辊套材料32Cr3Mo1V进行离散强化，通过添加合金粉末，提高基体组织的抗氧化性，利用细晶、固溶和晶界强化机理，在材料表面形成高于基体高温强度的和良好断裂韧性的离散强化点，以降低材料表面的裂纹萌生倾向，缩短疲劳裂纹长度和深度。同时拟采用YAG连续激光作为热源，利用激光局部加热、产生温度梯度高的特点，进行离散合金强化表面的热疲劳性能研究。通过与基体材料的对比，研究强化点的裂纹萌生倾向及对裂纹扩展的阻滞作用，揭示离散强化增强热疲劳性能机理，优化光电复合表面合金强化技术，指导铝铸轧辊套表面强化应用。

本项目研究了辊套材料的表面离散合金强化技术，包括光电复合强化技术和激光强化技术，期望在材料表面制备具有一定深度和抗氧化性的合金点，提高辊套材料的热疲劳性能。在光电复合离散合金强化技术研究中，发现稀土（La, Ce）氧化物能够压缩电弧，增大熔深，同时也容易造成合金粉末的飞溅，在电弧熄弧过程中容易出现结晶裂纹和低塑性工艺裂纹。而激光合金化过程工艺相对简单，Cr系合金粉末与基材的相容性好，合金点的熔深达到了0.4mm以上，500℃回火硬度为基材的2倍。搭建了扫描式YAG激光加热的热疲劳实验装置，通过光束整形和变换，获得了条状的匀强光斑。通过数值计算优化了激光输出功率和扫描速度，能够模拟实际辊套的最高温度、温度循环和应变疲劳条件。在32Cr3Mo1V辊套材料的热疲劳实验中，发现裂纹的扩展速率接近实际值（为实际值的一半）。在离散Cr合金化的辊套材料的热疲劳实验中，发现合金点具有高于基材的裂纹萌生抗力，并能够阻滞裂纹的扩展，使裂纹的扩展速率比基材下降了一个量级。通过SEM和EDS分析，发现Cr合金点表面形成了更加致密的氧化膜保护组织被进一步氧化，提高了合金点的热疲劳性能，其中Cr含量高于基材，O含量远低于基材。对合金点进行回火，研究不同硬度的合金点的热疲劳性能，发现高硬度和高Cr含量结合在一起，具有最佳的热疲劳性能。