As a new type of high manganese steel, the twinning induced plasticity （TWIP） steels have attracted a growing interest in the automotive industry due to their good performance. Thin plates of TWIP steel were welded by laser beam welding （LBW） and gas tungsten arc welding （GTAW）. The microstructure result shows that GTAW joint has obvious heat-affected zone （HAZ）, while the HAZ of LBW joint is almost invisible. The X-ray diffraction result shows that the phase compositions of both joints are austenitic and no phase transition occurs. Energy disper- sive spectrometry result shows that there is violent evaporation of Mn element in LBW joint, while the proportion of Mn element in GTAW joint is almost unchanged. Tensile tests and micro-hardness measurements were performed to take into account the mechanical properties of joints manufactured by the two different processes. The micro-hard- ness profiles of both joints present a typical saddle distribution, and the hardness of GTAW seam is lower than that of LBW seam. The failure positions of LBW joints are all located in base metal while the GTAW joints are all at the weld toe due to the softening of HAZ. By means of scanning electron microscopy, a typical ductile fracture is observed in LBW joint, while a brittle fracture with quasi-cleavage fracture characteristic is observed in GTAW joint.
The corrosion fatigue behavior of epoxy-coated Mg-3Al-1Zn alloy in gear oil was investigated. The corrosion and the fracture surfaces after fatigue test were analyzed by scanning electron microscopy(SEM) and the corrosion compositions were measured by energy-dispersive spectrometry(EDS). The fatigue properties and the crack initiation mechanisms of the specimens before and after epoxy coating treatment were discussed. The results indicate that the fatigue limit after epoxy coating treatment in gear oil is higher than that of the uncoated specimens. The epoxy coating is an excellent way to prevent direct contact between the Mg-3Al-1Zn alloy and surrounding environments. The mechanical properties of the epoxy coating layer are lower than that of magnesium alloy, which is the main reason for the fatigue crack initiation on the epoxy coating layer. In addition, the gear oil lubrication could lead to the flaking off of the epoxy-coated layer.
In order to simulate and study the mechanism of cement stabilized soils polluted by different contents of magnesium sulfate(MS), a series of tests were conducted on the cemented soil samples, including unconfined compression strength(UCS) tests of blocks, X-ray diffraction(XRD) phase analysis of powder samples, microstructure by scanning electronic microscopy(SEM),element composition by energy dispersive spectrometry(EDS), and pore distribution analysis by Image Processed Plus 6.0(IPP 6.0)software. The UCS test results show that UCS of cemented soils reaches the peak value when the MS content is 4.5 g/kg. While, the UCS for Sample MS4 having the MS content of 18.0 g/kg is the lowest among all tested samples. Based on the EDS analysis results,Sample MS4 has the greater contents for the three elements, oxygen(O), magnesium(Mg) and sulfur(S), than Sample MS1. From the XRD phase analysis, C-A-S-H(3Ca O·Al2O3·3Ca SO4·32H2O and 3Ca O·Al2O3·Ca SO4·18H2O), M-A-H(Mg O·Al2O3·H2O), M-S-H(Mg O·Si O2·H2O), Mg(OH)2 and Ca SO4 phase diffraction peaks are obviously intense due to the chemical action associated with the MS. The pore distribution analysis shows that the hydrated products change the distribution of cemented soil pores and the pores with average diameter(AD) of 2-50 μm play a key role in terms of the whole structure of cemented soil. The microscopic structure of the cemented soil with MS exhibits the intertwined and embedded characteristics between the cement and granular soils from the SEM images of cemented soils. The microstructure analysis shows that the magnesium sulfate acts as the additive, which is beneficial to the soil strength when the MS content is low(i.e., Sample MS2). However, higher MS amount involving a chemical action makes samples crystallize and expand, which is adverse to the UCS of cemented soils(i.e., Sample MS4).
The effect of the rare earth element Er on the microstructures and properties of Mg-Al intermetallic were studied in this experiment. Metallographic and X-ray diffraction（XRD） results showed that the microstructures of Mg-Al-Er alloys varied with Er content. The Mg-44Al-0.5Er and Mg-43.8Al-1.0Er alloys were both composed of Mg17Al12 matrix and Al3 Er phase, whereas Mg-43Al-3.0Er and Mg-42Al-5.0Er were composed of Mg17Al12 matrix, Al3 Er phase, and Mg-Mg17Al12 eutectic. The Mg-42Al-5.0Er alloy showed the highest microhardness, and the values remained nearly stable as Er content increased from 1.0 wt.% to 5.0 wt.%. The dispersed second phase Al3 Er caused the grain refinement of the Mg-Al-Er alloy, which was the main reason for the improvement in microhardness. The corrosion resistance of the Er-containing alloys initially increased and then decreased with increasing Er content. All the Er-containing alloys had the ability to suppress hydrogen evolution, which was the main reason for the higher corrosion resistance of the modified alloys than that of the Mg-44.3Al alloy. Considering the higher hardness and dispersity of the Al3 Er phase, Mg-43.8Al-1.0Er exhibited higher wear resistance than the as-cast Mg-44.3Al alloy.
The corrosion fatigue behavior of epoxy-coated Mg-3Al-1Zn alloy was investigated in air and 3.5 wt%NaCl solution. Epoxy coating as a new method was used to improve the corrosion fatigue property of the material.Results show that the fatigue limit（FL） of the coated specimens is higher than that of the uncoated specimens in3.5 wt% NaCl solution because of the strengthening and blocking functions of the epoxy coating. The FL of the coated specimens in 3.5 wt% NaCl solution is as high as that in air. It implies that the coated specimens are not as sensitive to the environment as the magnesium alloy. The low tensile strength and the short elongation of the pure epoxy coating lead to that the fatigue crack of the coated specimen is always initiated from the epoxy-coating film Pores and pinholes accelerate the fatigue crack initiation process. Pinholes are caused by the corrosion reactions between the epoxy coating and the NaCl solution.
Xiu-Li HeYing-Hui WeiLi-Feng HouZhi-Feng YanChun-Li GuoPeng-Ju Han
