Guo Feng Wang

Superplasticity and Sinter-Forging of Fine-Grained Si3N4-Si2N2O Composite

The Si3N4- Si2N2O composites are fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering(LPS) method in this article. XRD analysis shows that the sintered body consists of β-Si3N4 and Si2N2O. SEM experiment conforms that the average grain size of sintered body is less than 300nm. The superplastic deep-drawing forming can be proceed at a low temperature of 1550°C with a forming velocity of 0.2mm/min. There are only a few small sintered defects before forming, but there are a lot of cavity groups after forming. Cavitation failure occurs by nucleation, growth and interlinkage of cavities. The complex-shape gears can be formed by a sinter-forging technology when the sintering temperature is 1600°C and the superplastic forging temperature is 1550°C.

Microstructure Evolution and Fracture Behavior in Superplastic Deformation of Hot-Rolled AZ31 Mg Alloy Sheet

Fine-grained AZ31 magnesium alloy sheets were prepared through hot rolling process. The superplastic properties of hot-rolled AZ31 Mg alloy was examined by uniaxial tensile tests at a temperature range 250~450 and strain rate range 0.7×10 -3 ~1.4×10 -1 s -1 . Optical and scanning electronic microscope (SEM) were used to observe the microstructure evolution and fracture behavior in superplastic deformation of AZ31 Mg alloy and the values of deformation activation energy at various temperatures were calculated. It is demonstrated that, the hot-rolled AZ31 alloy begins to exhibit superplasticity from 300 and a maximum elongation of 362.5% is obtained at 400 and 0.7×10 -3 s -1 . In the temperature range 300~400 , the dominant super plastic deformation mechanism is grain boundary slide (GBS) controlled by grain boundary diffusion and the effects of temperatures on the fracture behavior of AZ31 Mg alloy are embodied by the transition of deformation mechanism from intracrystalline slip to GBS.

The Superplastic Forming Technology of Ti-6Al-4V Titanium Alloy Bellows

A new forming technology for bellows, which uses a combined superplastic forming (SPF) method by applying gas pressure and an axial compressive load, is developed. It can be used to fabricate large diameter U type titanium alloys bellows. Compared with conventional methods, the method has the advantage of low cost, high dimension accuracy and a high rate of finished products. During the SPF process, the tubular blank is restrained in a multi-layer die. The final dimensions of the bellows conform to corresponding dies. In order to obtain optimum thickness distribution of the components, the load route is divided into three stages free bulging, clamping and calibrating. Numerical simulations of this bellows SPF process are performed using a rigid visco-plasticity shell finite element code that was developed by the authors. Simulation results show great agreement with the experiments.

Superplasticity and Microstructure Evolution of Electrodeposited Nanocrystalline Nickel

Nanocrystalline pure nickel (nc-Ni) was produced by pulse electrodeposition and its superplastic properties at and above room temperature were investigated. The electrodeposited nickel has a narrow grain size distribution with a mean grain size of 70nm. Uniaxial tensile tests at room temperature showed that nc-Ni has a limited plasticity but high tensile strength up to 1GPa at strain rates between 10-5 and 10-2s-1. However, when the temperature increased to 420 and higher, test specimens showed uniform deformation and the elongation value was larger than 200%. A maximum elongation value of 380% was observed at 450°C and a strain rate of 1.67x10-3s-1, SEM and TEM were used to examine the microstructures of the as-deposited and deformed specimens. The results indicated that fracture was caused by intergranular cracking and most cracks were originated from the brittle oxide formed during the tensile test. Grain coarsening was observed in the deformed specimen. The role of temperature and strain on grain growth was evaluated by comparing the microstructure of deformed samples with that of samples statically annealed. Deformation mechanism was discussed based upon the deformed microstructure and strain rate jump tests.

Synthesis and Superplastic Deep Drawing of TiAl Alloy

TiAl alloys are powerful candidates for light weight high temperature structural materials because of their excellent high temperature strength, low density and good oxidation resistance. Unfortunately, TiAl alloys are difficult to machine and hot working due to ordered structure, which impede large-scale application. Synthesis and superplastic deep drawing of a TiAl alloy were studied. Mechanically synthesized fine crystalline Ti/Al composite powders as precursor of TiAl alloy. The TiAl alloy with fine grain via reactive sintering was obtained. The deformation behavior under biaxial stress by means of deep drawing was achieved. The optimum temperature is 1100°C≤T≤1150°C. The microstructures corresponding to deep drawing in the part were studied.

Current Assisted Superplastic Forming of Titanium Alloy Bellows

Titanium alloy bellows has advantages of light weight, excellent mechanical property, good heat resistance and corrosion resistance, etc. But the cold formability of titanium alloy is poor, so it is difficult to manufacture titanium alloy bellows by traditional mechanical forming and hydroforming. In this paper, current assisted superplastic forming technology was used to process titanium alloy bellows, which could overcome some shortcomings of traditional superplastic forming effectively, such as slow heating rate, high energy loss and low production efficiency. And titanium alloy bellows formed by this technology is of good quality with uniform wall thickness, light oxidation.

Superplastic Bulging of Nanocrystalline Ni-Co Alloy with Different Heating Methods

Superplasticity of nanocrystalline materials is a hot spot in the field of scientific research. In this paper, Ni-Co alloy was produced through pulse electrodeposition. Tensile tests were carried out to study the room temperature strength, high temperature plasticity. The superplastic formability under complex stress was evaluated through the superplastic bulging tests. The tests were studied through the methods of resistance heating and furnace temperature heating. The maximum ratios of height and diameter with different heating method were compared. Fracture behavior and microstructure were observed by the method of SEM.

Superplasticity of Nb-Si-Fe and TiAl Intermetallics Synthesized by Powder Metallurgy

Nb-16Si-2Fe alloy were processed by mechanical alloying (MA) and hot pressing sintering (HPS). Microstructure analysis revealed the presence of four phases: Nb solid solution (Nbss), three kinds of intermetallics Nb3Si, Nb5Si3 and Nb4Fe3Si5. The maximum elongation over 500% was obtained at 1450°C and strain rate of 2.31×10-4s-1. TiAl powder pre-alloyed was carried out on pulse current sintering equipment (PCS) with high heating rate. The effect of heating rate on microstructures and high temperature ductility was investigated. The results show that relatively high heating rate is beneficial for obtaining fine grained microstructures. And the resultant intermetallic alloy with equiaxed near gamma structures exhibits superplasticity at relatively low temperature.

Superplastic forming and diffusion bonding for four-layer sheets structure of nickel-base superalloy

The superplastic forming and diffusion bonding (SPF/DB) is applied in aviation and space flight field. The SPF/DB process with gas pressure control for dissimilar superalloy structure was studied. Diffusion bonding parameters, including bonding temperature T, pressure P, time t, affect the joining mechanism. When the bonded specimen with 50&m thick nickel foil interlayer was tensile at room temperature, shear fracture of the joints with nickel foil interlayer takes place at the GH4141 superalloy part. The SPF/DB of four-layer sheets structure was investigated. The optimum parameters for the SPF/DB process are: forming temperature T=1243K, forming pressure P=1MPa, forming time t=35min. The microstructure of the bonded samples was characterized. The microstructure shows an excellent bonding at the interfaces. The distribution of thickness after SPF/DB was investigated.

Gas pressure forming of amorphous Fe78Si9B13 alloy

The deformation behavior of gas pressure forming of amorphous Fe78Si9B13 alloy was investigated under equibiaxial tension. The gas pressure forming was carried out in the temperature range of 430°C~530°C below the crystallization temperature Tx and die apertures of 5mm~10mm. The dome height and amorphous ribbon thickness of deformed specimens at the pole was measured. It was found that amorphous Fe78Si9B13 alloy had exhibited good plasticity in the experimental temperature range. The near-semisphere specimens of the radius 5mm and the height 4.5mm were obtained from the gas-pressure forming at 450°C and 530°C for 30min, which is similar to the superplastic forming.