Effects of Ce Addition on Solidification Structure of a Low-Carbon 42CrMo4 Steel

Effects of Cerium (Ce) addition on solidification structure of a low-carbon 42CrMo4 steel was investigated. The addition of up to 0.067 wt.% of Ce in the steel produced greatly improved solidification structure with a suppressed columnar grain zone, finer grain size in an equiaxed grain zone and zero area fraction of casting shrinkage cavity. The added Ce occurred in the steel both in the form of Ce oxy-sulfide inclusions and as dissolved atomic Ce segregated together with other elements at prior austenite grain boundaries and at interdendritic spacing. The Ce oxy-sulfide inclusions were found to play a major role in the observed improved grain structure meanwhile dissolved Ce had pronounced effects on morphology of dendritic networks. The fraction of Ce dissolved in the melt appeared to bring about increase in fluidity of the molten steel, leading to total elimination of interdendritic shrinkage porosity in solidification structure of the steel with added Ce. Ce addition can be considered as a potential solution for grain structure refinement in heavy-weight castings of 42CrMo4 steel grade.

The Effect of Process Parameters on the Mechanical Properties of A356 Al-Alloy/ZrO2 Nanocomposite

In this study the effect of zirconia (ZrO2) nanoparticles (40 nm) in reinforcing A356 aluminum alloys as a base metal matrix were investigated. Zirconia nanopowders were stirred in the A356 matrix with different fraction ratios ranging from (0, 1, 2, 3, 5%) by weight at variable stirring speeds ranging from (270, 800, 1500, 2150 r.p.m) at a mushy zone (600°C) and liquid state (700°C) using a constant stirring time for one minute. The microstructure revealed the change of grains from dendritic to spherical shape with increasing stirring speed. The Scanning Electron Microscopy of the fractured surface revealed the presence of nanoparticles at the interdendritic spacing of the fracture surface and was confirmed with EDX analysis of these particles.The results of the study showed that the mechanical properties (strength, elongation and hardness) using ZrO2 as reinforcements were increased at the following parameters: 1500 r.p.m stirring speed in semi-solid state (600oC) and adding 3 wt.% of ZrO2.

Investigation of Structures in As-Cast Alloys from the Mg-Zn-Ca System

Alloys of nominal composition Mg-3Zn-xCa (x = 0, 0.2, 0.5, 0.7, 1.0, 1.3) wt.% have been prepared by resistance melting and casting into graphite mould under argon atmosphere. All investigated alloys have revealed α(Mg) dendritic microstructures with intermetallic phases distributed in interdendritic spacing. The Mg4Zn7 has been identified as the main phase which coexists with α(Mg) in the Mg-Zn alloy. Calcium addition causes the formation of ternary Ca2Mg6Zn3 hexagonal phase, denoted also as Ca3MgxZn15-x . The increase of Mg/Zn ratio from 0.59 in Mg-3Zn-0.2Ca wt.% alloy to 2.72 in Mg-3Zn-1.3Ca wt.% alloy in this phase is connected with an increase in the lattice parameters. The microhardness of the Ca containing alloys increases up to 67 HV (for the alloy containing 1.3 wag.% Ca) compared to the 50 HV for Mg-3Zn wt.% alloy.

Development of a τ-Type Mg-Zn-Al Alloy: An Investigation of the Microstructure and Solidification Characteristics

The practical phase constituent diagram has been used to determine the composition of a low alloying τ-type Mg-Zn-Al alloy, which has a nominal element content of 7wt%Zn and 3wt%Al (ZA73), as a basis for further improvement of τ-type alloy through appropriate thermal processing and micro-alloying. The microstructure and solidification characteristics of the alloy have been experimentally examined using optical microscopy, scanning electron microscopy, X-ray diffraction, and differential scanning calorimetry analysis. The results confirm that the as-microstructure of ZA73 alloy consists of globular equiaxed dendrite with τ phase as the only secondary phase evenly distributed in interdendritic spacing. Solidification sequence has been proposed with the help of DSC analysis and Mg-Zn-Al ternary liquidus projection phase diagram. Consistent with previous observed results for τ-type Mg-Zn-Al alloy, due to the decrease of element contents, ZA73 alloy has higher liquidus temperature (~627°C) and wider solidification range (~283°C), while at the mean time lower starting temperature of the second phase transformation (~354°C), compared to higher element containing ZA104 alloy. The phase constituent diagram has been shown to be a practical and effective tool for predicting the as-cast microstructural constituent of high zinc Mg-Zn-Al alloys under normal permanent mould solidification condition.