【作者】 江伟；

【导师】 周宏；

【作者基本信息】 吉林大学 ， 材料加工工程， 2009， 硕士

【摘要】 本文以HT200灰铸铁为研究对象,在贝壳软硬交替结构具有优良抗热疲劳性的自然界非光滑现象的启发下,通过激光器在长方体薄块状灰铸铁试样表面加工出一条条平行等间距的单元体来提高灰铸铁的抗热疲劳性能。研究单元体横截面积的变化对试样热疲劳抗性的影响;以单元体横截面积的大小为指标对激光器的加工参数进行了优化;研究了不同参数对单元体横截面积大小的影响规律;并研究了预热和退火工艺对单元体中的孔洞、裂纹、拉应力的影响。研究表明:试样的热疲劳裂纹扩展速度随着单元体横截面积的增加而显著减小。激光脉宽是影响单元体横截面积的最显著因素,脉宽增加,单元体横截面积显著增加;激光功率增加单元体横截面积逐渐增大;扫描速度加快使单元体的横截面积逐渐减小;频率和离焦量对单元体尺寸影响不显著。各因素对指标的影响次序为:脉宽>功率>扫描速度>频率>离焦量。对脉宽进行扩大化试验,确定的激光加工参数的最优值为:脉宽13 ms,功率为1482W,扫描速度0.5 mm/s,频率10Hz,离焦量5.5mm。对试样整体预热温度提高到250℃后,再激光处理可消除单元体表面的裂纹和孔洞,处理后单元体的晶粒略有增大,硬度下降7.42%。在同样的激光加工参数下,250℃预热后的试样的热疲劳性能相对于未预热的试样略有提高。预热后加工单元体再退火工艺可以去除单元体处93.3%的拉应力,退火后的试样的热疲劳性能相对于仿生非光滑未预热试样和仿生非光滑预热试样提高38%。更多还原

【Abstract】 Brake drum is the key part of truck braking system, the improvement of its performance can enhance the safety of vehicles and reduce maintenance cost. Modern society needs trucks having higher performance, such as higher speed, heavier load and some trucks have to more frequently work in variety harsh environments, these changes require brake drums having higher life. Gray cast iron HT200, is commonly used as the materials for brake drums in China. This kind cast iron has many excellent machining performances, which are why it can be widely used, but it contains a large number of graphite sheets having no strength, which is a kind of serious flaw. Brake drums are tore by braking force; the temperature of brake drums’ surface reaches to very high temperature during braking while drop to room temperature after braking; cracks are generated between high temperature and low temperature and the expanding of cracks result in the ultimate failure of brake drums.Our team has improved the thermal fatigue resistance of gray cast iron more than twice from the view of bionics, using a laser to process some parallel units in the gray cast iron surface to simulate the alternate hard and soft structure of shells. However, the laws of laser system’s five parameters impacting the thermal fatigue resistance of units have not been researched strictly. Some cracks, holes and biggish tensile stress would be generated in the surface of units, which seriously damaged the units’ thermal fatigue resistance. In this paper, cracks and holes in units were eliminated through preheating process, tensile stress was reduced sharply by annealing process, and the laws of preheating and annealing process impacting the units’ thermal fatigue properties were also studied.Through experiments and research analysis, the conclusions are as follows.1. By comparing the thermal fatigue property of samples whose units had different cross-sectional area, it was found that the thermal fatigue property of samples increased with the increased cross-sectional area of units. 2. Through optimization test of laser parameters, the laws of laser system’s five parameters impacting the thermal fatigue resistance of units had been researched strictly. It was found that the laser pulse width was the most important impact which affected the cross-sectional area of units. The cross-sectional area of units became larger obviously while the pulse width was increased; the cross-sectional area of units became larger with power increased; the cross-sectional area of units became smaller with scanning speed increased; the cross-sectional area of units was changed little while frequency and distance from the focused point changed. Factors are sorted as follows: pulse width> power> scanning speed> frequency> distance from the focused point. The optimal laser processing parameters are determined by the expansion test of the pulse width test. Minor other secondary factors are fixed and pulse width is changed only. The fixed final laser processing parameters are as follows: pulse width 13 ms, power of 1482W, scanning speed 0.5 mm / s, frequency of 10Hz, 5.5mm distance defocused.3. The cracks and holes in the surface of units could be eliminated by being preheated at 250℃firstly and then being scanned, moreover, the organization became a little bigger and hardness of units decreased 7.42%. Under the same laser processing parameters, compared with sample un-preheated, the thermal fatigue performance of preheated sample was increased very little. This was because although preheating process eliminated the cracks and holes, the higher tensile stress was not released. Tensile stress enhanced the growing of cracks and decreased the effect of the elimination of units’ cracks and holes which could enhance resistance to thermal fatigue.4. The tensile stress was eliminated 93.3% by annealing process and the annealing. The thermal fatigue performance of bionic non-smooth preheated and annealed sample was increased 38% compared with bionic un-preheated sample and bionic non-smooth preheated sample.更多还原