Bao-sheng Liu

Microstructure evolution of AZ91D induced by high energy shot peening

A nanostructured surface layer was fabricated on a AZ91D magnesium alloy by using a high-energy shot peening (HESP). HESP induced structure along the depth of the treated sample surface layer was characterized by means of X-ray diffractometer (XRD), transmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM). The experimental results show that a deformed layer of about 50μm has formed after HESP treatment and the average grain size increases from about 40 nm in the surface layer to about 200 nm at the depth of 40μm. The surface nanocrystallization can realize intercoordination of the dislocations slipping and dynamic recrystallization. The nanocrystalline grains have stacking faults and dislocation in their interiors. The microhardness of the top surface is about triplicate that of the coarse-grained matrix.

Formation Mechanism of Discoloration on Die-Cast AZ91D Components Surface After Chemical Conversion

A notebook (NB) computer component was manufactured from AZ91D Mg alloy by a die-casting process. After chemical conversion treatment, a discoloration was noted on the component surface. The source of this discoloration has been studied in detail by scanning electron microscopy, energy dispersive spectroscopy, and spark atomic absorption spectroscopy. The corrosion resistance was also measured by potentiodynamic polarization, hydrogen evolution and salt spray testing. The formation mechanism for the discoloration which was caused by the residue left behind by excess mold release agent sprayed during the die-casting was discussed in detail. After chemical conversion treatment, the residual-baked mold release agent was apparent on the component surface as “white ash.” Consequently, it degraded seriously both the appearance and the corrosion resistance of the manufactured component.

Microporosity formation in Mg alloys and its effect on protective coatings

Abstract A notebook computer component with a complex geometry was manufactured with a die cast process (DCP), using an AZ91D alloy. Chemical conversion and organic coatings were sequentially applied to provide protection against physical and chemical damage. Air content in the component, which gives rise to microporosity, was determined with a DCP computer simulation using MAGMA software. The surface layer characteristics of the component were also investigated using scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. The microporosity content was higher at the end of the filling process compared with the regions that filled first. Corrosion resistance was poorer for discontinuous conversion coatings that resulted from surface microporosity. Moreover, adhesion of an organic coating was degraded at areas with higher microporosity.

Protective compound coating on AZ91D Mg alloy fabricated by combination of cold spraying with die casting

A novel processing that leads to the formation of a protective compound coating upon the AZ91D Mg alloy by the combination of cold spraying of Al–Mg alloy on a die cavity surface with subsequent die casting was discussed. The microstructure and phase composition of the coating were investigated using X-ray diffraction and scanning electron microscope coupled with energy dispersive spectroscopy. Results revealed that the coating consisted mainly of β phase (Mg17Al12) plus some Al and integrated with the substrate by mechanical interlocking and slender metallurgical bonding formed during the die casting process. A high bond strength of 82±5 MPa for the coating was achieved. The hardness and wear resistance of the AZ91D Mg alloy were greatly improved due to the compound coating. Simultaneously, the results of immersion tests and electrochemical corrosion tests indicated that the coated AZ91D Mg alloy specimens had better corrosion resistance compared with the bare AZ91D alloy specimen.

Hydroxyapatite (HA) Modified Nanocoating Enhancement on AZ31 Mg Alloy by Combined Surface Mechanical Attrition Treatment and Electrochemical Deposition Approach

Hydroxyapatite (HA) nanocoating was electrodeposited on the surface mechanical attrition treated (SMATed) AZ31 magnesium alloy. Phases, morphologies and the adhesion of coating were characterized by X-ray diffraction, scanning electron microscopy (SEM) and 3D optical profiler. The corrosion resistance of the HA coating was tested by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the HA coating on SMATed sample had a better crystallization than that on original one. The thickness of HA coating increased from 25 to 40 μm. The bonding strength between HA coating and SMATed substrate was higher than that between the coating and untreated counterpart. Potentiodynamic polarization and EIS demonstrated that the corrosion current density of HA coating on SMATed substrate decreased by 30.84% than that on original. The corrosion potential shifted 80.3 mV to the positive direction. The corrosion resistance of coatings on SMATed sample was significantly enhanced. The immersion experiments showed that the HA coatings on SMATed sample exhibited a better biological activity.

Failure Investigation of an AlSi9Cu3 Alloy Die-Cast Cavity Insert

The examination of a fractured cavity insert, cooperated die to finish geometry intricate aluminum alloy die-casting product, is described. The dominating failure mechanism in the investigated insert is the combination of thermal fatigue cracking and stress concentration. A visual fractographic examination was performed at the cavity insert and crack initiated at the periphery adjacent to a sharp corner. Detailed scanning electron microscopy observations and energy-dispersive X-ray suggest that the crack growth was facilitated by a number of elements: oxidation of the cracks’ surfaces, filling of cast alloy, and high stress concentration of the sharp corner. The thermal cracks were also produced product around the inside wall of the cooling channel. Similarly, these cracks were also subjected to oxidation and filling with scale deposit and oxide. Finally, the schematic of the cavity insert fracture evolution during die-casting is discussed.