Guo Qing Zhang

Spray Forming and Thermal Processing for High Performance Superalloys

A unique pilot low-pressure spray forming plant was established and its spray atomisation and deposition process developed to study the new processing methods for high performance materials and to develop spray forming technology suitable for making sound superalloy preforms. The results indicated that high density (>99%) preforms (billets and rings) with little gas pick-ups and with the microstructural features of rapidly solidified superalloys, i.e. refined equiaxed grains and uniform microstructure, could be achieved after the optimisation of the spray atomisation and deposition process. The effects of subsequent thermal processing on the density, microstructure and mechanical properties of the spray formed superalloy were investigated. Compared to the turbine disks and rings made by wrought superalloys, the spray formed superalloys with identical chemistry showed significantly improved metallurgical quality, higher mechanical properties, and better hot workability.

Constitutive Characteristic of Hot Deformation for Spray Formed Superalloy GH738

The hot deformation behaviors of spray formed superalloy GH738 were investigated by using of Gleeble-3500 simulator in the temperature range of 950~1200, with a strain rate of 0.13~6.5s-1 and reduction of 50%. The corresponding flow curves were determined and hot deformed microstructures were observed. The results showed that the flow stress decreased with increasing deformation temperature or decreasing strain rate. A full dynamic recrystallization microstructures with fine-equiaxed grains were obtained at the temperature of 1100~1150 and strain rate of 2.6~6.5s-1. The hot deformation activation energy Q was 580.81kJ.mol-1, and the constitutive equation was derived by means of linear regression.

Powder Metallurgical Processing of Ti6Al4V Alloy

Ti6Al4V powders were produced by Argon gas atomization, the powder fraction ＜ 250μm was hot isostatically pressed (HIP) at 920°C and 140MPa. The properties of pre-alloyed powders and the compact were investigated in this paper. Powder particles are almost perfectly spherical. The microstructure of powder surface is approximate hexagonal cellular structure, the inner structure exhibits cellular αphase and needle-like martensiteα′ phase, these are resulting from the rapid solidification. After HIP, Ti6Al4V alloy has a Widmanstaten microstructure consisting of continuous grain boundary α（GBα）phase and β transformation structure, the grain size of GBα phase is in the range of 5～15μm . The tensile test at room temperature shows that strength of samples is 880MPa, the fracture surface exhibits obvious brittle cleavage fracture features including cleavage facets with river pattern and a few elongate dimples of different sizes and big voids at localized area.

Microstructure and Mechanical Properties of In718 Alloy Produced with Twin-Scanning Atomizer

Sound billets were produced at BIAM with a twin-scanning spray forming facility. Microstructure and mechanical properties of In718 alloy produced with this technology were examined. Both tensile strength and stress rupture properties are excellent with the rupture life even doubled after received a direct aging heat treatment.

Microstructures of Spray Formed Superalloy GH742

Microstructures and hot deformation behaviors of spray formed superalloy GH742 have been studied in this paper. The results indicate that the spray formed superalloy GH742 has high relative density, low oxygen pickup, refined microstructure, and good forgeability. The fully inhibited coarsening of microstructure in spray formed superalloy GH742 has also been found in this study. 32wt% γ¢ and 0.5wt% carbonitrides, (Nb,Ti)(C,N) and Ti(C,N) were found in the as-deposited GH742, and most of them are sub-micrometer.

Evaluation of Inclusions in P/M Superalloy

The non-metallic inclusions in master alloy, P/M superalloy and HIP powder billet were studied in this paper. The results show that the amount of inclusions in master alloy is higher than that of the superalloy powers. The EB-button analysis shows that the main non-metallic inclusions in both the master alloy and the HIP powder billet is Al2O3.The amount of the inclusion in master alloy is about 0.166cm2/kg and the size of most inclusions is in the range of 100μm to 200μm, while the maximum inclusion size reaches 400μm.In the P/M superalloy billet, the content of inclusion is only 0.01cm2/kg and the size of most inclusions is less than 50 μm.

Quantitative Analysis on Fatigue Fracture of Spray Formed GH738 Alloy

Low-cycle fatigue (LCF) fracture of the spray formed GH738 alloy was investigated under total strain controlled mode at 650°C. Basic theory for evaluating fatigue lives by fatigue striations has been introduced. The crack length a and the crack propagation rate da/dN were measured and the curve of da/dN in crack propagation zone was obtained with Paris formula and tabulation trapezoidal formula, respectively. The size and symmetry of rapid fracture zone of fatigue were also studied. The results showed that the relationship of a and N was linear with tabulation trapezoidal formula, and the da/dN and a was conic with Paris formula. At last, quantitative analysis of fatigue fracture was also discussed.

Preparation and Characterization of Spray Formed 2060 High Speed Steel

Spray forming has attracted considerable attention for the production of high speed steels due to its potential and priority in the microstructure refining and cost saving. In this study, high-quality large billets of 2060 high speed steel were successfully produced by spray forming process using a twin-atomizer facility. As-deposited billet was subsequently processed by hot forging, quenching in oil at 1180 °C and a triple tempering in the temperature range of 500-580 °C. The microstructures and hardness of the deposit and their subsequent development resulting from hot forging and heat treatment were investigated. This paper was designed to provide insight and have a better understanding of such a system for the steel. The results showed that the as-deposited microstructure was composed of the fine equiaxed grains with V-rich MC and W-Mo-rich M2C carbides non-uniformly distributed along the grain boundaries and inside the grains. M2C presented rod-like or unconnected net-shaped morphologies in the as-deposited microstructure. Following hot forging, metastable M2C carbides were completely decomposed into refined MC and M6C nearly spherical carbides uniformly distributed throughout the microstructure. A hardness value of 31HRC was attained for the spray deposited and hot forged samples. With increasing the tempering temperature, hardness was increased firstly and then decreased. Secondary hardening peak appeared at 540 °C for spray formed 2060 steel austenitized at 1180 °C, and the corresponding peak hardness reached 71HRC.

Influence of Master Alloy on the Cleanliness of Spray Formed Superalloy

Three kinds of master alloys, including scrap material, vacuum induction melting (VIM) ingot and electro slag remelting (ESR) ingot, were spray formed into different billets. The influence of master alloy on the cleanliness of spray formed superalloy was investigated by means of electron beam (EB) button melting and Scanning Electron Microscopy (SEM), on the basis of optimized process of spray forming and EB button melting. The results show that the inclusions in spray formed preform are mainly composed of alumina and magnesia, stem from master alloy and some refractory materials in the process of remelting. The cleaner the master alloy, the lower level of inclusion contents of the billet. Among three kinds of master alloys, the ESR ingot exhibit the cleanest melt surface in the process of re-melting and contains much smaller inclusions in EB button. The cleanliness of spray formed billet is better than ingot stack for deposition. Superclean sprayforming billet with smaller size inclusions (<100μm) can be attained by the ESR ingots.

Modeling of Flow Fields of Different Delivery Chamfers in Spray Forming Process

The computational fluid dynamic (CFD) software was used to calculate the velocity field in atomization chamber of spray forming equipment. The relationship between melt flow rates, gas aspiration of the atomizer and operating pressure are complex, and the above mentioned parameters are closely related to the atomization process. The influences of different delivery chamfers on gas flow field, which is determined by atomizer structure, were analyzed. Using K-epsilon model with a symmetrical domain, the gas dynamic of different delivery chamfer conditions were investigated. The results indicate that the sharp point of delivery tube causes detachment of flow field, and 56°, 45° and 34° chamfer conditions have same diffusion angle. Gas was aspirated from delivery tube when chamfer was 0°, which is beneficial to liquid metal flow in atomization process.