The Mechanisms of Resistance Spot Welding of Magnesium to Steel

Resistance spot welding of AZ31 magnesium alloys from different suppliers, AZ31-SA (from supplier A) and AZ31-SB (from supplier B), was studied and compared in this article. The mechanical properties and microstructures have been studied of welds made with a range of welding currents. For both groups of welds, the tension-shear fracture load (F _C) and fracture toughness (K _C) increased with the increase in welding current. The F _C and K _C of AZ31-SA welds were larger than those of AZ31-SB welds. The fracture surfaces of AZ31-SB welds were relatively flatter than those of AZ31-SA. Microstructural examination via optical microscope demonstrated that almost all weld nuggets comprised two different zones, the columnar dendritic zone (CDZ), which grew epitaxially from the fusion boundary, and the equiaxed dendritic zone (EDZ), which formed in the center of the nugget. The nature and extent of the CDZ seemed to be critical to the strength and toughness of spot welds because of its position adjacent to the inherent external circular crack-like notch of spot welds and the stress concentration in this region. The width and microstructure of the CDZ were different between AZ31-SA and AZ31-SB. The AZ31-SA alloy produced finer and shorter columnar dendrites, whereas the AZ31-SB alloy produced coarser and wider columnar dendrites. The width of the CDZ close to the notch decreased with the increase of current. The CDZ disappeared when the current was higher than a critical value, which was about 24 kA for AZ31-SA and 28 kA for AZ31-SB. The microhardness of the two base materials was the same, but within the CDZ and EDZ, the hardness was greater in AZ31-SA than AZ31-SB welds. It is believed that the different microstructures of spot welds between AZ31-SA and AZ31-SB resulted in different mechanical properties; in particular, K _C increased with the welding current because of the improved columnar-to-equiaxed transition.     

Microstructure Refinement After the Addition of Titanium Particles in AZ31 Magnesium Alloy Resistance Spot Welds

Microstructural evolution of AZ31 magnesium alloy welds without and with the addition of titanium powders during resistance spot welding was studied using optical microscopy, scanning electron microscopy, and transmission electron microscopy (TEM). The fusion zone of AZ31 magnesium alloy welds could be divided into columnar dendritic zone (CDZ) and equiaxed dendritic zone (EDZ). The well-developed CDZ in the vicinity of the fusion boundary was clearly restricted and the coarse EDZ in the central region was efficiently refined by adding titanium powders into the molten pool, compared with the as-received alloy welds. A microstructural analysis showed that these titanium particles of approximately 8 μm diameter acted as inoculants and promoted the nucleation of α-Mg grains and the formation of equiaxed dendritic grains during resistance spot welding. Tensile-shear testing was applied to evaluate the effect of titanium addition on the mechanical properties of welds. It was found that both strength and ductility of magnesium alloy welds were increased after the titanium addition. A TEM examination showed the existence of an orientation matching relationship between the added Ti particles and Mg matrix, i.e., [ 0 1[`1]0 ]<sub>\textMg</sub> //  [ 1[`2] 1[`3] ]<sub>\textTi</sub>  \textand ( 000 2 )<sub>\textMg</sub> //  ( 10[`1]0)_\textTi \left[ {0 1\bar{1}0} \right]_{\text{Mg}} // \, \left[ { 1\bar{2} 1\bar{3}} \right]_{\text{Ti}} \,{\text{and}}\,\left( {000 2} \right)_{\text{Mg}} // \, ( 10\bar{1}0)_{\text{Ti}} in some grains of Ti polycrystal particles. This local crystallographic matching could promote heterogeneous nucleation of the Mg matrix during welding. The diameter of the added Ti inoculant should be larger than 1.8 μm to make it a potent inoculant.     

Resistance Spot Welded AZ31 Magnesium Alloys,Part II: Effects of Welding Current on Microstructure and Mechanical Properties

Resistance spot welding of AZ31 magnesium alloys from different suppliers, AZ31-SA (from supplier A) and AZ31-SB (from supplier B), was studied and compared in this article. The mechanical properties and microstructures have been studied of welds made with a range of welding currents. For both groups of welds, the tension-shear fracture load (F _C) and fracture toughness (K _C) increased with the increase in welding current. The F _C and K _C of AZ31-SA welds were larger than those of AZ31-SB welds. The fracture surfaces of AZ31-SB welds were relatively flatter than those of AZ31-SA. Microstructural examination via optical microscope demonstrated that almost all weld nuggets comprised two different zones, the columnar dendritic zone (CDZ), which grew epitaxially from the fusion boundary, and the equiaxed dendritic zone (EDZ), which formed in the center of the nugget. The nature and extent of the CDZ seemed to be critical to the strength and toughness of spot welds because of its position adjacent to the inherent external circular crack-like notch of spot welds and the stress concentration in this region. The width and microstructure of the CDZ were different between AZ31-SA and AZ31-SB. The AZ31-SA alloy produced finer and shorter columnar dendrites, whereas the AZ31-SB alloy produced coarser and wider columnar dendrites. The width of the CDZ close to the notch decreased with the increase of current. The CDZ disappeared when the current was higher than a critical value, which was about 24 kA for AZ31-SA and 28 kA for AZ31-SB. The microhardness of the two base materials was the same, but within the CDZ and EDZ, the hardness was greater in AZ31-SA than AZ31-SB welds. It is believed that the different microstructures of spot welds between AZ31-SA and AZ31-SB resulted in different mechanical properties; in particular, K _C increased with the welding current because of the improved columnar-to-equiaxed transition.     

Fiber Laser Welded AZ31 Magnesium Alloy: The Effect of Welding Speed on Microstructure and Mechanical Properties

This study was aimed at characterizing microstructural change and evaluating tensile and fatigue properties of fiber laser welded AZ31B-H24 Mg alloy with special attention to the effect of welding speed. Laser welding led to the formation of equiaxed dendrites in the fusion zone and columnar dendrites near the fusion zone boundary along with divorced eutectic Mg₁₇Al₁₂ particles and recrystallized grains in the heat-affected zone. The lowest hardness across the weld appeared in the fusion zone. Although the yield strength, ductility, and fatigue life decreased, the hardening capacity increased after laser welding, with a joint efficiency reaching about 90 pct. A higher welding speed resulted in a narrower fusion zone, smaller grain size, higher yield strength, and longer fatigue life, as well as a slightly lower strain-hardening capacity mainly because of the smaller grain sizes. Tensile fracture occurred in the fusion zone, whereas fatigue failure appeared essentially in between the heat-affected zone and the fusion zone. Fatigue cracks initiated from the near-surface welding defects and propagated by the formation of fatigue striations together with secondary cracks.     

Effect of strontium on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy

The effect of strontium (Sr) on the microstructure, mechanical properties, and fracture behavior of AZ31 magnesium alloy and its sensitivity to cooling rate are investigated. Three phases—blocky-shaped Mg₁₇Al₁₂, acicular Mg₂₀Al₂₀Mn₅Sr, and insular Mg₁₆(Al,Zn)₂Sr—are identified in the Sr-containing AZ31 alloys. With increasing cooling rate, the blocky-shaped Mg₁₇Al₁₂ phase increases, the acicular Mg₂₀Al₂₀Mn₅Sr phase diminishes, and the insular Mg₁₆(Al,Zn)₂Sr phase is refined and granulated. The study suggests that the grain size decreases with increasing cooling rate for a given composition. However, the grain size decreases first, then increases, and finally decreases again with increasing Sr for a given cooling rate. The yield strength (σ _y ) of AZ31 magnesium alloy can be improved by grain refinement and expressed as σ _y =35.88+279.13d ^−1/2 according to the Hall-Petch relationship. The elongation increases when Sr is added up to 0.01 pct and then decreases with increasing Sr addition. Grain refinement changes the fracture behavior from quasicleavage failure for the original AZ31 alloy to mixed features of quasicleavage and microvoid coalescence fracture.     

Electron-beam welding behavior in Mg-Al-based alloys

The electron-beam welding (EBW) behaviors of pure Mg and the AZ31, AZ61, and AZ91 Mg alloys are examined in this study, in terms of fusion-zone characteristics, grain structures, texture evolution, and joint efficiency. With increasing A1 content, the Mg-based materials were found to be more easily fusion welded. The AZ91 alloy could be welded using a beam power of 2200 W and a weld speed of 16 mm/s, resulting in a weld depth of 29 mm with a fusion-zone aspect ratio of 8.2. The grains inside the fusion zone were nearly equiaxed in shape and ∼10 μm in size, due to the rapid cooling rate. Extended partial melting zones were observed in alloys with high solute contents, such as AZ61 and AZ91. The postweld tensile strength of the Mg alloys could recover back to ∼80 to 110 pct of the original strength. The texture in the fusion zone was traced by X-ray diffraction (XRD) and electron-backscattered diffraction (EBSD). The grain orientations inside the rapidly solidified electron-beam-welded fusion zones are still rather diversely distributed. The α ₁-, α ₂-, and α ₃-axes of some grains tend to align at 90 or 30 deg with respect to welding direction, and the c-axis tends to align along the plate normal direction. The influence from surface tension on the weld top-surface appearance and weld depth was not pronounced for the current four Mg materials. Instead, differences in the solidus temperatures and thermal conductivity should be the primary factors.     

Effect of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of squeeze-cast AZ91-X magnesium alloys

This study aims to investigate the effects of Y, Sr, and Nd additions on the microstructure and microfracture mechanism of the four squeeze-cast magnesium alloys based on the commercial AZ91 alloy. Microstructural observation, in situ fracture tests, and fractographic observation were conducted on the alloys to clarify the microfracture process. Microstructural analyses indicated that grain refinement could be achieved by small additions of alloying elements, although the discontinuously precipitated Mg₁₇Al₁₂ phases still existed on grain boundaries. From in situ fracture observation of an AZ91-Sr alloy, it was seen that coarse needle-shaped compound particles and Mg₁₇Al₁₂ phases located on the grain boundary provided easy intergranular fracture sites under low stress intensity factor levels, resulting in the drop in toughness. On the other hand, the AZ91-Y and AZ91-Nd alloys showed improved fracture toughness, since deformation and fracture paths proceeded into grains rather than to grain boundaries, as the planar slip bands and twinnings actively developed inside the grains. These findings suggested, on the basis of the well-developed planar slip bands and twinnings, that the small addition of Y or Nd was very effective in improving fracture toughness.     

Correlating the microstructure of the die-chill skin and the corrosion properties for a hot-chamber die-cast AZ91D magnesium alloy

The corrosion of a hot-chamber die-cast AZ91D thin plate (1.4 mm in thickness) was investigated in terms of its microstructure, to elucidate the role of die-chill skin in corrosion. The die-chill skin was composed of a thin layer of chill zone and a thick layer of an interdendritic Al-rich α-Mg/Al₁₂Mg₁₇ β-phase particle/α-Mg grain composite microstructures. The chill zone (4±1 μm in thickness) had fine columnar and equiaxed grains and contained a distribution of submicron Mg-Al-Zn intermetallic particles. Beneath the chill zone, Al₁₂Mg₁₇ β particles were irregularly shaped but did not have an interdendritic network morphology. Furthermore, Al-rich α phase (also known as eutectic α) was in the interdendritic network, which occupied a higher volume fraction than the β phase in the die-skin layer. Corrosion characteristics were studied via constant-immersion and electrochemical tests. Although previous studies have ascribed the fine microstructure to good corrosion resistance for the AZ91D alloy, the present study showed severe corrosion of the sample with a die skin in chloride solution. Moreover, the sample without the die skin on the surface corroded more slowly. The inferior corrosion performance of the die skin was considered to be related to the high volume fraction of the interdendritic network of Al-rich α phase contained in the die skin, owing to the high cooling rate during solidification. The Al-rich α phase does not increase the corrosion resistance of the AZ91D alloy.     

A solutal interaction mechanism for the columnar-to-equiaxed transition in alloy solidification

A multiphase/multiscale model is used to predict the columnar-to-equiaxed transition (CET) during solidification of binary alloys. The model consists of averaged energy and species conservation equations, coupled with nucleation and growth laws for dendritic structures. A new mechanism for the CET is proposed based on solutal interactions between the equiaxed grains and the advancing columnar front—as opposed to the commonly used mechanical blocking criterion. The resulting differences in the CET prediction are demonstrated for cases where a steady state can be assumed, and a revised isotherm velocity (V _T ) vs temperature gradient (G) map for the CET is presented. The model is validated by predicting the CET in previously performed unsteady, unidirectional solidification experiments involving Al-Si alloys of three different compositions. Good agreement is obtained between measured and predicted cooling curves. A parametric study is performed to investigate the dependence of the CET position on the nucleation undercooling and the density of nuclei in the equiaxed zone. Nucleation undercoolings are determined that provide the best agreement between measured and calculated CET positions. It is found that for all three alloy compositions, the nucleation undercoolings are very close to the maximum columnar dendrite tip undercoolings, indicating that the origin of the equiaxed grains may not be heterogeneous nucleation, but rather a breakdown or fragmentation of the columnar dendrites. An erratum to this article is available at .     

Effect of B on the microstructure and mechanical properties of mechanically milled TiAl alloys

The present study is concerned with γ-(Ti₅₂Al₄₈)_100−x B _x (x=0, 0.5, 2, 5) alloys produced by mechanical milling/vacuum hot pressing (VHPing) using melt-extracted powders. Microstructure of the as-vacuum hot pressed (VHPed) alloys exhibits a duplex equiaxed microstructure of α₂ and γ with a mean grain size of 200 nm. Besides α₂ and γ phases, binary and 0.5 pct B alloys contain Ti₂AlN and Al₂O₃ phases located along the grain boundaries and show appreciable coarsening in grain and dispersoid sizes during annealing treatment at 1300 °C for 5 hours. On the other hand, 2 pct B and 5 pct B alloys contain fine boride particles within the γ grains and show minimal coarsening during annealing. Room-temperature compressing tests of the as-VHPed alloys show low ductility, but very high yield strength >2100 MPa. After annealing treatment, mechanically milled alloys show much higher yield strength than conventional powder metallurgy and ingot metallurgy processed alloys, with equivalent ductility to ingot metallurgy processed alloys. The 5 pct B alloy with the smallest grain size shows higher yield strength than binary alloy up to the test temperature of 700 °C. At 850 °C, 5 pct B alloy shows much lower strength than the binary alloy, indicating that the deformation of fine 5 pct B alloy is dominated by the grain boundary sliding mechanism. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.     

Microstructure and Mechanical Properties of Fiber-Laser-Welded and Diode-Laser-Welded AZ31 Magnesium Alloy

The microstructures, tensile properties, strain hardening, and fatigue strength of fiber-laser-welded (FLW) and diode-laser-welded (DLW) AZ31B-H24 magnesium alloys were studied. Columnar dendrites near the fusion zone (FZ) boundary and equiaxed dendrites at the center of FZ, with divorced eutectic β-Mg₁₇Al₁₂ particles, were observed. The FLW joints had smaller dendrite cell sizes with a narrower FZ than the DLW joints. The heat-affected zone consisted of recrystallized grains. Although the DLW joints fractured at the center of FZ and exhibited lower yield strength (YS), ultimate tensile strength (UTS), and fatigue strength, the FLW joints failed at the fusion boundary and displayed only moderate reduction in the YS, UTS, and fatigue strength with a joint efficiency of ~91 pct. After welding, the strain rate sensitivity basically vanished, and the DLW joints exhibited higher strain-hardening capacity. Stage III hardening occurred after yielding in both base metal (BM) and welded samples. Dimple-like ductile fracture characteristics appeared in the BM, whereas some cleavage-like flat facets together with dimples and river marking were observed in the welded samples. Fatigue crack initiated from the specimen surface or near-surface defects, and crack propagation was characterized by the formation of fatigue striations along with secondary cracks.     

Effect of Ca and Rare Earth Elements on Impression Creep Properties of AZ91 Magnesium Alloy

Creep properties of AZ91 magnesium alloy and AZRC91 (AZ91 + 1 wt pct RE + 1.2 wt pct Ca) alloy were investigated using the impression creep method. It was shown that the creep properties of AZ91 alloy are significantly improved by adding Ca and rare earth (RE) elements. The improvement in creep resistance is mainly attributed to the reduction in the amount and continuity of eutectic β(Mg₁₇Al₁₂) phase as well as the formation of new Al₁₁RE₃ and Al₂Ca intermetallic compounds at interdendritic regions. It was found that the stress exponent of minimum creep rate, n, varies between 5.69 and 6 for AZ91 alloy and varies between 5.81 and 6.46 for AZRC91 alloy. Activation energies of 120.9 ± 8.9 kJ/mol and 100.6 ± 7.1 kJ/mol were obtained for AZ91 and AZRC91 alloys, respectively. It was shown that the lattice and pipe-diffusion-controlled dislocation climb are the dominant creep mechanisms for AZ91 and AZRC91 alloys, respectively. The constitutive equations, correlating the minimum creep rate with temperature and stress, were also developed for both alloys.     

Microstructural factors governing hardness in friction-stir welds of solid-solution-hardened Al alloys

Microstructural factors governing hardness in friction-stir welds of the solid-solution-hardened Al alloys 1080 and 5083 were examined by optical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The effect of grain boundary on the hardness was examined in an Al alloy 1080 which did not contain any second-phase particles. The weld of Al alloy 1080 had a slightly greater hardness in the stir zone than the base material. The maximum hardness was located in the thermomechanically affected zone (TMAZ). The stir zone consisted of recrystallized fine grains, while the TMAZ had a recovered grain structure. The increase in hardness in the stir zone can be explained by the Hall-Petch relationship. On the other hand, the hardness profiles in the weld of Al alloy 5083 were roughly homogeneous. Friction-stir welding created the fine recrystallized grains in the stir zone and recovered grains in the TMAZ in the weld of this alloy. The stir zone and the TMAZ had slightly higher dislocation densities than the base material. Many small Al₆(Mn,Fe) particles were detected in all the grains of the weld. The hardness profiles could not be explained by the Hall-Petch relationship, but rather by Orowan hardening. The results of the present study suggest that the hardness profile is mainly affected by the distribution of small particles in friction-stir welds of Al alloys containing many such particles.     

On the columnar to equiaxed transition in small ingots

The columnar to equiaxed transition (CET) in small ingots of, aluminum alloys was found to occur more easily for alloys with a larger value of the constitutional supercooling parameter (−mC _o (1-k)/k). The CET was found to be completely suppressed by increases in the mold temperature by preheating before casting. These results are discussed in terms of the model proposed by Burden and Hunt that the CET occurs by the effect of the thermal gradient, arising from the slow, solidification of equiaxed dendrites, which increases the undercooling of the columnar dendrites. The application of the model due to Burden and Hunt is shown to require, the use of the ‘big bang’ model for equiaxed nucleation on pouring. A higher density of the nuclei, that grow into equiaxed grains, formed by pouring with lower superheat and into a cold mold, gives a higher thermal gradient immediately in front of the growing columnar grains. Other evidence in favor of the model is briefly discussed.     

The Role of Carbon in Grain Refinement of Cast CrFeCoNi High-Entropy Alloys

As a promising engineering material, high-entropy alloys (HEAs) CrFeCoNi system has attracted extensive attention worldwide. Their cast alloys are of great importance because of their great formability of complex components, which can be further improved through the transition of the columnar to equiaxed grains and grain refinement. In the current work, the influence of C contents on the grain structures and mechanical properties of the as-cast high-entropy alloy CrFeCoNi was chosen as the target and systematically studied via a hybrid approach of the experiments and thermodynamic calculations. The alloys with various C additions were prepared by arc melting and drop cast. The as-cast macrostructure and microstructure were characterized using optical microscopy, scanning electron microscopy, and transmission electron microscopy. The cast HEAs transform from coarse columnar grains into equiaxed grains with the C level increased to ≥ 2 at. pct and the size of equiaxed grains is further decreased with the increasing C addition. It is revealed that the interdendritic segregation of Cr and C results in grain boundary precipitation of M₂₃C₆ carbides. The grain refinement is attributed to the additional constitutional supercoiling from the C addition. The yield stress and tensile strength at room temperature are improved due to the transition of columnar to equiaxed grains and grain refinement.     

Effect of dispersed Al<Subscript>3</Subscript>Sc particles on dynamic recrystallization in P/M 7000 series alloys

Al₃Sc particles are well known to be a recrystallization inhibitor in Al alloys. In this study, 0.4, 0.8 and 1.5 mass% Sc were added to 7000 series Al alloys in supersaturation by using powder metallurgy. By subjecting rapidly solidified powders of these alloys to a heat treatment at 773 K, Al₃Sc particles were precipitated in the matrix. Subsequently, hot extrusion was carried out at 773 K. During extrusion, continuous dynamic recrystallization (DRX) occurred and fine DRX grains with a diameter of 1 mm were formed. The number of DRX grains was the largest in the 1.5Sc alloy. Even though the amount of Sc added in this case was two times larger, the number of DRX grains in 0.4Sc was almost the same as that in 0.8Sc. Since continuous DRX occurs only under conditions where the grain boundary mobility is low, the number of DRX grains is strongly related to the pinning force on the initial grain boundary exerted by Al₃Sc particles. The pinning force varied with the diameter and volume fraction of Al₃Sc particles. When the pinning force was calculated from the diameter of Al₃Sc particles, which was measured by TEM, it was proven that the number of DRX grains was proportional to the calculated pinning force. Except in the case of 0.4Sc, the Al₃Sc particle diameter was twice that obtained at the maximum pinning force (d _max ). It is possible to promote continuous DRX by decreasing the Al₃Sc particle diameter in 0.8 and 1.5Sc alloys. Lowering the heating rate before heat treatment for degassing reduced the critical nucleus size for precipitation of Al₃Sc particles as well the diameter of Al₃Sc particles. Thus, the pinning force increased in 0.8Sc and 1.5Sc alloys, and the number of DRX grains also increased, as expected.     

Effect of samarium on microstructure and corrosion resistance of aged as-cast AZ92 magnesium alloy

The effects of samarium（Sm） on microstructure and corrosion resistance of AZ92 magnesium alloy were characterized and analyzed by scanning electronic microscopy, X-ray diffraction, mass loss test, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy and potentio-dynamic polarization test. The results showed that the added Sm could promote continuous precipitation of β-Mg17Al12 phase in grains, and meanwhile restrain discontinuous precipitation of the same phase along the grain boundaries. Thus, the precipitations distributed more uniformly in the aged AZ92 magnesium alloys. When the content of Sm was 0.5 wt.%, the corrosion resistance of aged AZ92 alloy tended to be the best, which was due to the β-phase distributes more homogeneous reducing the galvanic corrosion. The corrosion product film had more integrality and compactness than AZ92 alloys without Sm. However, it resulted in worse corrosion resistance of AZ92 alloy because of the formation of mass cathodic Al2 Sm phase coming from excess Sm in AZ92 alloy.