Study On The Cutting Performance of High-Speed Machining Zr-Based Bulk Metallic Glass

The cutting performance of high-speed machining (100-350 m/min) Zr57Cu20Al10Ni8Ti5 (at.%) bulk metallic glass (Zr57 BMG) was studied, as compared with industrial pure zirconium (Zr702). The effect of cutting speed on cutting force, surface roughness, surface morphology and chip morphology was analyzed. Although the strength of Zr57 BMG is much higher than that of Zr702, the difference in main cutting force is small, which can be attributed to the thermal softening of Zr57 BMG material during machining. The machined surface characteristics and the formation of chips were investigated. Different from low-speed machining, the groove marks and adhensions on machined surface evolve into wave patterns and molten droplets with the cutting speed increasing from 100 m/min to 350 m/min. The appearance of wave patterns tends to destroy the machined surfaces, and the worst quality was obtained at the speed of 250 m/min. The free surface morphology of the chips, with cutting speed smaller than 150 m/min, show obvious serration and molten droplets between shear bands. With the increase of cutting speed, oxidation on the chip surfaces occurred, and the chip surface was gradually covered by powder particles due to the melting of Zr57 BMG workpiece materials. The machined surfaces of Zr57 BMG maintain amorphous structures after high-speed machining, which shows excellent application potential for the processing of Zr57 BMG.

On the phenomena of partial crystallization of highly undercooled magnesium silicate molten droplets

The present work reports real-time observations of the phenomena of partial crystallization of one of the glass-forming materials, namely enstatite (MgSiO3) from its supercooled liquid droplet. Initially, the molten droplet has been held under purely non-contact conditions using the aerodynamic levitation technique. The desired levels of undercooling have been achieved by deliberately making the levitated molten droplet touch a thin molybdenum wire and hence to initiate heterogeneous nucleation from the point of contact. Influence of thermal parameters like undercooling, cooling rates and recalescence on the process of crystallization is investigated. To understand and report the morphological properties and extent of crystallinity, the solidified enstatite samples have been characterized using optical/scanning electron microscopy (SEM) and X-ray diffraction (XRD) respectively, which confirmed the formation of partially crystallized enstatite spherules and fully glass spherules. XRD showed sharp peaks of enstatite, which confirm crystallinity and a halo profile confirms the amorphous phase of enstatite. Based on the observations of several experiments, we propose the effect of thermal parameters such as levels of undercooling and recalescence on the partial crystallization, as well as partial glass formation from the initially molten droplets of enstatite composition.

Investigation of Liquid Breakup Process Solid Rocket Motor Part B: Vertical C-D Nozzle

Aluminized propellants are frequently used in solid rocket motors (SRMs) to increase specific impulse. However, as the propellant combusts, the aluminum is oxidized into aluminum oxide (Al2O3), it agglomerates into molten droplets that attach to the outside wall of the rocket nozzle. This phenomenon negatively impacts ballistics performance because the droplets remain attached to the inner wall of propulsion chambers. This buildup of particles tends to erode the wall, decreasing the performance and sustainability of the rocket. This study presents both experimental and computational fluid dynamics (CFD) to investigate the relationship between gas velocity and molten particle size for the vertically arrayed combustion chamber. Also, the Weber number and the Froude number are monitored to explain the breakup phenomenon and the condition of alumina flow in the whole testing channel. This study focused mainly on the vertical arrangement of the propulsion chamber with the cold experimental and simulation investigating the role of the liquid water in addition to a comparison with the horizontal chamber case. Unlike the horizontal setup, a greater number of droplets with smaller average droplet diameter present in the vertical setup; however, Froude number follows the same trend as for the horizontal C-D nozzle setup.

Soft Ferromagnetic Bulk Metallic Glass with Potential Self-Healing Ability

A new concept of soft ferromagnetic bulk metallic glass (BMG) with self-healing ability is proposed. The specific [Fe36Co36B19.2Si4.8Nb4]100−x(Ga)x (x = 0, 0.5, 1 and1.5) BMGs prepared by copper mold casting were investigated as a function of Ga content. The Ga-containing BMGs still hold soft magnetic properties and exhibit large plastic strain of 1.53% in compression. Local melting during shearing produces molten droplets of several µm size throughout the fracture surface. This concept of local melting during shearing can be utilized to produce BMGs with self-healing ability. The molten regions play a vital role in deflecting shear transformation zones, thereby enhancing the mechanical properties.

Characterization of mechanical behavior in repaired FC300 using directly deposited AISI-P21 and AISI-H13 metal powders

Remanufacturing is a promising technique for reducing manufacturing costs and material usage. This work presents a method for repairing casting parts using an additive metal-layer deposition process. To determine characteristics and mechanical properties of parts repaired using the proposed additive metal-layer-deposition-based method, tensile specimens were designed with grooves measuring 1 mm and 3 mm in depth. Two specific metal powders, AISI-P21 (SCM440) and AISI-H13 (SKD61), were melted using a highly focused laser, and molten droplets were subsequently built up layer-by-layer to fill-in the grooves. Mechanical and metallurgical characteristics of repaired parts were investigated via tensile and hardness tests and microstructural analyses. Experimental results demonstrate that the ultimate strength of specimens repaired using the proposed additive metal-layer deposition method measured approximately 9% lower compared with that of FC300. However, the mechanical strength of additive metal-layer deposition specimen was increased about 22% compared with that of welded specimen. Through this work, we can make a conclusion that the additive metal-layer deposition technique is well-suited for the repair and reproduction of castings.

The Morphology Analysis of Plasma-Sprayed Cast Iron Splats at Different Substrate Temperatures via Fractal Dimension and Circularity Methods

Plasma-sprayed cast iron splats were deposited onto polished aluminum substrates preheated to different temperatures ranging from 25°C to 250°C. The morphology of single splat was observed by a field emission scanning electron microscope. Quantitative characterization methods, including fractal dimension (FD) and circularity analyses of the splat profile, were employed to identify the difference in morphology of the splats with the change of the substrate temperature. The results showed that the substrate temperature has a significant effect on the spreading of molten droplets and the morphology of resultant splats through changing the solidification rate of the droplets. With the increment of substrate temperature, the homogeneous and sufficient spreading of the droplets resulted from low solidification rate reduces the splashing of the droplets. In addition, the evaporation of adsorbed moisture on the substrate improves the wettability between the spreading droplet and the substrate, then benefits the homogeneous spreading of the molten droplet. As a result, a distinct decline in the FD value was observed. It was also suggested that the FD analysis could be used to characterize the morphology of the splat more effectively while the circularity method was heavily dependent on the area of the splat.

Influence of the Titania Content on the Microstructure and Properties of Plasma Sprayed Alumina-Titania Coatings

The composite alumina-titania coatings have been widely used in industry fields to protect metallic components from wear, corrosion and thereby prolonging their service time. This paper presents an analysis of the microstructure and mechanical properties of Al2O3-TiO2 composite coatings, plasma sprayed on steel substrates from feedstocks containing various concentrations of the TiO2 powder. The coatings revealed lamellar microstructure, formed by the rapid solidification of molten droplets of the powder on previously deposited splats. The main phase identified in the coatings was γ-Al2O3 phase, the others were TiO2 and α-Al2O3, as well as amorphous phase. The results indicated significant improvement of hardness, Young’s modulus and fracture resistance of coatings with an increase in TiO2 from 3 wt. % to 13 wt. %. The friction coefficient was found to be the same for both coatings.