Study On The Preparation Crafts Of Biomimetic Gray Cast Iron And The Influence To Its Thermal Fatigue Property

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.