【作者】 孙健；

【导师】 邹宗树； 王晓鸣；

【作者基本信息】 东北大学 ， 钢铁冶金， 2011， 博士

【摘要】 采用表面处理技术,提高压铸模具相关的服役性能,是当前压铸模具研究、应用领域中最热门的方向之一。8407模具钢作为常用的铝合金压铸模具材料,在交变热应力的反复作用以及熔融铝合金的冲刷和腐蚀下,模具表面产生热熔损和热疲劳而失效。因此,找到一种能够同时提高8407钢的抗热熔损和抗热疲劳性能的表面处理新方法具有重要意义。常用表面处理技术如渗硼、氧化、渗氮等,都是通过在压铸模表面生成新的化合物层,避免熔融铝液和模具的直接接触,从而提高其抗热熔损性能。但是,这些化合物层与模具基体在热力学性能上的差异,容易导致应力集中,从而降低压铸模更为重要的热疲劳抗力。鉴于上述方法存在的缺陷,本论文采取先渗铝后氧化的复合处理方式对8407钢表面进行处理,期望通过在模具表面形成Fe-Al-O复合氧化物过渡层来提高其抗热熔损和抗热疲劳性能。首先,在理论分析的基础上进行了热浸镀铝实验,考察了浸镀温度和浸镀时间对渗铝层厚度的影响,并分析了渗铝层横截面组织与成分,提出了适宜的热浸镀铝条件为：750℃浸镀10min左右。在理论分析的基础上进行喷涂扩散法渗铝实验,考察了扩散温度和扩散时间对渗铝层厚度的影响,并分析了渗铝层横截面组织与成分,提出了适宜的喷涂扩散法渗铝条件：铝涂层厚度100μm左右,渗层厚度40～60μm范围内,扩散温度550℃,扩散时间4h左右。通过比较两种渗铝方法形成的渗铝层质量,确定选用喷涂扩散法对8407模具钢表面进行预渗铝处理更加合理。其后,在理论计算的基础上进行高温氧化实验,研究了不同气氛下渗铝试样的氧化行为,考察了氧化温度和氧化时间对氧化膜厚度的影响,并检测了氧化膜组织和物相,提出了适宜的高温氧化条件为：纯O2气氛下,570℃氧化3h以上。利用Fe-H2O和Al-H2O体系的电位-pH图确定Fe-Al合金表面能够同时生成Fe3O4和Al2O3氧化膜的条件为：溶液的pH值在4.6～8.3范围内。依此配制电解液,并进行了常温硬质阳极氧化实验,考察了磺基水杨酸浓度、氧化时间和电流密度对氧化膜厚度和硬度的影响,测定了氧化膜组织、成分和物相,并利用正交试验优化了硬质阳极氧化的参数条件,确定了在常温25℃条件下,硬质阳极氧化处理的最优参数条件为：磺基水杨酸浓度50g/L,氧化时间60min,电流密度2.5A/dm2。通过比较两种氧化方法形成的氧化膜质量,确定选用常温硬质阳极氧化方法对渗铝后的8407模具钢表面进行氧化处理更为合理。分别采用静态和动态热浸熔融ADC12铝合金溶液的方法,对比研究了表面未处理和表面渗铝氧化处理试样的热熔损行为,观察试样表面状况和横截面组织,并分析未处理试样熔损断面成分,利用熔损失重和熔损速率评价热熔损性能。结果表明,表面渗铝氧化处理能显著提高试样的静态和动态熔损抗力,使其综合熔损抗力明显改善。论文结合实验结果,分析了8407钢的热熔损机理,还从氧化膜性质、氧化膜与基体结合状态及氧化膜与铝液的润湿性三个方面,综合讨论了氧化膜的抗热熔损机理。最后,采用循环加热法进行热疲劳模拟实验,对比研究了表面未处理和表面渗铝氧化处理试样的热疲劳行为,观察试样表面产生的热疲劳裂纹形貌,根据主裂纹长度及其倒数评判热疲劳性能,考察了热疲劳主裂纹长度与热循环周次之间的关系。研究结果表明,表面渗铝氧化处理能够同时提高热疲劳裂纹萌生抗力和扩展抗力,使试样抗热疲劳性能得到改善。论文结合实验结果,分析了8407钢的热疲劳机理,还从热疲劳裂纹的萌生和扩展两个阶段,对氧化膜的抗热疲劳机理进行了探讨。更多还原

【Abstract】 The surface treatment to improve the associated performance of casting dies is one of the most interesting subjects at present. As a common material of casting die for aluminum alloy,8407die steel is suffering from the surface melting-loss and thermal fatigue failures due to the repeated action of alternating thermal stresses and the eroding of molten aluminum alloy. Consequently, it’s significant to find a new surface treatment method to improve both melting-loss and thermal fatigue resistance.The commenly used surface treatment technologies such as boriding, oxidation, nitridation and so on, can form a compound layer on the surface of casting die to impove its melting-loss resistance by preventing die from direct contact with molten aluminum. However, the difference between these compound layers and die substrate in terms of thermal properties leads stress concentration, thus deteriorating the more important thermal fatigue resistance of casting die. Considering the defects of the above-mentioned methods, aluminization and subsequent oxidation treatment of8407steel surface is proposed and studied, aiming at improving both the melting-loss and thermal fatigue resistance by forming a Fe-Al-O complex oxide layer in this work.First of all, hot-dip aluminization experiment was carried out based on the theoretical analysis. The influences of hot-dip temperature and time on aluminization layer thickness were investigated and the sectional microstructure and composition of aluminization layer were analyzed, arriving at the following appropriate aluminization condition:hot dipping for about10min at750C. Cold spray of aluminum and subsequent diffusion process was conducted on the basis of theoretical analysis. The influences of diffusion temperature and time on aluminization layer thickness were investigated and sectional microstructure and composition of aluminization layer were analyzed, arriving at the following appropriate aluminization condition:aluminum coat thickness of about100p,m, aluminization layer thickness in the range of40to60μm, diffusion temperature of550C and diffusion time of about4h. Comparing the aluminization layer quality obtained with these two aluminization methods, it’s more reasonable to conduct pre-aluminization treatment on8407die steel by means of spray and subsequent diffusion process.Secondly, the oxidation behaviors of aluminized specimens were studied under different atmospheres in the high temperature oxidation experiment based on the theoretical calculation. In addition, the influences of oxidation temperature and time on oxide film thickness were discussed and microstructure and mineral phases of the films were examined in the experiment, arriving at the following appropriate oxidation condition:oxidation for above3h at570℃under pure oxygen. The potential-pH diagrams of Fe-H2O and Al-H2O systems are applied to determine the conditions where both Fe3O4and Al2O3could form on the Fe-Al alloy surface as follows:solution pH in the range of4.6to8.3. The electrolyte solution was prepared according to this, and room temperature hard anodization experiment was carried out. The influences of sulfosalicylic acid concentration, oxidation time and current density on oxide film thickness and hardness were investigated and the microstructure, composition and mineral phases of the films were examined. The orthogonal tests were carried out to acquire the optimum condition at25℃as below:sulfosalicylic acid concentration of50g/L, oxidation time of60min and current density of2.5A/dm2. Comparing the oxide film quality formed with these two oxidation methods, it’s more reasonable to conduct oxidation process on aluminized8407steel by means of room temperature hard anodization.Surface untreated specimens and surface aluminization and subsequent oxidation treated specimens were immersed into molten ADC12alloy under static or dynamic conditions respectively to compare their melting-loss behaviors. In melting-loss test, surface condition and sectional microstructure of specimen were observed, and compositions of transverse section of untreated specimen were analyzed. Weight loss and melting-loss rate were adopted to evaluate melting-loss property. The results show that the surface aluminization and subsequent oxidation treatment can dramatically improve the melting-loss resistance under both static and dynamic conditions. The melting-loss mechanism of8407steel was analyzed with experiment results, and the anti-melting-loss mechanism of oxide film was discussed from three aspects, namely:property of oxide film, the combination status between oxide film and substrate, and the wettability between oxide film and molten aluminum.In the last part of this work, thermal fatigue test was performed with cycle heating method to compare thermal fatigue behaviors of samples with and without aluminization and subsequent oxidation treatment. In the test, thermal fatigue crack morphology formed on the surface was observed, and relationship between main crack lengths and cycles was discussed. The thermal fatigue property can be judged according to thermal fatigue main crack length and its reciprocal. The results show that the surface aluminization and subsequent oxidation treatment could improve both the initiating resistance and propagating resistance of thermal fatigue crack. The thermal fatigue mechanism of8407die steel was analyzed with experiment results, and the anti-thermal-fatigue mechanism of oxide film was clarified from two stages of thermal fatigue crack, one being initiating stage, the other being propagating stage.更多还原