پژوهشنامه ریخته گری
سال سوم شماره 4 (پیاپی 11، زمستان 1398)

Ni-base super alloy CMSX-4, as the 2nd generation of single crystal alloys, had been used since ‎‎1990 in different gas turbine blades. In this research, the samples were prepared from poly ‎crystal ingot alloy which was made with VIM and VAR techniques. The carrot of a single ‎crystal super alloy was made by Bridgman furnace. After casting the same solution and aging ‎treatment were carried out. The cast microstructure of the single crystal super alloy consists of many ‎directed dendrites toward (001) direction with some eutectic phase and a few amounts of ‎carbides. In heat-treated microstructure, all of the γ phases ordered as the cubic shape. The ‎misorientation was determined in order of 6 to 7 degrees by laue and RO-XRD techniques. ‎The tensile strength of CMSX-4 alloy was determined at the temperature of 25 and 870 °C. ‎The results showed that the UTS was 1060 and 390 MPa respectively for polycrystalline and ‎single crystal were 1066 and 1163 MPa. SEM observations confirmed that these behaviors ‎were affected due to grain boundaries and the precipitation’s morphology at high ‎temperatures. ‎

Hybrid nano-biocomposite of Mg-0.5Ca-0.8Mn (wt%) magnesium alloy reinforced by 2 wt% fluorapatite (FA) nanoparticles and 0.3 wt% graphene nanoplates (GNPs) were prepared via mechanical stir casting process. In order to eliminate the casting defects, remelting process was carried out under argon atmosphere. Mechanical and biocorrosion properties were investigated by tensile testing and electrochemical impedance spectroscopy in simulated body fluid (SBF) solution, respectively. Microstructural development including the size and distribution of the reinforcement particles, and matrix grain size were studied by optical microscopy and scanning electron microscopy (SEM). Mechanical test results revealed an increase of 27% in the ultimate tensile strength. Biocorrosion behavior approved the lower corrosion rate of the composite samples in comparison with unreinforced alloy. Optical micrographs confirmed the effect of reinforcement particles on twinning formation and reduction of composite grain size (175 μm) compared to that of unreinforced alloy (484 μm). SEM observations exhibited the formation of graphene scaffolds and their positive effect on the mechanical and biocorrosion properties.

In-situ Al-15Mg2Si composite due to their low density and good strength and wear properties are considered as potential candidates to substitute the Al and cast iron parts used in automotive and aerospace industries. The improved properties of these composites are governed by controlling the size and morphology (modification) of the primary Mg2Si particles. In the current study the effect of Cu addition (1.0, 3.0, and 5.0 wt. %) on metallurgical structure and quality index of Al-15Mg2Si composite was investigated. According to the results, Cu addition modified the morphology and reduced the average size of primary Mg2Si particles. The addition of Cu up to 3.0 wt. % improved the composite tensile strength by up to 34% whilst its further addition impaired the tensile properties. Moreover, the addition of Cu up to 1.0 wt. % improved the composite elongation by about 7%. This is while its further addition deteriorated the composite ductility. The maximum quality index was observed in 3.0 wt. % Cu containing composite where its quality index was 8% higher than that of the base sample. The microstructural observations, micro-hardness testing, precipitates chemical analysis, and fractography of fractured surfaces revealed that solid-solution-strengthening, dispersion hardening of Cu-containing precipitates, and modification of Mg2Si particles are the main factors improving the composite quality. Moreover, increasing the volume fraction of hard Cu-precipitates and micro-pores, negatively affected the composite tensile properties and quality index.

The prediction and control of solidification induced microstructure is an important issue during the product/process design stage in the selective laser melting additive manufacturing. It helps to avoid undesirable microstructures and possibly additional required post heat treatment, consequently improving the quality of manufactured parts. However, the direct numerical simulation of microstructure formation in this process is computationally very expensive, even by employing the state-of-the-art available computational resources. In the present study, an indirect approach based on empirical law is employed to predict the solidification microstructure. For this purpose, the macro-scale nonlinear heat equation include phase change effect is solved using the conventional finite element method and the local cooling rate and thermal gradient within the freezing interval is computed accordingly for Ti6-Al4-V alloy. Then, this information is projected on the empirical solidification microstructure map of this alloy to predict local microstructure, and the effects of process parameters like the laser power, laser effective radius (laser focus), scanning speed and scanning strategy on the solidification microstructure are investigated. According to the results, by decreasing the laser power, increasing the effective laser radius and laser scanning speed, the resulted grains morphologies can be varied gradually from the columnar to mixed columnar and equiaxed and completely equiaxed microstructures.

Casting of high strength Mg alloys has been considered in researchers studies. For this purpose, by addition of Ag instead of Zr, properties of the alloy can be modified. Thus, in this research, at the first thermodynamically simulation was done by the use of Thermo Calc. AB software, Then, for preparation of the alloy, pure commercial magnesium was melted under argon atmosphere at 740 oC. Then, Mg-30Zr master alloy was added to molten metal. Subsequently, silver and alkali elements were added to molten metal. After homogenizing for 15 minutes, molten metal was poured using a low pressure casting machine. In order to study the micro structural and phase studies, optical microscopy and scanning electron microscopy (SEM) and X-Ray diffraction apparatus was used. All the samples were T6 heat treated (solution treatment for 6 hours at 525 oC, quench in water and aged at 200 0C). Hardness measurements carried out using Brinell indenter. Analyses illustrated existing of 2.24 wt% Ag, 1.52 wt% alkali, and 0.55 wt% zirconium in the alloy. Clemex results showed that 93±1% of matrix structure is composed from 98% Mg metal and alfa eutectic precipitations are distributed within dendrites. EDS results showed that unlike zirconium, silver and alkali metals have remarkably dissolved in matrix phase. Finally hardness of alloy increased from 43 to 48 HB by solution and aging heat treatment.

In this paper, the effect of heat treatment on the microstructure behavior and hardness of aluminum nickel bronze alloy (C95500) are investigated. The alloys were heat treated in different heat treatment cycles including quenching, normalization, aging and annealing. The microstructure of the samples studied using light microscopy (OM) and Scanning Electron Microscopy (SEM) and Image J software to observe the percentage of phases in the structure. The hardness properties of the heat treated alloys as well as the microstructural micro hardness were evaluated and tested for the hardness of each phase and their comparison with each other and the effect of the phases on the overall hardness of the samples. The results show that the quenched samples and then aged are associated with increasing hard phases 'β and γ_2 (or δ), increasing these two phases in the alloy improves the hardness properties. The α-Widmanstätten phase is also a hard phase according to the micro hardening results, which increases it increasing hardness in the field. It should be noted that the formation of each of the phases in the microstructure will be subject to the heat treatment required to create that phase, such that aging causes the k-phase, the quenching to create the β phase as well as the normalize to the α-Widmanstätten phase.