Structures and Hardness of Materials Formed by Melting and Liquid Diffusion ofMg Alloy Substrate with Pure Al Surface+

Fumitaka Otsubo

Center for Instrumental Analysis, Kyushu Institute of Technology, Kitakyushu 804-8550, Japan

In this study, pure Al powder was compression molded on the surface of flame-retardant Mg alloy plate, and integration of pure Al and Mgalloy was attempted by melting and solidification in air. In order to integrate different materials by liquid diffusion of metal elements, thestructure, composition phase and hardness of the formed integrated materials were investigated.

It was found that heating the specimens for 180 s results in melting and liquid diffusion between Al and Mg. When heated and melted withpure Al down and Mg alloy up (Type B), shrinkage cavity was formed on the former Mg alloy side of the former Al/former Mg alloy interface.When heated at 450 s, the specimen was composed of Al­Mg system stable phases of Al3Mg2 and Al12Mg17, and Al­Mg system metastablephases of Al0.37Mg0.63 and Al0.1Mg0.9. In Type Awith pure Al up and Mg alloy down and Type B, stable phases of Al3Mg2 and Al12Mg17 wereformed on the former Al side. The constituent phase of the former Mg alloy side was composed of metastable phases of Al0.37Mg0.63 andAl0.1Mg0.9, but the structure morphologies of Type A and B differed near the surface of the former Mg alloy side, and the primary phase wasAl0.37Mg0.63 and Al0.1Mg0.9 respectively. The final solidification phase consisted of eutectic phases of Al0.37Mg0.63 and Al0.1Mg0.9. The hardnesson the former Al side was about 250HV, and the hardness on the former Mg alloy side had decreased. Type B was evaluated to be lower thanType A. [doi:10.2320/matertrans.F-M2020830]

(Received February 18, 2020; Accepted May 11, 2020; Published July 25, 2020)

Keywords: pure aluminum, magnesium alloy, melting, solidification, liquid diffusion, metastable phase, hardness

1. Introduction

Surface modification or composite of materials isperformed by various methods.1,2) These composites areintended to impart the necessary properties to the materialsurface, and industrially, an atmosphere control device andadvanced equipment for heating are required. The authorshave found a method of generating a coating layer consistingof particle stacks by cold rolling of composite materials.3,4)

Based on this result, we have been studying the developmentof a method to inexpensively and efficiently coat the surfaceof metallic materials.

Mg is the lightest metal for practical use, and has a higherspecific strength and specific rigidity than both steel and Al.On the other hand, although its use is spreading, there areproblems with corrosion resistance and plastic workability.Therefore, the authors have been investigating methods forforming an Al coating on Mg. Mg is not suitable for rollingbecause of its poor plastic workability. Therefore, the authorshave attempted to apply a melting and solidification process.In this process, disturbance and convection based on specificgravity differences with liquid diffusion and Marangoniconvection based on concentration differences and con-vection due to heating are dominant. Vacancy-basedmechanisms such as solid diffusion are rare in liquids, andthere is no specific energy barrier.5)

In this study, pure Al powder was compression-molded ona flame-retardant Mg alloy plate surface, and an attempt wasmade to integrate pure Al and Mg alloy by atmosphericmelting and solidification processes. Aiming at the integra-tion of metallic elements by liquid diffusion, the structure,phase structure, and hardness of the integrated materials wereinvestigated.

2. Experimental Procedure

A plate-like substrate with a thickness of 2mm and lateraldimensions of 30mm © 10mm was cut from a flame-retardant Mg alloy AMX602 ingot. One side of the substratewas blasted and a pure Al powder layer (particle size: 45 to10 µm) was formed on the substrate to produce a specimenwith a surface pressure of 163MPa using a hydraulic press(Sansho Industry: Mighty Press MT-100H) (Fig. 1). Thetarget thickness of the powder molding layer was about0.7mm. In addition, when a pure Al plate was used, it wasconfirmed by preliminary experiments that it could not beintegrated with the Mg alloy, so powder was used as the pureAl materials. Table 1 shows the chemical composition of theMg alloy. A high-frequency furnace and an electric furnacewere examined as the heating method for the specimen. Inthe case of the high-frequency furnace, heat is generated by

Powder

Substrate

Blasting

163MPa

Preform

Pedestal

Fig. 1 Schematics of preforming.

Table 1 Chemical composition of AXM602 alloy.

+This Paper was Originally Published in Japanese in J. JFS 92 (2020) 69­74.

Materials Transactions, Vol. 61, No. 8 (2020) pp. 1657 to 1662©2020 Japan Foundry Engineering Society

Joule heating by eddy currents due to free electrons in thesubstrate and melts. It was found that when heated at a rateof 700K/min, vibration due to thermal convection occurredin the substrate during melting, resulting in a lump withoutretaining its original shape. On the other hand, it wasconfirmed that heating with an electric furnace causedmelting and solidification while maintaining the originalshape. Therefore, priority was given to reducing thedeformation risk for the specimen, and an integrated materialwas produced by heating with an electric furnace. Theelectric furnace used was a tubular furnace manufactured byAsahi Rika Seisakusho, which has an atmosphere size of80mm in diameter and 300mm in length. A specimen wasplaced on a SUS304 steel plate (25 © 32 © 2.8 t) at the tip ofa quartz jig (Fig. 2), and the electric furnace was slid andinserted into the center of the furnace. The temperaturechanges from the start of heating of the specimen in theelectric furnace held at 993K was measured with a JIS-Ktype thermocouple welded to SUS304 steel plate (Fig. 3). Inpreliminary experiments, it was confirmed that the specimenwent through the process of heating, melting, deformation,and ignition with increasing heating time. Therefore, theheating and holding time was changed from 90 s, which isthe process of heating and melting, to a maximum of 500 s.The specimen was cooled outside the furnace by sliding theelectric furnace. The specimen was then devised and air-cooled so that no physical motion such as vibration occurred.The solidus and liquidus temperatures for the AMX602alloys and the melting point of the pure Al are 799K,6)

881K6) and 933K, respectively. In order to investigate theeffect of the specific gravity difference between Al and Mgon interdiffusion, Type A and Type B experiments were

conducted. Here, the case of heating with the Mg alloysubstrate below is referred to as Type A, and the case ofheating with the Al layer below is referred to as Type B.Appearance and cross-section observations were performedusing scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDS; Elionix: ERA-8800), and cross-sectional elemental analysis was carriedout using electron probe micro-analysis (EPMA; JEOL: JXA-8530F) during heating and melting. Also, the constituentcrystal phases were analyzed by X-ray diffraction using aCu target (Rigaku: SmartLab). Furthermore, the hardness ofthe cross section (load: 100 gf ) was measured with a microVickers hardness tester (MATSUZAWA: MMT-X7).

3. Experimental Results and Discussion

3.1 Changes in appearance and cross-sectional elementdistribution due to heating of specimen

Figure 4 shows an optical micrograph of the Mg alloy.The microstructure consists of the primary ¡Mg, the eutecticphase of Al2Ca, and ¡Mg in the final solidification phase.Figure 5 shows the change in appearance of the specimen onthe Al side when heated from 90 s to 270 s. In Type A andType B after 90 s, it is the same as before heating. In theType A specimen after 180 s, it can be seen that the surfacecolor has changed. At 270 s, the surface has turned black. Onthe other hand, the Type B specimen is not different at 180 sthan it was at 90 s. At 270 s, the surface turns black as in thecase of the Type A specimen. From the temperature profile in

Al/AMX602

SUS304Al/AMX602

Thermocouple

SUS304

Fig. 2 Schematics of top of quartz tube.

300

500

700

900

1100

0 100 200 300 400 500 600 700

Tem

pera

ture

,K

Time, s

Fig. 3 Change in temperature as function of heating time.

100μm

Fig. 4 Microstructure of AMX602.

Fig. 5 Appearance of specimens heated for each times.

F. Otsubo1658

Fig. 3, there is a temperature difference of about 25K for theType A specimen after 180 s and 270 s, which is consideredto be the cause of the discoloration.

Here, the relationship between the heating time and themelting of the specimen is considered assuming that there isno temperature difference or concentration difference in thethickness direction of the specimen. Based on the solidusand liquidus temperatures for the Mg alloy, the melting pointof pure Al and the temperature profile in Fig. 3, the time toreach the melting temperature in Mg alloy and Al is 50 sand 140 s, respectively. Therefore, it is expected that in thisexperiment, the Mg alloy side and the Al side melt on heatingfor 90 s or more and 180 s or more, respectively.

Figure 6 shows an EPMA image of the specimen crosssection. Figures 6(a) to 6(c) are characteristic X-ray imagesof Mg in the Type A specimen. The image in Fig. 6(a) showsthat Mg is not distributed on the surface of the Al layer,which is a black region. On the other hand, it can be seen thatMg is distributed in the Mg alloy side region of the Al layer.In addition, newly formed dendrite is observed on the Al sideof the Mg alloy. This indicates that it has melted andsolidified. Therefore, it is considered that Al diffused intothe Mg alloy and the melting point decreased. At 180 s inFig. 6(b) and 270 s in Fig. 6(c), it can be seen that Mg isdistributed over the entire thickness of the specimen, and astructure with new dendrite is seen. Furthermore, when anexternal force was applied to the specimen with tweezers, itdeformed fluidly. Therefore, it is assumed that the specimenmelted and liquid diffusion occurred. Figure 6(d) shows abackscattered electron image of the region shown inFig. 6(a), which confirms the surface side of the Al layer,the interdiffusion region of Al and Mg, the newly formeddendrite region, and the unmelted Mg alloy. Figures 6(e) and6(f ) are characteristic X-ray images of Mg in the Type Bspecimen. At 180 s in Fig. 6(e), it can be seen that theinterdiffusion of Al and Mg is not so advanced on the Alside. On the Al side of the Mg alloy, newly formed dendriteswere observed, as in Fig. 6(a). This also indicates that it hadmelted and solidified. As with the Type A specimen, it isthought that Al diffused in the Mg alloy and the melting point

decreased. Interdiffusion progresses with increasing temper-ature at 270 s in Fig. 6(f ), but compared to 270 s in Fig. 6(c)for Type A, there is a difference in Mg concentration on theAl side. Thus, diffusion of elements occurs during heating ofthe specimen, creating a concentration gradient. In addition,the melting point drops and selective melting occurs. Liquiddiffusion due to melting then also occurs.

3.2 Cross-sectional structure and element distributionFigure 7 shows continuous optical micrographs of the

entire cross section at the center of each the Type A andType B specimens by heating for 450 s. The specimen meltsand liquid diffusion of Al and Mg alloy elements occurs. Inaddition, the boundary between the former Mg alloy and theformer Al layer can be recognized. Comparing Type A andType B specimens, the Type B specimen has many voids onthe former Mg alloy side of the former Al layer/former Mgalloy interface. In these voids, primary dendrites crystallize,solidification of the residual melt progresses, the residualmelt disappears, and solidification is completed. In otherwords, it is considered that shrinkage occurs. Shrinkagecavity form in the final solidified region. Therefore, it isconsidered that the Type B specimen contained more meltthan the Type A specimen with the final solidifyingcomposition. When the inside of a shrinkage cavity wasobserved with SEM, primary dendrite was observed.Solidification occurs from the free and bottom surfaces. Asa result of observing the macrostructure of the entirelongitudinal section of the specimen, a shrinkage cavitywas found to be generated near the center of the specimen.

Figure 8 shows the distribution of Al and Mg from theformer Al side to the former Mg alloy by quantitativeanalysis using EPMA on the cross section of the specimenheated and held for 450 s. An electron beam with a diameterof 50 µm was scanned along the thickness direction of thespecimen cross section at 50 µm intervals. The concentrationdistribution of each element changes due to interdiffusionfrom the concentration distribution before heating. In theType A specimen, the concentration from the former Al sideis distributed in the range of 40 to 30 at% and 50 to 70 at%for Al and Mg, respectively. On the other hand, in the Type Bspecimen, Al and Mg are distributed in the range of 50 to20 at% and 50 to 80 at%, respectively. Therefore, the Type Aspecimen has stronger interdiffusion than the Type Bspecimen. In the Type B specimen, the diffusion of Al to

Fig. 6 EPMA images of specimens heated at different times. (a)­(c) Mg-K¡ images of Type A, (d) BSE image of (a), (e)­(f ) Mg-K¡ images ofType B specimen.

Fig. 7 Transvers-section of specimens heated at 450 s.

Structures and Hardness of Materials Formed by Melting and Liquid Diffusion of Mg Alloy Substrate with Pure Al Surface 1659

the Mg alloy due to the difference in specific gravity betweenAl and Mg is considered to be weak. Therefore, thedifference in concentration of Al and Mg in the thicknessdirection of the Type B specimen is larger than in the Type Aspecimen around 800 µm from the surface. In other words,in the Al­Mg system equilibrium diagram,7) the Type Bspecimen may have a composition in the region where theMg concentration is higher than that in the Type A specimen.Therefore, it can be inferred that in the region of 800 µm ormore from the surface in Fig. 8, the composition in whichvoids are formed, that is, the melt has the final solidifyingcomposition.

3.3 Constituent phases in specimenFigures 9 and 10 show X-ray diffraction patterns for

sequential surface layers removed from the former Al layerside of the Type A and Type B specimens heated and heldfor 450 s, respectively. From the analysis of these diffractionpatterns, the constituent phases in the specimen are stableand metastable Al3Mg2, Al12Mg17, Mg0.63Al0.378) andAl0.1Mg0.9.9,10) The constituent phases in the former Al sideconsist of all crystal phases confirmed by X-ray diffraction

in the Type A and Type B specimens. The constituent phasesin the former Mg alloy side are Al0.37Mg0.63 and Al0.1Mg0.9crystal phases in the Type A and Type B specimens. Thesecrystal phases are metastable phases that are not seen inthe Al­Mg system binary equilibrium diagram. Here,Al0.37Mg0.63 and Al0.1Mg0.9 are called MS-¡ and MS-¢,respectively. Stable phases of Al3Mg2 and Al12Mg17 aregenerated in the former Al side of the Type A specimen witha scale layer of about 30 to 40 µm thick peeled off. This isprobably because Mg diffused quickly into Al. On the otherhand, there is no Al12Mg17 phase on the former Al surfacein the Type B specimen, and Al3Mg2 and Al12Mg17 stablephases are formed on the surface with the scale layerremoved. This is probably because Mg diffused into Al moreslowly than in the case of the Type B specimen. In order tomake the Mg concentration distribution in Fig. 8 easy tounderstand for interdiffusion in a liquid, it is converted to astraight line by the least squares method. As a result, theconcentration range of Mg on the former Al side of theType A and Type B specimens is 44 at% to 63 at%. This isthe range where Al3Mg2 and Al12Mg17 are formed at roomtemperature in the Al­Mg system equilibrium phase diagram.The concentration range for Mg on the former Mg alloy sideof Type A and Type B is 63 at% to 80 at%. This is the rangewhere Mg and Al12Mg17 are generated at room temperaturein the Al­Mg system equilibrium phase diagram. It isconsidered that nonequilibrium solidification occurs at thecooling rate associated with air cooling11) in the Mgconcentration range on the former Mg alloy side. As a result,primary dendrite of MS-¡ or MS-¢ was formed, and then theeutectic phase was formed by MS-¡ and MS-¢.

Figure 11 shows the microstructure near the center ofType A and Type B specimens heated and held for 450 s. Inboth specimens, the upper part is the former Al layer andthe lower part is the former Mg alloy. When the structure ofthe Mg alloy substrate shown in Fig. 4 is integrated with pureAl by melt solidification, it shifts to the structure shown inFig. 11. The horizontal arrow in the figure indicates theboundary position between the former Al layer and theformer Mg alloy. In both specimens, dendrites (bright

Fig. 8 Change in content of Mg and Al in Type A and B specimens.

Fig. 9 XRD pattern of Type A specimen.

Fig. 10 XRD pattern of Type B specimen.

F. Otsubo1660

images) surrounded by broken lines from the vicinity of theinterface between the former Al layer and the former Mgalloy to the former Mg alloy are seen. It can be seen thatshrinkage cavity is generated in the Type B specimens. In theType A specimen, dendrites grown from the surface of theformer Mg alloy toward the former Al can be seen. On theother hand, dendrite (dark image) different from Type Asurrounded by the solid line grows from the surface ofthe former Mg alloy in the Type B specimen. The regionadjacent to the dendrite was confirmed to be a eutectic phaseby high-magnification optical microscopy. Figure 12 showsthe structure near the surface of the former Mg alloy side ofthe Type A and Type B specimens, which have differentprimary phases. Therefore, the Type A and Type B speci-mens have the same constituent phases on the former Mgalloy side but the microstructure is different. The surface nearthe former Mg alloy in the primary phases in the Type A andType B specimens was analyzed by SEM-EDS. The resultsare shown in Table 2. From the analysis results, the primaryphase in the Type A specimen is MS-¡, while the primaryphase in the Type B specimen is MS-¢, and the eutecticphase consists of MS-¡ and MS-¢.

3.4 Cross sectional hardness of specimenFigure 13 shows the micro Vickers hardness of the cross

section of the specimen heated and held for 450 s. It can beseen that the hardness of the former Al side is high for boththe Type A and Type B specimens, and the hardness isdecreased to the former Mg alloy side. The hardness ofthe former Al side is about 250HV, which decreases on theformer Mg alloy side, but the hardness of the Type Aspecimen is maintained from around 1500 µm from theformer Al surface. On the other hand, it can be seen that thehardness of the Type B specimen continues to decrease.Within this range, the Type A and Type B specimens consistof primary phases of MS-¡ and MS-¢. From Fig. 12, it can be

seen that the Type A and Type B specimens have largervolume fractions of MS-¡ and MS-¢, respectively. This isthought to be due to the hardness of the region.

4. Conclusions

(1) In this experiment, Al and Mg alloy elements diffuseby heating, and a concentration gradient is generated. In

Fig. 11 Optical micrograph of transvers-section of specimens heated at450 s. Horizontal arrow indicates boundary between former Al and formerMg alloy.

Resin

Fig. 12 Microstructure of cross-section of Type A (a) and Type B (b)specimens close to surface in former Mg alloy.

Table 2 Chemical composition obtained by EDS (at%).

Fig. 13 Change in hardness of Type A and B specimens as function ofdistance from surface of former Al layer.

Structures and Hardness of Materials Formed by Melting and Liquid Diffusion of Mg Alloy Substrate with Pure Al Surface 1661

addition, the melting point drops and selective meltingoccurs. Liquid diffusion due to melting also occurs,and through this process, it is thought that diffusion ofelements is promoted.

(2) In the case of heating for 450 s, the Type B specimencontains voids on the former Mg alloy side of theformer Al/former Mg alloy interface. These voids arethought to be due to shrinkage cavity. In addition, theType A specimen exhibits stronger interdiffusion thanthe Type B specimen, and this shrinkage is thought tobe caused by the difference in specific gravity betweenAl and Mg.

(3) When heated for 450 s, the specimens consist ofAl3Mg2, Al12Mg17, Al0.37Mg0.63, and Al0.1Mg0.9 Al­Mg system stable and metastable phases. In both theType A and B specimens, it is considered that stablephases of Al3Mg2 and Al12Mg17 were formed on theformer Al side as the result of rapid diffusion of Mginto Al. The constituent phases in the former Mgalloy side are metastable phases of Al0.37Mg0.63 andAl0.1Mg0.9.

(4) Structures near the surface of the former Mg alloy aredifferent in the Type A and Type B specimens afterheating for 450 s. The primary crystal phases areAl0.37Mg0.63 and Al0.1Mg0.9, respectively, and thebalance consists of eutectic phases of Al0.37Mg0.63 andAl0.1Mg0.9. The hardness on the former Al side is about250HV, and the hardness near the surface on the formerMg alloy side decreases, and the Type B specimen wasrated lower than the Type A specimen. The Type A and

Type B specimens have large volume fractions of MS-¡and MS-¢, respectively. This is thought to be due to thedifference in hardness.

Acknowledgments

We would like to express our sincere gratitude to thetechnical staff of the Center for Instrument Analysis ofKyushu Institute of Technology, for their cooperation inthis analysis. We would also like to thank Yurika Omori(currently Sankyu Inc.) and Daisuke Hotogi (currentlyAdvanex Inc.) of Kyushu Institute of Technology, Schoolof Engineering, for their cooperation in this experiment.

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