AZ31 Base Metal Research Articles

Effect of Weld Currents on Microstructure, Corrosion Behavior of AZ31 Magnesium Alloy

Resistance spot welding (RSW) of magnesium alloys is very attractive due to not require filler metal. AZ31 Mg alloy sheets were exposed to RSW by constant weld electrode pressure, constant weld times, and numerous weld currents. The potentiodynamic polarization technique was used for the investigation of the corrosion behavior of the alloys. Results showed that the dendritic structure was promoted at the weld zone. AZ31 base metal is more resistant to corrosion than welded alloys. However, when compared with each other, icorr values of the welded alloys decrease as a function of weld current increases. The corrosion rate was predominantly influenced by the distribution of intermetallic and alloying elements, and RSW weld varied, which was in response to altering the joint microstructure.

Effect of mono and hybrid ceramic reinforcement particles on the tribological behavior of the AZ31 matrix surface composites developed by friction stir processing

The effects of the size and morphology of the reinforcement particles on hardness and tribological behaviors of the AZ31 Mg alloy matrix composites were studied. Different ceramic compounds, including boron carbide (B4C), tungsten carbide (WC), and Zirconia (ZrO2) were selected as the reinforcement materials for developing mono composites. The average sizes of the B4C, WC, and ZrO2 particles were about 150 μm, 5 μm, and 35 nm, respectively. Besides, hybrid reinforcements composed of the B4C + ZrO2 and WC + ZrO2 powders were employed to develop hybrid composites. All the composite were fabricated using the friction stir processing (FSP) technique. Investigating the microstructure of the composites by secondary electron microscopy (SEM) analysis showed a homogenous distribution of the reinforcement particles in the AZ31 Mg alloy matrix. Microhardness measurements revealed that the hardness of AZ31/ZrO2 nanocomposite is about 120% higher than that of AZ31 base metal. According to the results of the dry sliding wear tests, the AZ31/B4C and AZ31/ZrO2 composites had a maximum wear resistance and a minimum friction coefficient average, respectively. Combining the B4C and WC reinforcements with the ZrO2 nanoparticles caused an improvement in wear resistance and friction performances of the hybrid composites. SEM observations of the worn surfaces and debris resulted from wearing of the samples after 500 m sliding distance under the normal load of 10 N, revealed that the severe and mild abrasive mechanisms are dominant.

Tribological behavior of AZ31/ZrO2 surface nanocomposites developed by friction stir processing

Abstract The present study focused on the poor wear resistance of the AZ31 magnesium alloy and improving its tribological applications. Surface nanocomposites reinforced with nano-sized ZrO2 particles on the AZ31 magnesium alloy substrate were fabricated by friction stir processing (FSP) in two different pass number. The average grain size of the as-received AZ31 base metal reduced from 11 μm to 2 μm in the case of AZ31/ZrO2 surface nanocomposite developed by four passes of FSP. Secondary electron microscopy (SEM) images of the 4-pass FSPed nanocomposite showed that the ZrO2 nanoparticles distributed homogenously in the magnesium matrix. Microhardness measurements revealed a significant improvement by adding ZrO2 nanoparticles and also increasing in FSP pass number. The microhardness of the AZ31 base metal improved about 90% in the case of four passes FSPed AZ31/ZrO2 surface nanocomposite. The wear performance of the samples was evaluated using a reciprocal wear machine and AISI 52100 steel with the hardness of 63 HRC as the counterpart. Investigating the wear performance and friction coefficient of the samples revealed enhanced tribological behavior in FSPed samples. The wear rate of the AZ31 base metal reduced about 40% for the 4-pass FSPed AZ31/ZrO2 nanocomposite. The average friction coefficient of the 4-pass FSPed nanocomposites was about half of the base metal. The SEM observations of the worn surface, wear track cross-section, counterpart tip and wear debris indicated that the wear mechanism changed from severe abrasion and adhesion for AZ31 base metal to mild abrasion for the 4-pass FSPed AZ31/ZrO2 nanocomposite.

Study on Microstructure and Properties of AZ31/6061 GTAW Butt Joints According to Mg-Cd-Al Ternary System

AZ31 Mg and 6061 Al were successfully joined by GTAW method with Cd as filler metal. The tensile strength of the joints reached 170 MPa through this alloying method according to Mg-Cd-Al ternary system. The microstructure, phases, element distribution and fracture mechanism of alloyed joints were studied. The results suggest that the fusion zone is mainly composed of CdMg compounds and Al-based solid solution. A transitional zone thickness of 40-80 μm with few primary Al12Mg17 and fine Al12Mg17-MCMS (MCMS represents a mixture of CdMg and CdMg3 compounds) eutectic is formed between the AZ31 base metal and fusion zone. The fracture surface presents three distinct regions because of the different distribution of Al12Mg17 compounds in the transitional zone. The alloying with Cd filler metal can disperse Al12Mg17 intermetallic compounds and form CdMg ordered compounds with excellent plasticity. Thus, the mechanical properties of alloyed joints have been significantly enhanced.

Effect of microstructure and microtexture on mechanical properties and fracture behaviour of friction stir-welded AZ31/AZ61 joint

Defect-free AZ31/AZ61 joint was obtained by friction stir welding. The surface characteristics, yield strength, ultimate tensile strength, grain size, microtexture, and dislocation density were characterized by experimental observations. Under low heat input, fine equiaxed grains with an average grain size of 3.5 μm were observed in the stir zone (SZ) of the AZ31/AZ61 joint. The grains formed through dynamic recrystallization. Second-phase particles observed in this region were small. The ultimate tensile strength of the AZ31/AZ61 weld seam reached 96.1% of that of the AZ31 base metal (BM). The yield strength of the joint exceeded that of the AZ31 BM due to the second-phase particle and fine-grain strengthening. The fracture was located on the AZ31–SZ-side with a (0001) plane at an angle of 45° to the tensile direction.

The mechanical properties and interface bonding mechanism of immiscible Mg/steel by laser–tungsten inert gas welding with filler wire

Immiscible Mg/steel was welded by laser–tungsten inert gas (TIG) butt fusion welding method with AZ61 filler wires. A better joint was obtained by optimizing the process parameters. The tensile strength of Mg/steel joints was improved with the increase of laser power before the weld burn–through. The interface bonding strength reached 238.1 MPa, about 94.5% of AZ31 base metal, and the joint fractured approaching Mg/steel interface. The Mg/steel interface layer was mainly composed of a continuous 50 nm–thick high temperature (HT)–Al11(Mn, Fe)4 intermetallic compound layer and approximately 350 nm–thick α–Fe(Al) solid solution. The orientation relationships (OR) of α–Fe(Al)/HT–Al11(Mn, Fe)4 system and Mg/HT–Al11(Mn, Fe)4 system matched favorable with (011)α–Fe(Al)||(060)HT–Al11(Mn, Fe)4 and [11̅0]α–Fe(Al)||[100]HT–Al11(Mn, Fe)4, (011¯2)Mg||(060)HT–Al11(Mn, Fe)4 and [101¯0]Mg||[100]HT–Al11(Mn, Fe)4, respectively. The growth model of the interface layer along the Mg/steel interface was discussed.

Weldability and Distortion of Mg AZ31-to-Galvanized Steel SPOT Plug Welding Joint by Cold Metal Transfer Method

In this study, cold metal transfer (CMT) plug welding of 1 mm thick Mg AZ31 to 1 mm thick hot-dipped galvanized mild steel (i.e., Q235) was studied. Welding tests were performed and the process variables optimized with Mg AZ61 wire and 100% argon shielding gas for a plug weld located in the center of the 25 mm overlap region. It was found that it is feasible to join 1 mm thick Mg AZ31 workpiece to 1 mm thick galvanized mild steel using CMT plug welding. The optimized process variables for CMT plug welding Mg AZ31-to-galvanized mild steel were a wire feed speed of 10.5 m/min, a predrilled hole with a diameter of 8 mm in Mg AZ31 workpiece and a welding time of 0.8 s. CMT plug welded Mg AZ31-to-galvanized mild steel joints were composed of the fusion zone between Mg AZ31 base metal and Mg weld metal, Mg weld metal (i.e., combined base metal, filler wire and Zn coating), and the brazing interface between magnesium weld metal and galvanized mild steel. The brazing interface mainly consisted of Al, Zn, Mg, Si intermetallic compounds and oxides (i.e., Fe3Al, Mg2Si, MgZn, and MgZn2), and magnesium solid solution. The static strength of CMT welded-brazed Mg AZ31-galvanized steel was determined primarily by the strength and area of the brazed interface and thickness of the intermetallic reaction layer.

Microstructure Evolution of Zn-Al Cladding Fabricated on AZ31 Magnesium Alloy

The surface of AZ31 magnesium substrate is coated by Zn-Al alloys using cast-penetrated cladding. The transverse section of alloy cladding is composed of cladding zone, diffusional zone, and bonding zone. The microstructure evolution, phase constitution, and chemical composition of the transition layer are studied. The experimental results exhibit cladding zone contained dendrite matrix and interdendritic eutectic structures. The plume eutectic structure and columnar eutectic structure are formed in diffusional zone and bonding zone, respectively. Zn and Al solid solutions gradually decrease and disappear owing to the diffusion of magnesium atom and the changes of magnesium element concentration. Mg7Zn3 phases are generated rapidly due to the interdiffusion of zinc and magnesium atoms rapidly in the diffusional zone and bonding zone. As Mg-Zn eutectic phases hinder the movement of Mg and Al atoms, the Mg-Al intermetallic compounds are eliminated completely. The microstructure is transformed into Mg solid solution and Mg7Zn3 eutectic structure to combine with AZ31 base metal.

High Temperature Plasticity of Bimetallic Magnesium and Aluminum Friction Stir Welded Joints

The high temperature deformation of a bimetallic AZ31/AA6061 Friction Stir Welded joint was investigated in the present study by constant load creep experiments carried out at 473 K (200 °C). The microstructural analysis revealed the strongly inhomogeneous nature of the weld, which was characterized by an extremely fine grain size in the magnesium-rich zones and by the extensive presence of intermetallic phases. In the high stress regime, the creep strain was concentrated in the refined and particle-rich microstructure of the weld zone, while the AA6061 base metal remained undeformed. In the low stress regime, deformation became more homogeneously distributed between the AZ31 base metal and the weld zone. The creep behavior of the weld was found to obey the constitutive equation describing the minimum creep rate dependence on applied stress for the base AZ31, slightly modified to take into account the finer microstructure and the role of secondary phase particles, i.e., the retardation of grain growth and the obstruction of grain boundary sliding.

A6061-T6/AZ31 이종금속 마찰교반접합부의 기계적 특성에 미치는 툴 삽입 위치의 영향

Butt-joints of A6061-T6 and AZ31 plates were successfully manufactured by using a friction stir welding method. Off-set free joints and off-set joints were fabricated to investigate the effect of the tool plunge position on the mechanical properties of the joints. Hardness test, tensile test and charpy impact test was performed to evaluate the mechanical properties of the joints. Off-set condition resulted in increase of mechanical properties of the joints. The variation of the hardness distribution in the SZ was also stabilized in the off-set condition. Tensile strength of the off-set joint was about 85% against to that of the AZ31 base metal. Impact absorption energy by Charpy-Impact-Test of the off-set joint also increased by almost 2 times against to that of the AZ31 base metal.

Microstructure and mechanical properties of friction stir welded dissimilar Mg alloys of ZK60–AZ31

Dissimilar friction stir welding (FSW) of ZK60 and AZ31 magnesium alloys was studied. Two kinds of material arrangements, with ZK60 or AZ31 in advancing side (AS), were applied. The results showed that the dissimilar plates with both material arrangements were joined successfully. No obvious defect was observed. Microstructure and texture variation across the joints were examined by scanning electron microscope (SEM) and electron backscatter diffraction (EBSD). The grains in stir zone (SZ) were significantly refined during FSW (∼8.7μm in AZ31 side and ∼2.7μm in ZK60 side). The second phases in ZK60 side were broken up, which contributed to grain refinement. Two-dimensional (2D) hardness maps across the normal direction (ND)—transverse direction (TD) sections in the joints were obtained. Clear boundary was observed between SZ and AZ31 base metal, but it was less clear between SZ and ZK60 base metal. The mixture of materials, grain refinement and broken up of particles were responsible for the sharp increasing of hardness between SZ and AZ31 base metal. The transverse ultimate tensile strength (UTS) of the joints was 78–82% of AZ31 base metal. The joints consistently fractured in the transition region of the AZ31 side. The sudden hardness decrease from SZ to AZ31 base metal might result in deformation incompatibility at the interface, which is considered as the main cause for the failing in the AZ31 side. In addition, the activation of twinning in SZ-side (with c-axis parallel to TD) and slip in the region adjacent to the SZ-side (with c-axis at ∼45° angle to TD) aggregated the deformation incompatibility in the transition region of the AZ31 side. As a result, the break of the specimens started at the boundary of transition zone (TZ)/SZ. Many extension twins were observed in AZ31–SZ-side after fracture. Although the ZK60–SZ-side presented the similar texture with the AZ31–SZ-side, much less extension twins were observed in the former because of the hindering effects by the refined grains and secondary particles.

Bonding Interface Formation between Mg Alloy and Steel by Liquid-phase Bonding using the Ag Interlayer

Liquid-phase bonding between a Mg alloy (AZ31) and low-carbon steel was attempted at 773 K (500 °C) using Ag as an interlayer that forms a eutectic melt with the Mg alloy at this temperature. On the AZ31 side, eutectic melting and subsequent isothermal solidification were observed, and it was confirmed that the solidification of the eutectic liquid was promoted by the diffusion of Ag into the AZ31 base metal. On the steel side, Al was transported from AZ31 during the eutectic melting and isothermal solidification. This transported Al was enriched at the steel surface and reacted with steel to form a uniform, thin Fe-Al intermetallic compound layer. After the isothermal solidification, strong bonding was achieved via the thin intermetallic compound layer between AZ31 and steel, and no Ag remained at the bonding interface. The strength of the joint was found to be higher than the yield strength of AZ31.