Seiji Saikawa

Effect of Al and Mn Contents on Microstructure in AM-Series Magnesium Alloys

Magnesium alloys containing aluminum have been used for industrial materials due to their lightweight and recyclability. And the Mg-Al based alloys are usually used for the industrial production. The Mg17Al12 phase is reported as the precipitate formed in the Mg-Al alloys during aging after the solution heat treatment, which is the discontinuous precipitate in the grain boundary and continuous precipitate in the matrix. The Mg-Al alloys are divided into AZ-series alloys with the addition of Zn, and AM-series alloys with the addition of Mn, respectively. AM-series and AZ-series Mg-Al alloys have been used for industrial products widely, particularly for AM-series alloys because of better toughness and impact absorption properties than AZ-series alloys. However, there is few report about the effect of Al and Mn contents on age-hardening behavior and microstructure of AM-series alloys. The propose of this study is to investigate the difference of the age-hardening behavior and microstructures of AM-series alloys using hardness test and scanning electron microscopy (SEM) observation.

Effect of Casting Method and Al Contents on Microstructure in AM-Type Magnesium Alloys

Magnesium alloys have received considerable attention because of their lightweight and recyclability. AM-type and AZ-type Mg-Al alloys have been used for industrial products widely, particularly for AM-type alloys because of the better toughness and impact absorption properties than AZ-type alloys. However, there is little report about the effect of casting method on age-hardening behavior and microstructure of AM-type alloys. The purpose of this study is to investigate the difference of the age-hardening behavior and microstructures of three AM-type alloys cast with steel, copper and sand molds using hardness test and scanning electron microscopy (SEM) observation. Furthermore, the effect of Al content is also investigated in this study using three alloys of AM30 (3%Al), AM60 (6%Al) and AM90 (9%Al).

Microstructure Observation of AM60 Magnesium Alloy Solidified by Rapidly Quench

In solidification theory, with a slow cooling rate such as sand mold casting, it is easy to segregate the solute aluminum near the grain boundary of primary α-Mg phase under the solidification in Mg-Al system alloys. Thus, volume fraction of none-equilibrium crystallized β-Mg17Al12 phase showed the higher value compared with metal mold casting with faster cooling rate. However, in our microstructure observation results, the volume fraction of β phase in permanent mold castings was larger than that of sand mold castings. In the present study, these contradictory behavior was investigated by observation of as-solidified microstructure obtained from rapid cooling castings at the just below the solidus temperature of 723, 773 and 823K.

Microstructure and Aging Behavior in AM60 Magnesium Alloy Cast into Sand and Permanent Molds

AM60 magnesium alloy castings gave the solution treatment at 688K for 86.4ks. After that, aging treatment was carried out at three temperatures of 473, 498 and 523K. The age hardening curve obtained, hardness of all the specimens in the condition of peak aging was increased by decreasing the aging temperature. In the condition of long aging time, a cellular precipitation grows up from grain boundary to crystal grain. Fine cellular precipitation and intergranular precipitation obviously occurs at the lower aging temperature.

The Effect of Sc Addition on Microstructure in Mg-Gd Alloys

Traditionally, Mg-Gd alloys have been strengthened by dispersed precipitates. Several reports are available about Sc addition to Mg alloys for improving a creep resistance. In this research, aging behavior of Mg-Gd, Mg-Sc and Mg-Gd-Sc alloys including the same amount of solute elements were investigated to understand the effect of Sc on microstructures and mechanical properties during aging. Hardness measurement revealed that Sc addition delayed to form precipitate. Close inspection of TEM micrographs, β” phase formed at an initial stage of aging and β’ phase was observed at a peak-aged stage in Mg-Gd and Mg-Gd-Sc alloys. In Mg-Sc alloy, there is no evidence of precipitate formation during aging at 473K.

Effect of Sr Addition on the Solidification Structure in Al-6mass%Mg-3mass%Si Alloy

Al-Mg-Si system alloy have good strength and high ductility without heat treatment. However, the castability in this alloy is inferior to other aluminum alloy, in particular, hot-tearing is easy to occur during solidification. In our previous study, hot-tearing was not occurred in the case of 0.04%Sr addition to this alloy, because of the remarkable refinement of eutectic Mg2Si phase. In this study, in order to clarify the mechanism of the change of the crystallized eutectic Mg2Si morphology, the effect of Sr addition on the solidification structure in Al-6%Mg-3%Si alloy was investigated. By Sr addition to this alloy, the change of the nucleation mode from homogeneous to heterogeneous was occurred with the temperature drop at the start of eutectic reaction, and the great change of eutectic growth mode from facet to non-facet was thought to be a main reason improving of hot-tearing.

Age Hardening Behavior of Al-Li Alloys Produced by Sand Mold Process

Al-Li alloys have higher mechanical properties and more lightweight than other conventional aluminum alloys. Therefore , it is focused as a good material for weight reduction of industrial fields. However, since the Al-Li alloy are highly active and hard to cast, there has been limited research on casting. In this study, age-hardening behavior of Al-2.5mass%Li alloys cast into sand and metal mold were investigated. All alloys cast into Y-block shape sand mold, and then artificial aged after solution treated at 743K for 36ks. Because of difference in quantity of precipitation by metastable δ’(Al3Li) phase, peak hardness of metal mold casting is higher than that of sand molds castings.

Mechanical Properties of Al-10 Mass % Si-Mg High-Pressure Die-Casting Alloy with Different Mg Content

The influence of Mg content on Al-10 mass % Si-0.05, 0.30 and 0.60 mass % Mg alloys castings were produced by high-pressure die-casting process, used by microstructure observation and evaluation of mechanical properties. With increasing Mg content, the main constituent phases (primary α-Al phase and eutectic phase) were not changed, but a small amount of phases crystallized at last stage of solidification (β-Al5FeSi, π-Al8Si6Mg3Fe and Mg2Si) were changed in the kind and the volume. In the mechanical property, 0.2%PS and UTS were increased, elongation and absorbed energy were decreased with increasing Mg content or difference of heat treatment. Because the fracture mode in primary α-Al phase was changed from ductile to brittle by precipitation strengthening.

TEM Observation of HPT-Processed Cu-Added Excess Mg-Type Al-Mg-Si Alloys

Severe plastic deformation (SPD) techniques, such as equal channel angular pressing (ECAP), accumulative roll bonding (ARB) and high pressure torsion (HPT) have been extensively investigated to achieve. SPD techniques make use of plastic deformation process where no change in the cross-sectional dimension of a work piece occurs during straining. In this work, the effect of HPT on the aging behavior and microstructure in excess Mg-type Al-Mg-Si alloys including Cu are investigated. These alloys were investigated by hardness test and transmission electron microscopy (TEM) observation.The results show that processing by HPT leads to significant grain refinement with a grain size of 200-250nm.An age-hardening phenomenon is observed at 343K and 373K for the Al-Mg-Si alloys with HPT. Some precipitates were observed on grain boundaries.

Age-Hardening Behavior of Mg-Al-Zn Alloys Produced by Sand Mold Casting

In recent years, Mg-Al-Zn system alloy has been used for the parts in the automobile for weight reductions. The age-hardening behavior of Mg-6mass%Al (-1mass%Zn)-0.3mass%Mn alloys sand mold castings were investigated by Vickers hardness measurement and optical microscopic observation. Both alloys were solution-treated and then isothermal-aged at 473, 498 and 523K. For each aging temperature, both alloys were indicated age-hardening obviously, and decreased the value of maximum hardness and shorten time to maximum hardness with heighten aging temperature. Age-hardening curves for both alloys approximately showed the same change of hardness. However, these optical micrographs after aging treatment are observed different microstructure. In case of AM60 magnesium alloy, discontinuous precipitation preferentially occurred in aged sample. The volume fraction of discontinuous precipitation was larger than that of continuous precipitation. On the other hand, in case of AZ61 magnesium alloy, the volume fraction of continuous precipitation was larger than that of discontinuous precipitation. Furthermore, over-aged sample of both alloys were consisted of discontinuous precipitation, continuous precipitation and pre-precipitation area.