Microstructure Investigation of WC-Based Coatings Prepared by HVOF onto AZ31 Substrate

In this paper, three commercial cermet powders, WC-Co-Cr, WC-Co and WC-Cr3C2-Ni, were sprayed by the High Velocity Oxy Fuel (HVOF) method onto magnesium alloy AZ31 substrate. The coatings were investigated in terms of their microstructure, phase analysis and residual stress. The manufactured coatings were analyzed extensively using optical microscopy (OM), X-ray diffraction (XRD), scanning (SEM) and transmission electron microscopy (TEM). Based on microstructure studies, it was noted that the coatings show satisfactory homogeneity. XRD analysis shows that in WC-Co, WC-Co-Cr and WC-Cr3C2-Ni coatings, main peaks are related to WC. Weaker peaks such as W2C, Co0.9W0.1, Co and W for WC-Co and W2C, Cr3C2 and Cr7C3 for WC-Cr3C2-Ni also occur. In all cermet coatings, linear stress showed compressive nature. In WC-Co and WC-Cr3C2-Ni, residual stress had a similar value, while in WC-Co-Cr, linear stress was lower. It was also proved that spraying onto magnesium substrate causes shear stress in the WC phase, most likely due to the low elastic modulus of magnesium alloy substrate.

EFFECTS OF OVERLOAD AND SUPPLYING OIL ON CRACK GROWTH BEHAVIOR IN MG ALLOYS

A single overload was applied during the crack growth process under constant stress amplitude, and retardation of crack growth was observed in the case of magnesium alloys as well as carbon steel, aluminum alloys, etc. The retardation of crack growth was related to crack closure, the fracture surface roughness, and crack tip deformation. In addition, the effects of supplying oil into the crack on crack growth behavior of an overloaded specimen were investigated in this study. The crack growth rate in the case of supplying oil became lower than in the case without supplying oil. In the case of the magnesium alloy AZ31, powder of oxide magnesium appeared from the crack after overloading. It is one of the typical behaviors of AZ31. In the case of AZ31 and AZX912, the crack growth behavior after overloading was slightly different due to the deformation of the crack tip.

Silane-Modified Graphene Oxide Composite as a Promising Corrosion-Inhibiting Film for Magnesium Alloy AZ31

In this study, (3-aminopropyl)-triethoxysilane–modified graphene oxide (GO) composite thin films were synthesized on magnesium alloy AZ31 substrate. The structure, composition, and morphology of silane-GO films were analyzed. Electrochemical measurements and immersion tests showed that silane-GO coatings provide effective protection for magnesium alloy substrates, owing to the good barrier property of the layered GO, and decrease the defects on the GO film surface due to the silane modification. In addition, the corrosion product between the outer silane-GO film and Mg alloy substrate also improved the corrosion resistance of the Mg alloy. Thus, silane-GO composite thin films provide an effective approach for protecting the lightweight metal substrate.

Machining Temperature and Accuracy of Magnesium Alloy AZ31 with Deep-Hole Small Drilling

In recent years, magnesium-based materials have become expected to replace conventional engineering plastics as next-generation industrial materials to protect the global environment. However, in the production technology, problems of cracking and unstable accuracy in drilled hole shapes persist in plastic molding and machine tool processing; many studies have been conducted to address these problems. In dry machining ignition can be caused by the material, so wet machining is the prevalent method. However, it is necessary to establish a machining method with improved environmental parameters, considering the impact of oil mist and waste oil treatment on woks. In this study, the relationship between machining temperature and the accuracy of hole shapes in magnesium alloy AZ31 is investigated with four types of drills: high-speed steel, cemented carbide (K-Base), diamond-like carbon (DLC; K-Base), and TiN-coated cemented carbide (K-Base). The drill tip angle is set to 116°, 118°, or 120°. The work material used is the extruded AZ31 magnesium alloy. To evaluate the hole shape accuracy, squares of 80 × 80 mm are used. The cutting temperature is measured over an area of 12 × 30 mm. The work material is drilled using a dry method with a 3-mm-diameter drill having the aspect ratio (L/D) of 10. The tool protrusion length of 50 mm and cutting speed of 20 m/min are fixed, and the tool feed rate and drill step amount are changed. The experiment is repeated 3 times. The burr generated around the loophole on the back surface of the test material after the test is evaluated with a criterion burr height H of 0.02 mm. Furthermore, the average roughness (Ra) of the centerline is measured on the inner surface of the hole with a contact-type roughness meter. The results show that when using the three drill point angles of 116°, 118°, and 120° in the drill step, no burrs form at the exit of the drill hole. Carbide tools form burrs when the feed rate exceeds 30 mm/min and the step amount exceeds 20 mm. TiN tools are highly accurate up to a tip angle of 118°, while DLC tools have lower cutting forces and yield better finished surfaces than the other tools.