Hideharu Shimizu

Biaxial Tensile Deformation Behavior and Microstructural Evolutions of Superplasticity in AZ31 Magnesium Alloy

Magnesium alloys show promise in meeting the demand for materials of lighter weight and higher rigidity. Mg alloys are hard to process and normally require grain refining for improved formability and mechanical properties. To process these fine-grained Mg alloys effectively, it is important to relate their load stress and mechanical properties to changes in their microstructures. Using a biaxial tensile machine and cruciform specimens, to evaluate the mechanical properties, microstructure, and plasticity, in a high temperature biaxial stress state, used of AZ31 Mg alloy sheet. With biaxial deformation, grain boundary slide occurred more frequently than with uniaxial deformation, causing grain boundary separation and formation of micro-voids between the grains. In the vicinity of the cracks and at the locations of grain boundary separation, although deformation temperature at higher than the recrystallization temperature, fine grains (about 2 )m) showing in duplex grain structures were formed locally. The formation of duplex grain structures as a result of local formation of fine grains during the deformation process is a major issue to be solved from the viewpoint of plasticity processing.

Improvement of the Fatigue Characteristic of AZ31 Magnesium Alloy by Microstructures Control

High-strain conditions as a means of microstructure control have recently been investigated to improve the ductility and enhance the strength of magnesium alloys. The level of superplastic deformation and the fatigue properties of the wrought materials have also been studied. In comparison, only a small number of such reports are available on cast materials. As a part of the search for applications of magnesium alloys, comparisons of structural changes and mechanical properties should be made between wrought and cast materials. In the present study, the grain refinement of cast and extruded materials made from commercially available AZ31 magnesium alloy was conducted using a multi-axial alternative forging method. The relationships between the structural changes and working processes and the relationships between changes in the mechanical properties as well as grain sizes and fatigue properties are discussed. Both the cast and the extruded materials tended to exhibit uniform crystalline structures with an increasing number of working cycles. Dynamic recrystallization was observed during both working and static recrystallization during both reheating and holding. When an equivalent strain of 0.6 was applied, the localized formation of ultra-fine grains of 0.5 μm was observed. The tensile strength and yield stress had maximum values in the initial stage of the multi-axial alternative forging. Although ductility improved with higher numbers of working cycles, the strength decreased. This can be explained by the dynamic and static recrystallization processes and work softening.

Study on Liquid Zinc Embrittlement of Hot-dip Galvanizing Bolts, IV. Effects of Cooling Condition on Liquid Zinc Embrittlement Cracking and Plating Performance of Hot-Dip Galvanizing Bolts.

Hot-dip galvanized bolts tend to crack during galvanizing at the under-head fillet or thread root on which stress is concentrated. In this study, we investigated the influence of the temperature of cooling water on the occurrence of cracking. We further examined the plating performances under different cooling conditions. The main results obtained are as follows.(1) Heating of cooling water to a higher temperature is found effective against cracking.(2) The shower-cooling method is effective against cracking, with almost no difference from the conventional cooling method in plating performance such as plating microstructure or cramping torque coefficient.

Effect of Hot-dip Galvanizing Condition on Crack Nucleation of Bolts.

A crack sometimes occurs at the underhead fillet of a bolt in the hot dip galvanizing process. In the preceeding paper, the effect of pretreatment in this process on crack nucleation was investigated experimentally.In this study, to investigate the crack nucleation time in hot-dip galvanizing and the effect of cooling condition on crack nucleation at the hot dip galvanizing process, some experiments and thermal stress analysis by FEM were performed. The main results are as follows.(1) The crack occurred at the underhead fillet of a bolt which had been dipped in a zinc bath over 10 seconds.(2) The crack occurred at the underhead fillet of a bolt when the bolt had been cooled by water, but it did not occur when the bolt had been cooled by air.(3) It was found by numerical calculation of FEM that high tensile thermal stress occurred during the cooling process after the hot dip galvanizing process.

Effect of pretreatment on crack nucleation of hot dip galvanized bolts.

Crack nucleation sometimes occurs at the underhead fillet of a bolt when it is galvanized. The phenomenon is generally known as liquid zinc embrittlement, but the effect of the pretreatment such as the acid pickling on liquid zinc embrittlement is not well known.In the present study, to investigate the effect of the pretreatment on liquid zinc embrittlement, the observations of the crack on the longitudinal section and the fracture surface at the underhead fillet of bolts which were subjected to various pretreatments were performed by using an EPMA and an optical microscope. The main results are as follows:(1) The acid pickling accelerated crack nucleation.(2) The value of pH in fluxing affected the crack nucleation.(3) The shot blasting had the effect of preventing crack nucleation.