Study on Diffusion Bonding of Cast Irons

球墨铸铁是一种极难熔接的材料，采用了扩散焊。用镍箔作为插入金属或不使用镍箔在820、900和950的温度下进行1.8 k的焊接。然后在720℃下进行10.8 ks的铁化处理。研究了扩散键的冲击和扭转特性。得到的主要结果如下。(1)由于界面处石墨析出较多，直接结合的接头的结合韧性很低。当铸铁与镍箔结合时，结合韧性得到明显提高。厚度为50 μ m的镍箔接头的吸收能与母材的吸收能基本相同。(2)石墨颗粒数量较少的铸铁，其接头的吸收能随结合温度的升高而降低，直接结合的效果较好。而当铸铁中石墨颗粒数大于250粒/mm^2时，随着结合温度的升高，接头的吸收能增加。这是考虑如下。随着石墨颗粒数量的增加，铸铁的铁素化速度加快，铁素化过程中石墨的析出主要发生在铸铁中的石墨颗粒上，而不是在结合界面上。结果表明，通过减少石墨在界面处的析出，提高了结合韧度。(3)直接结合的铸铁接头进行扭转试验，在相同应力下，扭转疲劳的耐久性很大程度上是分散的。这是由于石墨在键合界面处析出所致。厚度为10 μ m的镍箔接头的耐久性几乎不分散，疲劳极限为160 MPa。

Spheroidal graphite cast irons which are a very difficult material to fusion weld were diffusion bonded. The joints were fabricated with Ni foil as an insert metal or without it at 820, 900 and 950 ﾟC for 1.8 ks. And then they were annealed to ferritize the cast iron matrix at 720 ﾟC for 10.8 ks. Impact and torsion characteristics of the diffusion bonds have been investigated. Main results obtained are as follows.(1) The bond toughness of the joints directly bonded was very low due to much precipitation of graphites at the bond interface. When the cast irons were bonded with Ni foil as an insert metal, however, the bond toughness was strongly improved. The absorbed energy of the joints with Ni foil of 50 mu m in thickness was nearly the same as that of the base metal.(2) The absorbed energy of joints of cast irons with lower number of graphite particles, the cast irons were directly bonded, decreased with an increase in bonding temperature. When the number of graphite particles was more than 250 particles/mm^2 in the cast iron, however, the absorbed energy of the joints increased with the increase in bonding temperature. This is considered as follow. As the ferritizing speed of the cast iron increased with an increase in number of graphite particles, precipitation of graphites during the ferritizing was mainly developed on the graphite particles in the cast iron rather than at the bond interface. As a result, the bond toughness was improved by a decrease of precipitation of graphite at the bond interface.(3) When the cast iron joints directly bonded were torsion tested, and endurance of the torsion fatigue was largely scattered at the same stress. This is due to the precipitation of graphites at the bond interface. The endurance of the joints with Ni foil of 10 mu m in thickness was hardly scattered, and the fatigue limit was 160 MPa.