Microstructures and Mechanical Responses of Powder 

Metallurgy Noncombustive Magnesium Extruded 

Alloy by Rapid Solidification Process in 

Mass Production 

by Katsuyoshi Kondoh, EL-Sayed Ayman Hamada, Hisashi Imai, 

and Junko Umeda 

ARL-CR-647 May 2010 

prepared by 

Joining and Welding Research Institute 

Osaka University 

11-1 Mihogaoka 

Ibaragi, Osaka, 567-0047, Japan 

under contract 

FA-5209-09-P-0158 

Approved for public release; distribution is unlimited. 
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Army Research Laboratory 

Aberdeen Proving Ground, MD 21005-5066 

ARL-CR-647 May 2010 

Microstructures and Mechanical Responses of Powder 

Metallurgy Noncombustive Magnesium Extruded 

Alloy by Rapid Solidification Process in 

Mass Production 

Katsuyoshi Kondoh, EL-Sayed Ayman Hamada, Hisashi Imai, 

and Junko Umeda 

Joining and Welding Research Institute 

prepared by 

Joining and Welding Research Institute 

Osaka University 

11-1 Mihogaoka 

Ibaragi, Osaka, 567-0047, Japan 

under contract 

FA-5209-09-P-0158 

Approved for public release; distribution is unlimited. 
ii

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4. TITLE AND SUBTITLE 

Microstructures and Mechanical Responses of Powder Metallurgy Noncombustive 

Magnesium Extruded Alloy by Rapid Solidification Process in Mass Production 

5a. CONTRACT NUMBER 

FA-5209-09-P-0158 

5b. GRANT NUMBER 

5c. PROGRAM ELEMENT NUMBER 

6. AUTHOR(S) 

Katsuyoshi Kondoh, EL-Sayed Ayman Hamada, Hisashi Imai, and Junko Umeda 

5d. PROJECT NUMBER 

1L162618AH80 

5e. TASK NUMBER 

5f. WORK UNIT NUMBER 

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Joining and Welding Research Institute 

Osaka University 

11-1 Mihogaoka 

Ibaragi, Osaka, 567-0047, Japan 

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13. SUPPLEMENTARY NOTES 

14. ABSTRACT 

Spinning water atomization process (SWAP), one of the rapid solidification processes, promised to produce coarse noncombustible

magnesium alloy powder 1–4 mm long and have fine α-Mg grains and Al2Ca intermetallic compounds. SWAP had economical and safe 

benefits in producing coarse Mg alloy powders with very fine microstructures in the mass production process due to its extreme high 

solidification rate compared to the conventional atomization process. AMX602 (Mg-6%Al-0.5%Mn-2%Ca) powders were compacted at room 

temperature. Their green compacts, with a relative density of about 85%, were heated at 573–673 K for 300 s in Ar gas atmosphere and 

immediately consolidated by hot extrusion. Microstructure observation and evaluation of mechanical properties of the extruded AMX602 alloys 

were carried out. The uniform and fine microstructures, with grains <0.45–0.8 μm via dynamic recrystallization during hot extrusion, were 

observed and much smaller in comparison to the extruded AMX602 alloy fabricated from cast ingot. The extremely fine intermetallic 

compounds, 200–500 nm in diameter, were uniformly distributed in the matrix of powder metallurgy extruded alloys. These microstructures 

caused excellent mechanical properties of the wrought alloys. For example, in the case of AMX602 alloys extruded at 573 K, tensile strength 

of 447 MPa, yield stress of 425 MPa, and 9.6% elongation were obtained. 

15. SUBJECT TERMS

rapid solidification, noncombustive magnesium alloy, powder, hot extrusion, water atomization, AMX602, mechanical 

response, microstructure 

16. SECURITY CLASSIFICATION OF: 

17. LIMITATION 

OF ABSTRACT 

UU 

18. NUMBER 

OF PAGES 

26 

19a. NAME OF RESPONSIBLE PERSON

Tyrone Jones 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified 

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Unclassified 

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410-278-6223 

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18 
iii

Contents 

List of Figures iv

List of Tables v

Acknowledgments vi

1. Introduction 1

2. Experimental 2

2.1 Raw Materials..................................................................................................................2

2.2 Powder Consolidation .....................................................................................................3

2.3 Evaluation ........................................................................................................................3

3. Results and Discussion 3

4. Conclusions 10

5. References 11

Distribution List 13 
iv

List of Figures 

Figure 1. (a) Illustration of SWAP equipment to produce rapidly solidified Mg alloy 

powders and (b) morphology of coarse noncombustive magnesium alloy powder 

(AMX602 prepared by SWAP). ................................................................................................2

Figure 2. Optical microstructures of input raw materials: (a) as-cast ingot billet, (b) asreceived fine SWAP powder, and (c) coarse powder. ...............................................................4

Figure 3. Differential thermal analysis profiles of AMX602 cast ingot billet and as-received 

coarse SWAP powder. ...............................................................................................................5

Figure 4. X-ray diffraction patterns of input raw material and AXM602 alloys extruded at 

573 and 673 K, using SWAP powder compact (a) and cast ingot billet (b). .............................5

Figure 5. Scanning electron microscope observation of AMX602 alloys extruded at 573 and 

673 K, using SWAP powder compact (a-1 and a-2) and cast ingot billet (b-1 and b-2). .........6

Figure 6. SEM-EDS analysis on intermetallic compounds of AMX602 alloy extruded at 573 

K using cast ingot billet (a–f).....................................................................................................7

Figure 7. Dependence of micro-Vicker’s hardness on grain size of extruded AMX602 alloys. ....8

Figure 8. Tensile strength, yield stress, and elongation dependence on preheating 

temperature. ...............................................................................................................................9

Figure 9. SEM observation on fractured surface of tensile test specimen extruded at 623 K 

using powder compact (a) and cast ingot (b). ............................................................................9
v

List of Tables 

Table 1. Chemical compositions of noncombustive magnesium alloy powders. ...........................2
vi 

Acknowledgments 

I would like to thank Mr. Tyrone Jones of the U.S. Army Research Laboratory for the technical 

vision and collaboration in this international program. Mr. Jones initiated this successful 

program through the International Technology Center - Pacific (ITC-PAC) and set technical key 

performance parameters that have led to the development of a high-strength, high-ductility 

lightweight magnesium alloy for applied structural applications. I would also like to thank Dr. 

Jitendra Singh of ITC-PAC for his drafting and management of this research contract. 
1 

1. Introduction 

Magnesium has a low density of 1.8 kg/m3, and its alloy is lightest of the industrial metals. It has 

a higher specific tensile strength than other metals, such as aluminum, iron, and titanium alloys 

(1). Therefore, it promises to save energy and reduce air pollutants, such as CO2, SOx, and NOx, 

through weight reduction as structural components used in automobiles, motorcycles, and 

airplanes. In general, the strengthening of materials is useful for the weight reduction of their 

components. Grain refinement is one of the effectively strengthening processes. For example, 

the Hall-Petch equation is well known to quantitatively express the effect of the grain size on the 

yield stress of the materials (2). In this equation, a larger Hall-Petch constant, corresponding to 

its gradient of the equation, is more effective to increase the strengthening effect. When 

comparing the constant of magnesium and aluminum, the former is about twice as large as the 

latter (3). Accordingly, the grain refinement process is often employed to improve the 

mechanical properties of magnesium alloys. For example, hot extrusion, rolling, forging, highpressure torsion, accumulative roll bonding, and roll compaction, which are typical severe plastic 

working processes, are useful to refine grains of the matrix via dynamic recrystallization (4–8). 

Strong texture, however, is formed in the matrix at the same time these plastic working processes 

are applied to the magnesium alloys. It is concluded that the severe plastic deformation is 

available to increase the tensile property by grain refinement but causes anisotropy in the 

magnesium alloys (9). 

On the other hand, a rapid solidification process is useful to prepare ultrafine microstructures of 

metal powders, such as iron, aluminum, magnesium, copper, and their alloys (10–13). 

Nonequilibrium phases and amorphous and metallic glass structures can be also built up in the 

rapidly solidified metal powders (14–16). They also promise to improve the mechanical and 

physical properties of their consolidated materials. In general, microstructure refinement by the 

rapid solidification process strongly depends on the particle size. For example, the finer powders 

are significantly more effective in forming smaller grain size. It is obvious, however, that finer 

powders of active metals, such as aluminum, magnesium, and titanium, are very dangerous to 

handle. It is also difficult to consolidate them via the solid-state sintering process because of 

their thermally stable oxide films covering the powder surfaces. Therefore, from the previous 

two points of view, it is necessary to handle and consolidate the fine magnesium powders, 

ribbons, and flakes under a vacuum condition. 

In this study, the microstructure observation and structural analysis were carried out on 1–4-mmlong coarse magnesium powders prepared by spinning water atomization process (SWAP). This 

process has already been commercialized to produce stainless steel fine particles and highstrength aluminum alloy powders. The noncombustive magnesium alloy (17) AMX602 

(Mg-6%Al-0.5%Mn-2%Ca) was selected for the matrix alloy. While consolidating AMX602 
2 

powders by hot extrusion, the effect of the temperature in the preheating process before extrusion 

on the microstructural and mechanical properties of the extruded AMX602 powder metallurgy 

(P/M) alloys was investigated in detail. The extruded material fabricated using AMX602 cast 

ingot was employed as the reference. P/M AMX602 alloys showed extremely fine grains with a 

diameter of 0.45–0.8 μm, a good balance of tensile strength with 422 MPa, and 14.2% elongation 

at room temperature. 

2. Experimental 

2.1 Raw Materials 

In SWAP powder preparation, illustrated in figure 1a, the noncombustive AMX602 magnesium 

alloy ingot was melted at 1053 K in the ceramic crucible. A protective layer of inert gas 

surrounded the ingot during the melting phase of SWAP preparation. The molten metals were 

directly streamed into the spinning water from the crucible nozzle. Table 1 shows chemical 

compositions of AMX602 alloy powders prepared by SWAP. The required calcium content of 

2.09% was satisfied. The impurity content of Fe and Cu was successfully controlled to less than 

0.005% because they are corrosive elements against magnesium alloys. As shown in figure 1b, 

the coarse AMX602 powders are 1–4 mm long and have an irregular shape. The cast ingot with 

the same composition was also prepared as a reference input material. 

Figure 1. (a) Illustration of SWAP equipment to produce rapidly solidified Mg alloy powders and 

(b) morphology of coarse noncombustive magnesium alloy powder (AMX602 prepared 

by SWAP). 

Table 1. Chemical compositions of noncombustive magnesium alloy powders. 

Al Zn Mn Fe Si Cu Ca Mg 

6.01 0.007 0.26 0.002 0.038 0.004 2.09 Balance 
3 

2.2 Powder Consolidation 

The powder was consolidated at room temperature by using a 2000-kN hydraulic press machine 

to fabricate the green compact, which had a relative density of 85% and a diameter of 42 mm. 

The columnar compact and cast ingot were heated at 573–673 K for 180 s in an argon gas 

atmosphere and immediately consolidated into full density by hot extrusion. The extrusion ratio 

of 37 and 1-m/s extrusion speed were used in this study. 

2.3 Evaluation 

To evaluate the thermal stability of the microstructures of raw powders, the differential thermal 

analysis (DTA, Shimadzu DTG-60) and x-ray diffraction (XRD, Shimadzu XRD-6100) analysis 

were carried out. An optical microscope (Olympus, BX-51P) and a scanning electron 

microscope (SEM, JOEL JSM-6500F) with energy-dispersed x-ray spectrometer (EDS, JOEL 

EX-64175JMU) were used to investigate the grain size and intermetallic compounds of the raw 

powders and their extruded alloys. The image-scanning software (Image Pro-Plus II) was 

applied to the photos to estimate the grain size distribution. The micro-Vicker’s hardness of 

powders and wrought materials was measured by using the hardness tester (Shimadzu HMV-2T). 

The tensile test specimens were machined from the extruded alloys and evaluated at room 

temperature under a strain rate of 5*10–4/s. The fractured surfaces of the tensile test specimens 

were observed by SEM to investigate the fracturing mechanism of the magnesium alloys. 

3. Results and Discussion 

Figure 2 shows SWAP-prepared optical microstructures of AMX602 cast ingot (figure 2a), asreceived fine powders <0.5 mm (figure 2b), and coarse powders over 1 mm (figure 2c). The cast 

ingot material consists of coarse α-Mg grains 60–150 μm in diameter, and some intermetallic 

compounds are observed at their grain boundaries. As shown in (figure 2b), fine powders reveal 

small dendrite structures formed during the rapid solidification of molten Mg alloy droplets after 

atomization. A mean dendrite arm spacing (DAS) of fine powders (figure 2b) is 0.97 μm. The 

coarse powders shown in figure 2c indicate large grains 4–8 μm in diameter caused by a reduced 

solidification rate during atomization. The grain mean size is 6.7 μm. According to the 

following equation (18), the DAS value suggests the estimated solidification rate (R) of the fine 

AMX602 powders. 

0.31 35.5    R λ :DAS(μm), R:Solidification rate (K/s) (1) 

As a result, the solidification rate of fine and coarse Mg alloy powders via SWAP is 1.1 

× 105 K/s and 6.8 × 103 K/s, respectively. The mean particle size of fine and coarse powders via 

SWAP is 0.36 and 1.87 mm, respectively. They are much larger than the other alloy powders 

(100–200 μm) via the conventional atomization process. This indicates that SWAP Mg alloy 
4 

Figure 2. Optical microstructures of input raw materials: (a) 

as-cast ingot billet, (b) as-received fine SWAP 

powder, and (c) coarse powder. 

powders are safe to handle and show very fine microstructures due to high solidification rare as 

mentioned above. Therefore, SWAP has an economical benefit when applied to the mass 

production process of powder metallurgy Mg alloys. On the other hand, the cast AMX602 alloy 

had a very small solidification rate of 0.6 K/s, calculated by the previous equation, which was 

equal to that of the conventional cast Mg alloys (19). Furthermore, no intermetallic compounds 

were detected at the grain boundaries in either powder due to very high rapid solidification rate. 

Figure 3 shows DTA profiles of the atomized coarse powders and cast ingot. The extremely 

large exothermic peaks detected at 723 K in both profiles are caused by changing the heating rate 

from 10 to 5 K/min. The large endothermic peaks are detected at 798–800 K in both and 

correspond to a melting point of Al2Ca intermetallics (17). The endothermic peak at about 880 K 

is due to the appearance of a liquid phase of this alloy. According to these results, the extrusion 

temperature of the atomized powders and cast ingot should be controlled to less than melting 

point of Al2Ca intermetallics at 798 K. In this study, the preheating temperature of the green 

compact and cast ingot material is set at 573–673 K. 

Figure 4 shows XRD patterns of the input raw material and its extruded alloys at 573 and 673 K. 

In the case of SWAP powders shown in figure 4a, the magnesium oxide (MgO) peak is detected 

at 2θ=43°, which is caused by oxidation of molten Mg alloy droplets during water atomization. 

MgO films covering the powder surfaces are thermally stable. However, it is easy to break them 

by applying a very low pressure in consolidation because of their porous structures (20). No 

Al2Ca intermetallic peak is shown in the profile of the raw powder, but it is detected in the 

extruded alloys. This means that Al and Ca are solid-soluted in the matrix of the as-received raw 

powder by rapid solidification. However, Al2Ca compounds are precipitated due to the elevated 

temperature during hot extrusion. The peak intensity of Al2Ca gradually increases with increases
5 

Figure 3. Differential thermal analysis profiles 

of AMX602 cast ingot billet and asreceived coarse SWAP powder. 

Figure 4. X-ray diffraction patterns of input raw material and AXM602 alloys extruded at 573 

and 673 K, using SWAP powder compact (a) and cast ingot billet (b). 

in the extrusion temperature. That is, the amount of precipitated Al2Ca intermetallics increases 

by a longer thermal history in preheating. On the other hand, the cast ingot material before 

extrusion contains coarse Al2Ca intermetallics crystallized during casting (21), as shown in 

figure 4b. There is no remarkable difference in the profile of the wrought alloys extruded at 

573–673 K.

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

273 373 473 573 673 773 873 973

Temperature, T / K

DTA / uV mg-1

Atomized powder Atomized powder

Cast ingot Cast ingot

Exo.

Heating rate; 5 K/min.

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

273 373 473 573 673 773 873 973

Temperature, T / K

DTA / uV mg-1

Atomized powder Atomized powder

Cast ingot Cast ingot

Exo.

Heating rate; 5 K/min.
6 

Figure 5 shows SEM observation results on the extruded AMX602 alloys using the powder 

compact (figures 5a-1 and 5a-2) and cast ingot billet (figures 5b-1 and 5b-2). The extrusion 

temperatures are 573 and 673 K. The powder extruded materials show extremely fine grains 

0.3–1.1 μm in diameter compared to the cast ingot extruded ones shown in figures 5b-1 and 5b-2. 

Regarding the effect of the extrusion temperature on the grain size of the powder extruded 

material, the mean grain size of the wrought alloy at 573 K, calculated by the image scanning 

software, is 0.45 μm, and the extruded alloy at 673 K is 0.8 μm. The longer thermal history 

during preheating in the latter at 673 K causes a small grain growth after dynamic 

recrystallization in hot extrusion (22). The amount of precipitated Al2Ca intermetallic 

compounds, which corresponds to fine white particles 100–300 nm distributed in the matrix, is 

larger for the extruded alloy at 673 K than that of the wrought alloy at 573 K. 

Figure 5. Scanning electron microscope observation of AMX602 alloys 

extruded at 573 and 673 K, using SWAP powder compact (a-1 and 

a-2) and cast ingot billet (b-1 and b-2). 

On the other hand, figure 5b-1 indicates that the AMX602 wrought alloy extruded at 573 K using 

the cast ingot billet consists of fine α-Mg grains 1–3 μm in diameter via dynamic recrystallization. 

Some large grains 5–10 μm in diameter are observed in the matrix. The mean grain size of the 

extruded materials at 573 K is 1.96 μm. However, when employing the higher preheating 

temperature at 673 K, many coarse grains over 10 μm exist in the matrix, as shown in figure 

5b-2, and a few fine grains <3 μm in diameter are observed. The mean grain size is 3.29 μm. 
7 

Compared to the microstructure shown in figure 5b-1, the grain growth and coarsening of figure 

5b-2 occurred during extrusion at the higher temperature of 673 K after dynamic recrystallization. 

Both wrought alloys using cast ingots contain coarse intermetallic compounds with irregular 

morphologies. Compared to AMX602 cast ingot material, some small compounds are observed 

because the severe plastic deformation during hot extrusion caused the fragmentation of coarse 

and brittle intermetallics distributed in the matrix. 

As shown in figure 6, SEM-EDS analysis results on the specimen extruded at 573 K, shown in 

figure 5b-1, indicate the intermetallic dispersoids are mainly Al2Ca compounds and exist at grain 

boundaries. A very few small compounds with spherical shapes correspond to Al-Mn 

intermetallics. Both intermetallics are typical of the conventional AMX602 cast ingot (17, 21). 

Figure 6. SEM-EDS analysis on intermetallic compounds of AMX602 alloy extruded at 573 K using cast ingot 

billet (a–f). 

Figure 7 shows a microhardness dependence on the grain size of the extruded AMX602 alloys 

fabricated by using SWAP powder compacts and cast ingot billets. Micro-Vicker’s hardness of 

each material shown in figure 5 is as follows: 113 Hv (figure 5a-1), 94.3 Hv (figure 5a-2), 

77.0 Hv (figure 5b-1), and 69.9 Hv (figure 5b-2). The hardness of wrought alloys using 

atomized powder compacts is higher than that using the cast ingot. This is due to the extremely 

fine grains and very small intermetallic compounds of the materials mentioned in figure 5. It 

reveals that the microhardness is proportional to α-Mg grain size, d–0.5, and the Hall-Petch 

relationship is shown in these data. 
8 

Figure 7. Dependence of micro-Vicker’s hardness on 

grain size of extruded AMX602 alloys. 

Figure 8 indicates a dependence of ultimate tensile strength (UTS) and yield tensile strength 

(YTS) of the extruded AMX602 alloys on the preheating temperature. UTS and YTS of the 

wrought alloys using the cast ingot billets were 298–311 and 204–251 MPa, respectively. It is 

reported that the addition of Ca in AM60 alloys is effective not only for its noncombustive 

properties but also its mechanical properties because coarse intermetallics of Al2Ca with 

irregular shapes’, as shown in figures 5b-1 and 5b-2, cause the decrease of the tensile properties 

(17, 22). Therefore, when using the cast ingot billet, the maximum content of Ca is 2% by mass. 

On the other hand, the alloys using the SWAP powder compacts show an extremely high 

strength of 391–452 MPa UTS and 358–428 MPa YTS. This is due to the very fine α-Mg grains 

and Al2Ca intermetallics shown in figures 5a-1 and 5a-2. In particular, small Al2Ca precipitated 

compounds formed by rapid solidification are significantly useful for the dispersion 

strengthening effect of these magnesium alloys. Concerning their dependence on the 

temperature, both materials reveal the decrease of UTS and YTS with increase in the preheating 

temperature due to the grain growth and coarsening, as shown in figure 5. In particular, the 

decrement of the strength of SWAP powder extruded alloys is larger than that of cast ingot 

extruded materials. The fine microstructures via dynamic recrystallization of the former are very 

sensitive to the temperature (23), and the grain growth by solid diffusion easily occurs when a 

higher preheating temperature is applied. 
9 

Figure 8. Tensile strength, yield stress, and elongation dependence on preheating temperature. 

As shown in figure 8b, the elongation increases with increasing temperature. The SWAP powder 

extruded alloy at 623 K shows a good balance of 422 MPa UTS and 14.2% elongation. As 

shown in figure 9a, fine dimpled fracture patterns, which mean typical fractures inside α-Mg 

grains, are observed, with no fracture at the primary particle boundaries in evidence by SEM 

observation. The fragmentation of intermetallic compounds at the fractured surface is not 

observed. Therefore, this material shows a high strength and good ductility. On the other hand, 

figure 9b indicates the fractured surface of the cast ingot extruded alloy at 623 K. It also shows 

dimpled fractured patterns. However, the coarse brittle intermetallic compounds include some 

cracks marked with white arrows, and they correspond to the initiation and propagation of the 

fracture. 

Figure 9. SEM observation on fractured surface of tensile test specimen extruded at 623 K 

using powder compact (a) and cast ingot (b). 
10 

4. Conclusions 

The noncombustive AMX602 magnesium alloys were fabricated by extruding the green 

compacts of rapidly solidified coarse powders 1–4 mm long via SWAP. They showed extremely 

fine α-Mg grains 0.3–1.1 μm in diameter via dynamic recrystallization. Fine Al2Ca compounds 

with a particles size of 100–300 nm were precipitated during hot extrusion and uniformly 

distributed in the matrix. Compared to the AMX602 extruded alloys using the cast ingot billets, 

TS and YS of the powder extruded materials showed a significant increase of 30%–45%. The 

optimization of the preheating temperature before hot extrusion was effective to form very fine 

recrystallized α-Mg grains and intermetallic dispersoids. For example, a good balance of 422 

MPa TS and 14.2% elongation was obtained when employing the preheating temperature at 623 K. 
11 

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