The microstructure and mechanical properties of nodular cast iron produced by the melted metal die forging process (MMDF for short) were experimentally researched.The main obtained results are : the nodular cast iron produced by this process can be machined as easy as traditional one only if holding at the remained temperature for 4 h ; its strength and plasticity are obviously higher than those of traditional ones ; the graphite size arrived at grade 8 , and the graphite spheroidizing arrived at grade 1 or 2 , but a streamline molded distribution of the graphite slightly appeared.Both of the strength and plasticity increased with the pressure when the pressure holding time was larger than its critical value.A new way to produce high properties nodular cast iron was provided.
The particle motion behavior affects the distribution of particles in the metal matrix and finally determines the mechanical properties of the particle reinforced metal-matrix composites. To obtain a uniform distribution of TiC particles and excellent strengthening effect in 20%TiC/ZG270-500 composites fabricated by lost foam-squeeze casting(LFSC), the particle motion behavior in the gas gap and the conditions of the particles getting into the molten steel were investigated. The results show that the airflow velocity(v1) and TiC particle motion velocity(uP) change little with the pouring temperature(TP), increase with an increase in metal filling velocity(vP), ratios of cross sections of in-gate/orifice(AP/A0) and orifice/mould cavity(A0/A1), but the increase trend of u P is more intense. The airflow pressure(P1) changes little with TP and A0/A1, but increases with the increasing of vP and AP/A0. Besides, there is a critical velocity(+|3yLGcosθ/ρpdp|1/2) for the particles getting into the molten steel. The higher the particle motion velocity, the easier the particles get into the molten steel and the more uniform the distribution of the particles in the steel matrix. When TP = 1,873 K, vP = 30 mm·s-1, AP/A0 =10 and A0/A1 = 0.02 in this study, the biggest TiC particle motion velocity(20.59 m·s-1) can be gained, and the steel matrix with the most uniformly distributed TiC particles and fine grains are obtained.
In squeeze casting process,the essence of mold-filling and feeding is rheological solidification of the alloy melt under pressure.Microstructure evolution is inevitable in this process,which affects the mold-filling and feeding in turn.In this work,the Archimedes spiral sample prepared by indirect squeeze casting was applied to investigate the microstructure evolution during the rheological process under pressure.The results showed that the primaryα-Al phase was transformed from fine rosette-like or granular structure to coarse platelet-like structure with the increase of spiral length.However,the primaryα-Al grain size of the starting point had a slight growing trend compared with that at the position of 140 mm away from the starting point.The volume fraction of the primaryα-Al phase increased from 45.57%to 70.35%along the spiral length direction,demonstrating that the experimental pressure improved the rheological ability of the alloy melt to some extent.Furthermore,the eutectic Si phase was varied from a fine granular or wormlike structure to a coarse platelet-like or needle-like structure,and the dispersion of eutectic Si particles was also varied along the spiral length direction.This microstructure evolution was mainly owing to the comprehensive action of rheological solidification and pressurized solidification.More specifically,the microstructure evolution strongly depended on the pressure and rheological velocity during the rheological process,however,the effect of rheological distance was relatively small.
