Effects of homogenisation conditions on semisolid microstructures of Al–Mg–Si–Mn alloys produced by D-SSF process

Abstract

The effects of different heating rates to a homogenisation temperature on the semisolid microstructure of Al–Mg–Si–Mn alloys are investigated. It is found that the size, morphology and distribution of the α-Al₁₂Mn₃Si₂ intermetallic compound (Mn containing dispersoid) depend on the heating rate in the homogenisation process. Fine spherical and homogeneously distributed Mn containing dispersoid particles are found in the slow heated samples (0˙7°C min^?1), while inhomogeneously distributed coarser particles with a rod-like shape are found in the rapid heated samples (110°C min^?1). The homogenised sample is deformed by 60% cold rolling. It is found that the recrystallised and semisolid grain sizes of the rapid heated sample are smaller than those of the slow heated sample in all conditions. Compared with the M4 alloy (0˙4 mass-%Mn), the M7 alloy (0˙72 mass-%Mn) has much finer semisolid grain size and smaller values of the shape factor close to 1. The Mn containing dispersoid greatly affects the semisolid grain size of the alloys. The results in this work show that the rapid heating in the homogenisation process is useful to produce high quality semisolid products of the Al–Mg–Si–Mn alloys.     

Effects of Sn on microstructure of as-cast and as-extruded Mg–9Li alloys

The effects of Sn addition on the microstructure of as-cast and as-extruded Mg–9Li alloys were investigated. The results show that α-Mg, β-Li, Li₂MgSn, and Mg₂Sn are primary phases in the microstructures of the as-cast and as-extruded Mg–9Li–xSn (x=0, 5; in mass fraction, %) alloys. Li₂MgSn phase evolves from continuously net-like structure in the as-cast state to fine granular in the as-extruded state. After the extrusion, Mg–9Li–5Sn alloy has finer microstructures. Li₂MgSn or Mg₂Sn compound can act as the heterogeneous nucleation sites for dynamic recrystallization during the extrusion due to the crystallography matching relationship. Extrusion deformation leads to dynamic recrystallization, which results in the grain refinement and uniform distribution. The as-extruded Mg–9Li–5Sn alloy possesses the lowest grain size of 45.9 μm.     

Microstructures and Mechanical Properties of Mg–xAl–1Sn–0.3Mn( x = 1, 3, 5) Alloy Sheets

The influence of Al alloying on the microstructures and the mechanical properties of Mg–x Al–1 Sn–0.3 Mn alloy sheets was investigated. The microstructure of Mg– x Al–1 Sn–0.3 Mn consisted of α-Mg and Mg 17 Al 12 precipitates. Alloying with Al increased the amount of Mg_(17)Al_(12) and the average grain size. Uniaxial tensile tests were carried out along the extrusion direction(ED), the transverse direction(TD) and 45° toward the ED. Mg–5 Al–1 Sn–0.3 Mn alloy sheet exhibited the best combination of mechanical properties along the ED: a yield strength of 142 MPa, an ultimate tensile strength of 282 MPa and an elongation of 23%. The good performance of Mg–5 Al–1 Sn–0.3 Mn sheet was mainly attributed to the large quantity of Mg_(17)Al_(12) precipitates and a weak basal texture. Annealing caused static dynamic recrystallization, refined the grain size and enhanced the mechanical properties: yield strength of 186 MPa, ultimate tensile strength of 304 MPa, elongation of 21% along ED. Both strength and ductility were enhanced by Al alloying.     

Effect of Mn Modification on the Corrosion Susceptibility of Mg–Mn Alloys by Magnesium Scrap

The microstructure and anti-corrosion behavior of Mg–Mn alloys by magnesium scrap have been investigated in this study.The results show that the size of the Fe-rich particles in magnesium scrap decreases but the quantity increases with the Mn addition.Although the presence of Mn-containing Fe-rich particles with unique symbiotic structure can eff ectively weaken the micro-galvanic corrosion,the presence of more free Fe(Fe-rich particles) does not necessarily lead to severe corrosion of the alloy.The corrosion susceptibility of Mg–Mn–Fe alloy primarily depends on the solubility of iron in the Mg matrix,while it can be significantly reduced by suitable Mn addition.Besides,the tolerance limit of the Fe impurity can be expressed as Fe_(max) = 0.0083 Mn(relative to the iron solubility).Only when the Fe/Mn ratio is below 0.0083 can the alloy have excellent corrosion resistance,with the corrosion rate changing in the scope of 0.38 ± 0.09 to 0.54 ± 0.15 mg/cm~2 day and i_(corr) from 3 to 9 × 10~(–4) A/cm~2.     

Effects of Ga and Sn on the electrochemical behavior of Mg–6Al–1Zn as anode materials

The microstructure and electrochemical behavior of Mg–6Al–1Zn, Mg–6Al–1Zn–1Ga, Mg–6Al–1Zn–1Sn, and Mg–6Al–1Zn–0.5Sn–0.5Ga as anode materials in a 3.5 wt% NaCl solution are compared systematically. The results show that Sn alloying refines the second-phases of Mg–6Zn–1Al by promoting tiny granular Mg₁₇Al₁₂ phases containing Sn, and inspires their disperse distribution. However, the Ga results in the formation of semicontinuous reticular Ga containing Mg₁₇Al₁₂ phases. The comparison of discharge tests indicates that Mg–6Al–1Zn–1Sn has the highest discharge activity, and Mg–6Al–1Zn–1Ga displays the largest hydrogen evolution corrosion resistance in 3.5 wt% NaCl solution at 298 K. The synergy of Ga and Sn can shorten discharge activation time and promote low discharge potential. In addition, the utilization efficiencies of the alloys decrease as follows: Mg–6Al–1Zn–1Ga > Mg–6Al–1Zn–0.5Sn–0.5Ga > Mg–6Al–1Zn–1Sn > Mg–6Zn–1Al. This study illustrates that the Mg–6Al–1Zn–0.5Sn–0.5Ga alloy has acceptable utilization efficiency and desirable electrochemical activity, which implies that doping Ga and Sn obtains a balance between discharge activity and utilization efficiencies.     

Evolution of Microstructure,Residual Stress,and Tensile Properties of Mg–Zn–Y–La–Zr Magnesium Alloy Processed by Extrusion

The microstructure, texture, residual stress, and tensile properties of Mg–6 Zn–2 Y–1 La–0.5 Zr(wt%) magnesium alloy were investigated before and after extrusion process, which performed at 300 °C and 400 °C. The microstructural characterizations indicated that the as-cast alloy was comprised of α-Mg, Mg–Zn, Mg–Zn–La, and Mg–Zn–Y phases. During homogenization at 400 °C for 24 h, most of the secondary phases exhibited partial dissolution. Extrusion process led to a remarkable grain refi nement due to dynamic recrystallization(DRX). The degree of DRX and the DRXed grain size increased with increasing extrusion temperature. The homogenized alloy did not show a preferential crystallographic orientation, whereas the extruded alloys showed strong basal texture. The extrusion process led to a signifi cant improvement on the compressive residual stress and mechanical properties. The alloy extruded at 300 °C exhibited the highest basal texture intensity, the compressive residual stress and hardness, and yield and tensile strengths among the studied alloys.     

Investigation on the Effect of Sub-Zero Treatment on Micro-Hardness and Microstructure of GTAW Welded Al–Si–Mg–Mn Alloy

In this work, the effect of sub-zero treatment on the mechanical properties of an Al–Si–Mg–Mn alloy welded by GTAW (gas tungsten arc welding) leads to significant softening in the welded region. The latter is due to melting and resolidification in the welded region, which have resulted in decomposition of the strengthening precipitates. The experiments were performed on GTAW welded plates of 6 mm thickness by varying the heat inputs, namely, of 370, 317.1, 277.5, 246.4, and 222 J/mm, and sub-zero treatment time periods. The Sub-Zero treatment was performed at–45°C using dry ice; hardness and microstructure investigations were performed in the welded region of the Al?Si–Mg–Mn alloy that was studied in two different conditions, namely, as-welded and in that formed after post weld sub-zero treatment with artificial aging. It was found that the post weld Sub-Zero treatment followed by artificial aging had led to realization of significantly higher hardness values in the welded region due to the recurrence of the precipitation sequence.     

Effect of microstructure on discharge performance of Al–0.8Sn–0.05Ga–0.9Mg–1.0Zn (wt%) alloy as anode for seawater-activated battery

The electrochemical performance and discharge behavior of Al–0.8Sn–0.05Ga–0.9Mg–1.0Zn (wt%) alloys in as-cast, homogenized, and annealed states were investigated through electrochemical means, corrosion rate test, and discharge test in a 3.5 wt% NaCl solution. Results suggest that the discharge performance of this alloy is enhanced by rolling and subsequent annealing treatment. This is attributed to the fact that the microstructure of the alloy is greatly improved through rolling and subsequent annealing treatment. The 400°C-annealed alloy exhibits the most excellent discharge activity than alloys in other states, which is due to more regions being activated by a finer and more uniform Sn-rich phase. Furthermore, the anode efficiency of the 400°C-annealed alloy is higher than that of as-cast and homogeneous alloys due to the more uniform distribution of Sn in the aluminum matrix.     

Influence of Initial Microstructure on the Strengthening Effect of Extruded Mg–8Sn–4Zn–2Al Alloys

The Mg–8Sn–4Zn–2Al(TZA842, in wt%) alloys with different initial microstructure(as-cast-AC and homogenization treatment-HT) subjected to hot extrusion. Also, the strengthening responses to AC and HT for the extruded TZA842 alloys were reported. The results revealed that the alloy subjected to HT shows finer grain size, more homogenous microstructure and weaker basal texture than those of counterpart subjected to AC. In addition, compared with TZA842-AC alloy, precipitates were finer and uniformly dispersed in TZA842-HT owing to the utilization of HT. Moreover, the TZA842-HT alloy showed higher yield strength of 200 MPa, ultimate tensile strength of 290 MPa and elongation(EL) of17.9% than those of TZA842-AC, which was mainly attributed to the combined effects of grain boundary strengthening,precipitation strengthening, solid solution strengthening and weak texture. Strengthening mechanism for both alloys was discussed in detail.     

Effects of Cooling Rate and Component on the Microstructure and Mechanical Properties of Mg–Zn–Y Alloys

The microstructure and mechanical properties of Mg–6Zn–1Y and Mg–6Zn–3Y(wt%) alloys under different cooling rates were investigated. The results show that the second dendrite arm spacing(SDAS) of Mg–6Zn–1Y and Mg–6Zn–3Y is reduced by 32 and 30% with increasing cooling rates(Rc) from 10.2 to 23 K/s, which can be predicted using a empirical model of SDAS=68 R 0:45:45cand SDAS=73 R 0c, respectively. The compressive strength of both alloys increases with increasing the cooling rate, which is attributed to the increase of volume fraction(Vf) of secondary phases under high cooling rate. The interaction of the cooling rate and component with SDAS has been theoretically analyzed using interdependence theory.     

Effects of Sb Addition on Microstructural Evolution and Mechanical Properties of Mg–9Al–5Sn Alloy

The Mg–9Al–5Sn-xSb(x=0.0,0.3,0.6,1.0,1.5 wt%) alloys were prepared by a simple alloying process followed by hot extrusion with an extrusion ratio of 28.2. The effects of Sb additions on the microstructure and mechanical properties of the Mg–9 Al–5 Sn alloys were investigated by optical microscopy, X-ray diffraction, transmission electron microscopy, scanning electron microscopy equipped with an energy-dispersive X-ray spectrometer. The results indicated that the phases α-Mg matrix, Mg_2_Sn, Mg_3Sb_2 and Mg_17 Al_12 exist in the as-cast Sb-containing alloys. Sb addition results in the precipitation of Mg_3Sb_2. The dendritic size of these alloys decreases with the addition of Sb. Both their ultimate tensile strength and yield strength of extruded alloys increase, and their elongation decreases gradually with increasing the content of Sb. The better mechanical properties of the as-extruded alloys were achieved due to the refined grains and the formation of dispersive second phases Mg_3Sb_2.     

Microstructure,martensitic transformation and mechanical properties of Ni–Mn–Sn alloys by substituting Fe for Ni

The effects of partial substitution of Fe element for Ni element on the structure, martensitic transformation and mechanical properties of Ni_50–xFe_xMn₃₈Sn₁₂ (x=0 and 3%, molar fraction) ferromagnetic shape memory alloys were investigated. Experimental results indicate that by substitution of Fe for Ni, the microstructure and crystal structure of the alloys change at room temperature. Compared with Ni₅₀Mn₃₈Sn₁₂ alloy, the martensitic transformation starting temperature of Ni₄₇Fe₃Mn₃₈Sn₁₂ alloy is decreased by 32.5 K. It is also found that martensitic transformation occurs over a broad temperature window from 288.9 to 352.2 K. It is found that the mechanical properties of Ni–Mn–Sn alloy can be significantly improved by Fe addition. The Ni₄₇Fe₃Mn₃₈Sn₁₂ alloy achieves a maximum compressive strength of 855 MPa with a fracture strain of 11%. Moreover, the mechanism of the mechanical property improvement is clarified. Fe doping changes the fracture type from intergranular fracture of Ni₅₀Mn₃₈Sn₁₂ alloy to transgranular cleavage fracture of Ni₄₇Fe₃Mn₃₈Sn₁₂ alloys.     

Effects of duty ratio on properties of micro-arc film on Ti–3Zr–2Sn–3Mo–25Nb

Porous titanium oxide layers, which are important features for improving the biological activity of Ti implants with bone tissues, have been obtained through the technique of micro-arc oxidation (MAO). Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to evaluate the micromorphology and crystalline structure of these oxide films, and the chemical compositions were measured by electron dispersive X-ray spectroscopy (EDS). TiO₂ layers presented the crystalline phases of rutile and anatase. During the micro-arc oxidation treatment, Ca and P ions were incorporated into the oxide layer, and incorporation of Ca and P with the Ca/P content (%) of around 1.38 is similar to that in the human body. Nb₂O₅ was also identified in the treatment samples. The corrosion resistance was evaluated using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation curves. In the electrochemical corrosion tests, the treated samples presented lower values of corrosion current density than untreated Ti, indicating a better corrosion resistance. Diffusion phenomena were present in the process of corrosion.     

Effects of Zinc and Calcium Concentration on the Microstructure and Mechanical Properties of Hot-Rolled Mg–Zn–Ca Sheets

Four kinds of Mg alloys with different Zn and Ca concentration were selected to analyze the effect of Zn and Ca concentration on the microstructure and the mechanical properties of Mg–Zn–Ca alloys. It was found that Zn and Ca concentration has a great influence on the volume fraction, the morphology and the size of second phase. The Mg–1.95Zn–0.75Ca(wt%) alloy with the highest volume fraction, continuous network and largest size of Ca2Mg6Zn3 phase showed the lowest elongation to failure of about 7%, while the Mg–0.73Zn–0.12Ca(wt%) alloy with the lowest volume fraction and smallest size of Ca2Mg6Zn3 phase showed the highest elongation to failure of about 37%. It was suggested that uniform elongations of the Mg–Zn–Ca alloys were sensitive to the volume fraction of the Ca2Mg6Zn3 phases, especially the network Ca2Mg6Zn3phases; post-uniform elongations were dependent on the size of the Ca2Mg6Zn3 phase, especially the size of network Ca2Mg6Zn3 phase. Reduction in Zn and Ca concentration was an effective way to improve the roomtemperature ductility of weak textured Mg–Zn–Ca alloys.     

Effects of Passes on Microstructure Evolution and Mechanical Properties of Mg–Gd–Y–Zn–Zr Alloy During Multidirectional Forging

The multidirectional forging(MDF) process was conducted at temperature of 753 K to optimize the mechanical properties of as-homogenized Mg–13 Gd–4 Y–2 Zn–0.6 Zr alloy containing long-period stacking ordered phase. The effects of MDF passes on microstructure evolution and mechanical properties were also investigated. The results show that both the volume fraction of dynamic recrystallization(DRX) grains and mechanical properties of the deformed alloy enhanced with MDF passes increasing till seven passes. The average grain size decreased from 76 to 2.24 lm after seven passes, while the average grain size increased to 7.12 lm after nine passes. The microstructure after seven passes demonstrated randomly oriented fine DRX grains and larger basal(0001)\11"20[ Schmid factor of 0.31. The superior mechanical properties at room temperature(RT) with ultimate tensile strength(UTS) of 416 MPa and fracture elongation of 4.12% can be obtained after seven passes. The mechanical properties at RT after nine passes are inferior to those after seven passes due to the coarsening of DRX grains, which can be ascribed to the static recovery resulting from the repeated heating at the interval of MDF passes. The elevated temperature mechanical properties of the deformed alloy after seven passes and nine passes were investigated. When test temperature was below 523 K, the elevated temperature tensile yield strength and UTS after seven passes are superior to those after nine passes, while they are inferior to that after nine passes as temperature exceeds523 K.     

Microstructure modification and precipitation strengthening for Mg–6Zn–1Mn–4Sn–0.5Ca through extrusion and aging treatment

The microstructure revolution and mechanical properties of as-extruded and peak-aged Mg–6Zn–1Mn– 4Sn–0.5Ca (ZMT614–0.5Ca) alloy were studied by OM, SEM, TEM, hardness testing and tensile testing. The results showed that the as-cast ZMT614–0.5Ca alloy mainly consisted of α-Mg, Mg–Zn and CaMgSn phase. The hot extrusion process effectively refined the microstructure and led to a completely dynamic recrystallized microstructure. The average grain size of as-extruded alloy was ˜4.85 μm. After solution treatment, remained CaMgSn with high melting point played a significant role in pinning effect and impeding the migration of grain boundary. After aging treatment, peak-aged ZMT614–0.5Ca alloy exhibited a good combination of strength and ductility, with yield strength, ultimate tensile strength and elongation being 338 MPa, 383 MPa and 7.5%, respectively. The yield strength of the alloy increased significantly by around 36% compared with that in as-extruded condition, which should be attributed to the precipitation strengthening of β' phase.     

Microstructure and Tensile Properties of a Nb–Mo Microalloyed 6.5Mn Alloy Processed by Intercritical Annealing and Quenching and Partitioning

The transformation behavior, microstructural evolution and mechanical properties were compared in a coldrolled Nb–Mo microalloyed 6.5Mn alloy after intercritical annealing(IA) and quenching and partitioning(Q P),respectively. The thermodynamic calculation and theoretical analysis were used to determine the optimal heat treatment parameters. The Q P samples exhibited relatively higher strength with relatively low ductility, mainly due to the hard martensite matrix, which resulted in continuous yielding behavior upon loading, whereas the IA samples showed the significantly improved ductility, which benefited from the more sufficient transformation-induced plasticity(TRIP) effects and the softer ultrafine ferrite matrix. The dependence of yield point elongation(YPE) of IA samples on grain size demonstrated that the YPE value was in the reverse proportional relationship to the average grain size, which agreed well with theoretical analysis.     

Effects of extrusion and heat treatments on microstructure and mechanical properties of Mg–8Zn–1Al–0.5Cu–0.5Mn alloy

The effects of extrusion and heat treatments on the microstructure and mechanical properties of Mg–8Zn–1Al–0.5Cu– 0.5Mn magnesium alloy were investigated. Bimodal microstructure is formed in this alloy when it is extruded at 230 and 260 °C, and complete DRX occurs at the extruding temperature of 290 °C. The basal texture of as-extruded alloys is reduced gradually with increasing extrusion temperature due to the larger volume fraction of recrystallized structure at higher temperatures. For the alloy extruded at 290 °C, four different heat treatments routes were investigated. After solution + aging treatments, the grains sizes become larger. Finer and far more densely dispersed precipitates are found in the alloy with solution + double-aging treatments compared with alloy with solution + single-aging treatment. Tensile properties are enhanced remarkably by solution + double-aging treatment with the yield strength, tensile strength and elongation being 298 MPa, 348 MPa and 18%, respectively. This is attributed to the combined effects of fine dynamically recrystallized grains and the uniformly distributed finer precipitates.     

Effects of ECAP Extrusion on the Microstructure,Mechanical Properties and Biodegradability of Mg–2Zn–xGd–0.5Zr Alloys

Effects of equal channel angular pressing(ECAP) extrusion on the microstructure, mechanical properties and biodegradability of Mg–2Zn– xGd–0.5Zr( x=0,0.5,1,2 wt%) alloys were studied in this work. Microstructure analysis, tensile test at ambient temperature, immersion test and electrochemical test in Hank's solution were carried out. The results showed that Gd could further enhance the grain refinement during the ECAP extrusion. Both Gd addition and ECAP extrusion could improve the mechanical properties of the alloys, and the extrusion played the dominant role. Minor addition of Gd(0.5–1 wt%) could obviously enhance the corrosion resistance of the alloys. To some extent, ECAP extrusion improved the corrosion resistance of the alloys due to the change of second phases distribution and the refinement of grains. Further increase in extrusion pass was detrimental to the improvement of the corrosion resistance as a result of increment of the grain boundaries.     

Effects of Cooling Rate on Precipitate Evolution and Residual Stresses in Al–Si–Mn–Mg Casting Alloy

The residual stresses with different heat treatment conditions have been measured and correlated with the microstructural behavior of AA365. 30 and 100 K/min cooling of AA365 inhibited the transformation of precipitates under 773 K, respectively. The alloy cooled at 30 and 100 K/min exhibited tensile residual stresses of 6.2 and 5.4 MPa, respectively, while the alloy cooled at 1 and 10 K/min showed compressive stresses of ??12.8 and ??10.3 MPa, respectively. The formation β′, β″, and other intermetallic compounds affected the compressive residual stresses, and that the fracture of the brittle intermetallic phases could reduce the extent of residual stresses in the lattice through plastic deformation.