Peijie Li

Preparation of Al–TiC composites by self-propagating high-temperature synthesis

Al–TiC composites were produced by a casting route assisted by self-propagating high-temperature synthesis (SHS). Fluxes were used to activate a SHS reaction in the aluminum melt and promote higher Ti and C yield in the composites. The TiCx stoichiometry corresponded to C/Ti ratios from 1.0 to 0.8.

Effect of fluxes on structure formation of SHS Al-Ti-B grain refiner

Abstract The Al–5%Ti–1%B grain refiner was prepared by the casting route with the use of self-propagating high-temperature synthesis (SHS). The activity of the SHS reaction was increased through the use of two fluxes, Na 3 AlF 6 and K 1–3 AlF 6 , which were found to have a great effect on the structural characteristics of Al 3 Ti phase. The K 1–3 AlF 6 flux caused the formation of needle-like intermetallic particles with larger size, whereas the Na 3 AlF 6 resulted in producing smaller particles with blocky morphology. In the absence of fluxes, some silicon was incorporated in the lattice of this phase.

A new insight into high-strength Ti62Nb12.2Fe13.6Co6.4Al5.8 alloys with bimodal microstructure fabricated by semi-solid sintering

It is well known that semi-solid forming could only obtain coarse-grained microstructure in a few alloy systems with a low melting point, such as aluminum and magnesium alloys. This work presents that semi-solid forming could also produce novel bimodal microstructure composed of nanostructured matrix and micro-sized (CoFe)Ti2 twins in a titanium alloy, Ti62Nb12.2Fe13.6Co6.4Al5.8. The semi-solid sintering induced by eutectic transformation to form a bimodal microstructure in Ti62Nb12.2Fe13.6Co6.4Al5.8 alloy is a fundamentally different approach from other known methods. The fabricated alloy exhibits high yield strength of 1790 MPa and plastic strain of 15.5%. The novel idea provides a new insight into obtaining nano-grain or bimodal microstructure in alloy systems with high melting point by semi-solid forming and into fabricating high-performance metallic alloys in structural applications.

Electron theory study on mechanism of action of cobalt in Fe-Co-Cr based high-alloy steel

The valence electron structure of martensite in FeCoCr based high-alloy steel is calculated using the empirical electron theory of solids and molecules (EET). The results show that the incorporation of cobalt leads to a rise in nA from 0.3835 to 0.4684, which enhances the bonding forces between atoms in α-Fe matrix. Meanwhile, the incorporation of Co changes the valence electron structure of segregated structure units formed by C and other alloying elements, and increases nA for the segregated units containing C-Me significantly, resulting in changing the precipitation behavior of the secondary phases during tempering and strengthening the resistance to tempering.

Characteristics of defect bands and their formation mechanisms in A356 wheel fabricated by horizontal squeeze casting

Abstract Based on the opinion that a decreasing solid fraction gradient in the cross-section develops during the intensification stage, the shear stress in the solidifying alloy was analysed, and a mechanism for dilatant band formation was discussed. That is, when the shear stress at one position of the stable grain network reaches a critical value, the network collapses and the band forms. The characteristics of defect bands at different locations of an A356 wheel fabricated by horizontal squeeze casting were then studied. Three types of defect bands, i.e. skin related band, dilatant band and separate band, were observed. Combining the process and conditions of defect band formation, several types of defect bands in the casting were discussed. Furthermore, the influence of the structural characteristics of castings on the defect bands, including thickness, geometry and location, was summarised.

Ultrafast consolidation of bulk nanocrystalline titanium alloy through ultrasonic vibration

Nanocrystalline (NC) materials have fascinating physical and chemical properties, thereby they exhibit great prospects in academic and industrial fields. Highly efficient approaches for fabricating bulk NC materials have been pursued extensively over past decades. However, the instability of nanograin, which is sensitive to processing parameters (such as temperature and time), is always a challenging issue to be solved and remains to date. Herein, we report an ultrafast nanostructuring strategy, namely ultrasonic vibration consolidation (UVC). The strategy utilizes internal friction heat, generated from mutually rubbing between Ti-based metallic glass powders, to heat the glassy alloy rapidly through its supercooled liquid regime, and accelerated viscous flow bonds the powders together. Consequently, bulk NC-Ti alloy with grain size ranging from 10 to 70 nm and nearly full density is consolidated in 2 seconds. The novel consolidation approach proposed here offers a general and highly efficient pathway for manufacturing bulk nanomaterials.

Tribological Behavior of Graphene Reinforced 600 °C High Temperature Titanium Alloy Matrix Composite

Graphene reinforced high temperature titanium matrix composite synthesized by spark plasma sintering (SPS) was prepared from mixed powders of graphene oxide (GO) and 600 °C high temperature titanium alloy (TA29). SEM, EDS, XRD, Raman spectroscopy, UMT and SRV tribotester were employed to investigate the microstructure and tribological properties. Microstructural studies indicated that the added graphene oxide was substantially decomposed into graphene during the sintering process and did not cause a significant increase in oxygen content of the TA29 matrix. Graphene was observed to remain after the sintering process and be evenly dispersed in the TA29 matrix. The friction coefficient and wear rate under room temperature and light load were reduced by 15 and 60% respectively, because the graphene dispersed in the matrix was proved to improve the conditions of the frictional interface by self-lubricating effect. The wear rate under high temperature and heavy load was reduced by about 25% for the increasing in the yield strength of graphene reinforced TA29 matrix composite.

Effect of Dual-Sided Laser Peening Modes on Residual Stress Distribution of Aero-engine Titanium Blades

For thin-walled structure components, dual-sided laser peening modes greatly affect surface integrity of the components. In this study, simplified compressor blade made of TC17 titanium alloy was chosen as the studied object. The effect of simultaneous and non-simultaneous dual-sided laser peening on residual stress distributions along surface and depth were studied through numerical simulation. The influencing mechanisms of dual-sided laser peening modes on residual stress distribution along depth were investigated by means of wave propagation theory. This study is instructive in parameter choosing for laser peening of thin-walled structure components based on better understanding the effect of dual-sided laser peening modes on residual stress distribution.

Laser Ablation of Aluminum and Titanium Alloys Under Glass Confinement

Single-pulse laser ablation of aluminum and titanium alloys under glass confinement is investigated. Processing parameters such as laser intensity and gap width between targets and glass are varied. Surface morphologies and crater profiles of the alloys after laser ablation are studied by SEM and white-light interferometric microscope, respectively. The effects of gap width and laser intensity on surface morphologies and crater profiles of targets are systematically analyzed. In addition, the generation of porous surface structures in Ti target is regarded as the result of phase explosion in the molten surface layer of targets and the thermodynamic condition in our experiment for phase explosion is evaluated.