Bong Koo Park

Effects of Ca Addition on Microstructure and Mechanical Properties of Mg-RE-Zn Casting Alloy

Influences of Ca addition on microstructures and mechanical properties at room and elevated temperatures up to 300oC have been investigated for EZ43 (Mg-4%RE-3%Zn)-(0~1.2)%Ca permanent mould casting alloys, based on experimental results from optical micrography, X-ray diffractometry, scanning electron microscopy combined with energy dispersive X-ray spectroscopy and mechanical tests. With an increase in Ca content, yield strength is increased gradually at all temperatures, whereas elongation shows a decreasing tendency. High level of tensile strength around 150MPa is maintained until 250oC in the EZ43-0.8Ca and EZ43-1.2Ca alloys, meaning that Ca is very effective for improving elevated temperature strength of the Mg-RE-Zn alloy. The microstructural examinations reveal that Ca refines the a grains markedly, increases discontinuity of Mg12RE network intermetallic compound and dissolves into the Mg12RE phase. The refined a grains and Ca-containing Mg12RE with higher thermal stability are considered to be responsible for the improved mechanical properties at room and elevated temperatures for the EZ43-Ca alloys.

Microstructures and Mechanical Properties of Mg-Zn-RE-Ca Casting Alloys

Influences of Ca addition on microstructures and mechanical properties at room and elevated temperatures up to 300oC were investigated for ZE41 (Mg-4%Zn-1%Zn)-(0~1.0)%Ca permanent mould casting alloys, based on the results from X-ray diffractometry (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) and tensile tests. The microstructure of ZE41 alloy is characterized by dendritic α-(Mg) grains surrounded by Mg7Zn3-based eutectic network phase. The average size of α grains decreases continuously with an increase in Ca content. The Ca-containing ZE41 alloys have Mg7Zn3-(RE)-(Ca) eutectic phase, in which Ca is distributed inhomogeneously owing to its strong segregation power. It is noteworthy that tensile yield strength (YS) for the ZE41 alloy was enhanced with increasing Ca content at all temperatures up to 300oC, which demonstrates that Ca can play a beneficial role in improving its tensile strength at room and elevated temperatures. The ZE41 alloy with 0.5%Ca showed the highest ultimate tensile strength (UTS) at room temperature, but in the range of 175 to 300oC, the higher the Ca content, the greater the UTS. The improved tensile strength of the Ca-containing ZE41 alloy would be caused by the refined α grains and higher thermal stability of the Mg7Zn3-(RE)-(Ca) eutectic phase.

Microstructure and Properties of Mg-Al Based Casting Alloys Modified with Minor Alloying Elements

Minor alloying elements such as RE, Sb, and Si were added to Mg-Al based commercial alloys and their effects on the mechanical properties, corrosion resistance, and castability were investigated.The result show that small amount of cerium-rich misch metal (RE) addition with antimony enhances the heat resistance of AZ91 alloy effectively without deteriorating other good properties. The addition of Si with RE was also found to improve the heat resistance and casting capabilities (fluidity and hot cracking resistance) of AM50 alloy.

Microstructure and Tensile Creep Behavior of Mg-Nd-RE-Ca Casting Alloys

Influences of Ca addition on microstructures and mechanical properties at room and elevated temperatures have been investigated for Mg-1.5%Nd-1.0%RE-0.5%Zn-(0~1.0)%Ca casting alloys, on basis of experimental results from X-ray diffractometry (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS), tensile and creep tests. Microstructures of the alloys are characterized by dendritic α-(Mg) grains surrounded by Mg12Nd-Zn-(Ca) eutectic network phase. The average size of α grains decreases gradually with an increase in Ca content. At room temperature, yield strength (YS) is enhanced with increasing Ca content with a decrease in ultimate tensile strength (UTS) and elongation to fracture, whereas the Ca addition leads to greater YS and UTS at 175oC. The tensile creep strain and secondary creep rate, measured at 150 and 200oC under 100MPa for 100hrs, become lower with the increase in Ca content. The obtained tensile properties at elevated temperature demonstrate that the addition of Ca plays a role in improving high temperature mechanical properties including creep resistance for the Mg-Nd-RE-Zn-(Ca) alloys. In view of microstructural evolution, this would be attributed to the refined primary α grains and higher thermal stability of the Mg12Nd-Zn-Ca eutectic strengthening phase.

Development of Mg-Al-Zn Based Diecasting Alloys for Elevated Temperature Applications

Effort has been devoted to develop new heat resistant diecasting alloys based on Mg-Al- Zn system in this research. Small amounts of cerium-rich misch metal and antimony additions to AZ91 alloy could enhance the tensile strength at an elevated temperature while keeping the good castability. The increase of Zn content in Mg-8(wt%)Al-xZn-0.5RE-0.5Sb alloys, was observed to significantly increase the yield strength at 175oC although the castability such as fluidity and hot cracking resistance was slightly decreased. Intensive microstructural investigation on the new Mg- Al-Zn diecasting alloys was also carried out.

Die-Casting Capabilities of Heat Resistant Mg-Al-Ca Alloys

Various amounts of Ca were added to AZ91D magnesium alloy, and their effects on the die-casting abilities were investigated. It was observed that fluidity as die filling ability tends to decrease by Ca additions except for about 2%Ca. This reduction of fluidity by Ca was more significant at high superheats probably due to the high affinity between Ca and oxygen. Contrary to expectation, hot cracking resistance was found to increase by Ca additions. High Ca alloys showed some die-sticking tendency. However, the tendency was not observed below 2%Ca.

Improved Heat and Corrosion Resistance of Mg-Al-Zn Alloys by Additions of Minor Alloying Elements

Small amounts of minor alloying elements such as RE and Sr were added to Mg- 8wt%Al-5wt%Zn (AZ91D+4%Zn), and their effects on the microstructure, mechanical properties and corrosion resistance were investigated. The microstucture of the investigated alloys could be characterized by dendritic Mg, Mg17Al12, a quasi-crystalline Zn-rich phase, and Al4RE (if RE is added). Although the tensile strength of alloys was not improved, the creep strength was significantly enhanced by the additions of minor alloying elements. No apparent influence of the additions could be found on the corrosion resistance.

Heat Resistance of Mg-Nd Based Casting Alloys Containing Gadolinium and Yttrium

Influences of Gd or Y addition on microstructures and tensile properties at room and elevated temperatures were investigated for T6-treated Mg-3%Nd-0.5%Zn-0.4%Zr casting alloys, on the basis of experimental results from X-ray diffractometry, scanning electron microscopy, transmission electron microscopy, energy dispersive spectroscopy, tensile and creep tests. Microstructures of the T6-treated alloys are characterized by recrystallized α-(Mg) grains containing various nano-sized precipitates such as Mg41Nd5, Zn2Zr3 and Zr particles. In T6 condition, most of added Gd and Y elements are dissolved in Mg41Nd5 precipitates rather than formation of new phases. Tensile properties and creep resistance of the Mg-3%Nd-0.5%Zn-0.4%Zr alloy are remarkably increased by the addition of Gd or Y, but their efficiencies are substantially the same.

Microstructure and Mechanical Properties of Cast and Semi-Solid Formed Mg-Al-Zn Alloys

Various Mg-Al-Zn alloys with different Al and Zn contents were fabricated by conventional casting and semi-solid forming process. And the microstructure and mechanical properties of the alloys were investigated. In Mg-4%Al-(5~7)%Zn alloys most of the grain boundary phases were found to be Mg-Al-Zn while in Mg-6%Al or Mg-8%Al based alloys the Mg- Al-Zn phase coexisted with Mg17Al12 at grain boundaries. At room temperature the semi-solid formed alloys showed significantly higher tensile properties, especially elongation, than the conventionally cast alloys.

Formability of Mg-Zn-Zr Based Alloy Sheets at Elevated Temperatures

Mg-3%Zn-0.2%Zr based alloy sheets with various alloying elements additions were fabricated through thermo-mechanical process, and their microstructure and mechanical properties were investigated at room and elevated temperatures. CCV(conical cup value) and V-bend tests were also carried out to evaluate the formability of the fabricated alloy sheets. The experimental results showed that small amounts of Sn or Sr additions could improve the elongation at elevated temperatures, even though the room temperature tensile properties were slightly deteriorated by the Sr addition.