Stress relaxation behavior of general-purpose wrought Mg-3Al-1Zn (mass%, AZ31) alloy was investigated by bolt load retention test. Rolled AZ31 alloy sheets, which had high density of dislocations and fine grain structure, showed large stress drop during the test, presumably owing to the enhanced dynamic recrystallization or grain boundary sliding. Unlike the rolled samples, an extruded AZ31 alloy had coarse-grained recrystallized microstructure, resulting in the relatively low stress drop during the test. For comparison, the stress relaxation behavior of a commercial rolled Al-Mg alloy (A5052-H34) sheet was evaluated. Although high density of dislocations existed in the aluminum sheet, dynamic recovery and dynamic recrystallization were suppressed, which led to the limited stress drop during the test.

The active deformation mode (slip systems and twinning) of extruded AZ31B alloy (Mg-3Al-1Zn, mass%) during compression at RT, 100°C and 150°C was investigated by visco-plastic self-consistent (VPSC) simulation. Compression tests were first performed to obtain compression curve of AZ31B. The VPSC model was fitted to the compression curves to estimate the Voce hardening parameters which are required for the VPSC simulation. In the case of compression along the extrusion direction, work-hardening occurred rapidly with the transition of dominant deformation modes at all temperatures. In the case of compression to the other directions, basal＜a＞slip was dominant throughout the deformation at all compression temperatures. In addition, prismatic＜a＞slip and tensile twin were active in the early stage of deformation, and pyramidal＜c＋a＞slip became active in the later stage of deformation. Pyramidal＜c＋a＞slip became more active with increasing temperature from RT to 100°C, whereas basal＜a＞nd prismatic＜a＞slips became more active with increasing temperature from 100°C to 150°C. These different trends in the change of the active slip system with increasing temperature can be attributed to the CRSS maximum of the pyramidal＜c＋a＞slip located around 100°C.

The stress corrosion cracking (SCC) behavior of an extruded AZ31 alloy plate with a large planar anisotropy was investigated using constant load test in 0.01 mol/L NaCl solution at 35 ºC. Due to the texture with a large spread of (0001) orientation in the transverse direction (TD), the extruded AZ31 alloy plate exhibited a much higher yield strength (169 vs. 70 MPa) along the extrusion direction (ED) compared with the TD. The threshold stress for SCC was slightly higher for the ED specimen compared with the TD specimen (90 vs. 70 MPa) due to the much higher yield strength. However, the ratio between threshold stress and yield strength was much larger for the TD specimen (100% vs. 53%), indicating its low susceptibility to SCC. Corrosion grooves formed perpendicular to the tensile direction and along the interstices of Mg(OH)₂ corrosion product film. Corrosion grooves acting as fracture initiation sites tended to propagate along non-basal planes, particularly, prismatic plane, due to lower corrosion resistance compared with basal plane. The higher resistance to SCC for the TD specimen may be attributed to the higher probability of basal plane on the exposed surfaces of corrosion grooves.

The effect of weld geometry on fatigue properties of flame-resistant magnesium alloy welded joints was evaluated using computational and data science methods. First, a workflow for fatigue life prediction of flame-resistant magnesium alloy welded joints was proposed. The workflow consists of thermo-elastic-plastic analysis simulating welding, macroscopic stress field analysis to identify stress concentration zones, crack initiation analysis using crystal plasticity analysis and Tanaka-Mura model, and crack propagation analysis using finite element method and Paris’ law. The fatigue life calculations were repeatedly performed by using the proposed workflow for different shapes of weld toe, excess weld metal and undercut, and the results were compiled into a database. A surrogate model was developed from the database by machine learning to predict the fatigue indicator parameter (FIP), which is an index of crack initiation life obtained by the Tanaka-Mura model. The effect of weld geometry on fatigue life was evaluated by exploring the surrogate model using a Markov chain Monte Carlo method. The results obtained are consistent with the previous findings that toe radius and undercut depth have a strong influence on fatigue.

Rotating bending fatigue tests were conducted for Mg-Al-Ca alloy thick plates with different compositions (extruded Mg-4%Al-1%Ca (AX41, 20 mm thickness) and extruded Mg-9%Al-1%Zn-2%Ca (AZX912, 14 mm thickness), and microstructural observation was made to clarify the effects of microstructural factors on the fatigue properties of the alloys. Strong basal texture, in which basal plane was inclined to the transverse direction, was formed in both AX41 and AZX912. Hence, the yield stress of AX41 and AZX912 exhibited large anisotropy. The results of rotary bending fatigue test showed larger anisotropy of fatigue strength in AX41 as well as the tensile tests. It was suggestedthat the strong texture formation has a large effect on fatigue strength similar to the tensile tests. The fatigue strength of AZX912 thick plate obtained by rotary bending fatigue test was comparable to that of AZX912 medium plate (3 mm thickness) obtained by plane bending fatigue test. The reason was that the coarse second-phase particles more than 30 μm were found in the AZX912 medium plate as well as in the AZX912 thick plate, although in small amounts.

This study investigates the fatigue properties, Charpy impact energy, and fracture toughness of three flame-retardant magnesium alloys with the different amount of aluminum solid solution. These properties strongly depended on the amount of aluminum dissolved in the magnesium alloy. Micro-strain analysis was used to evaluate the degree of damage during fatigue tests. This method could successfully evaluate the degree of damage by focusing on the principal slip plane rather than the crystallographic planes. In addition, we examined the influence of welding, wherein AZX811 filler metals containing Ga were used, on Charpy impact energy and fracture toughness. The Charpy impact energy of the welded joint was lower than that of the base metal with the same amount of aluminum solid solution. However, the fracture toughness value of the welded joint was almost the same as that of the base metal. This was because the welding defects decreased the Charpy impact absorbed energy. However, they did not affect the fracture toughness value owing to the microscopic elastoplastic behavior of the crack tip. Furthermore, a proportional relationship between the impact-absorbed energy and fracture toughness value was determined for both the base metal and welds.

For the application of thixomolded AZ91D magnesium alloy to structural parts, the effects of carbon nanoparticle addition and phosphate anodization on fatigue properties were evaluated by rotary bending fatigue tests. Mechanical properties such as the tensile and fatigue strengths were improved by only 0.1 mass% of the carbon nanoparticle addition because of the refinement of crystal grains. Compared to the fatigue strength of un-treated specimens, the fatigue strength was reduced by phosphate anodization with and without carbon nanoparticle addition. In this case, pre-treatment before anodization reduced the fatigue strength by about 20 MPa. In phosphate anodization, there is concern that the plasma generated during electrolysis may heat the specimen surface and affect the microstructure. However, in this experiment, the effect of heating was negligible. It was expected that pores of a few microns in the anodic oxide film would adversely affect fatigue properties.

Tensile tests at various strain rates in humid air (HA) and dry nitrogen gas (DNG) were carried out to investigate the effects of strain rate and humidity on tensile properties of AZ31 and AZ61 magnesium alloys. In both alloys, the elongation in DNG increased with decreasing the strain rate. When the AZ31 alloy was tested at a strain rate of 1.39 ×10^‒6s^‒1 in HA, it showed a loss of elongation due to quasi-cleavage fracture. The degradation of ductility was observed in the AZ61 alloy as well as the AZ31 alloy. The embrittlement sensitivity was larger in the AZ61 alloy compared to the AZ31 alloy. In HA, hydrogen atoms arisen from chemical reaction between magnesium surface and water vapor were presumed to be absorbed into the specimen and result in hydrogen embrittlement.

Ignition temperatures and dust explosion properties (minimum explosible concentration and minimum ignition energy) of machined chips of flame retardant magnesium alloys (AZX611 (Mg-6Al-1Zn-1Ca, mass%), AXM4102 (Mg-4Al-1Ca-0.2Mn) and AZX912 (Mg-9Al-1Zn-2Ca)) and aluminum alloy (A6005C (Al-0.6Si-0.6Mg)) were investigated. Ignition temperature of the flame-retardant magnesium alloy chips (＜300 μm) measured by DTA (Differential Thermal Analysis) slightly increased with increasing calcium concentration, regardless of particle size of chips. As a result of the tests of minimum explosible concentration, the flame-retardant magnesium alloys exhibited significantly lower values than A6005C. In particular, AZX912 had lower value than the other magnesium alloys, due to smaller particle size of the AZX912 chips than the other magnesium alloys. The minimum ignition energy of AZX611 and A6005C was also evaluated. As a result, a significantly lower value was observed in AZX611 compared to A6005C. On the other hand, the value of AZX611 was higher than the literature value due to larger particle size. A series of experiments indicated that the dust explosion properties were strongly dependent on particle size rather than alloy composition (calcium concentration).

Porous aluminum, which has many pores inside, is lightweight and has high shock absorption properties. However, once porous aluminum is impacted and deformed, the cell walls are collapsed and difficult to reuse. In this study, we attempted re-foaming of deformed porous aluminum using the undecomposed foaming agent remained during the porous aluminum fabrication process. In our previous study, it was found that porous aluminum can be re-foamed by compressing and densifying with FSP before optical heating. In this study, we attempted further foaming of refoamed porous aluminum by optical heating after densification by compression and FSP. The amount of foaming gradually decreased with each repeated foaming, and the pore structures were not the same as that of the initial foaming, but further foaming was achieved after re-foaming. From the XRD analysis, it was observed that the undecomposed foaming agent remained after re-foaming.