Investigation of the Soldering Reaction in Magnesium High Pressure Die Casting Dies

INVESTIGATION OF THE SOLDERING REACTION IN MAGNESIUM HIGH PRESSURE DIE CASTING DIES

C. Tang CRC for Cast Metal Manufacture (CAST) Industrial Research Institute, Swinbourne University of Technology (IRIS) Manufacturing Science and Technology, CSIRO (CMST) Locked Bag 9 Preston VIC 3072 Australia

M. Z. Jahedi CRC for Cast Metal Manufacture (CAST) Manufacturing Science and Technology, CSIRO (CMST) Locked Bag 9 Preston VIC 3072 Australia

M. Brandt CRC for Cast Metal Manufacture (CAST) Industrial Research Institute, Swinbourne University of Technology (IRIS) P.O. Box 218 Hawthorn VIC 3122 Australia

Abstract Soldering in high pressure die casting (HPDC) of magnesium alloys is believed not to occur because of the poor afﬁnity between magnesium and iron. However, information from industry showed that a soldered layer is formed

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between the magnesium alloy and the die material. To understand this phenomenon, a series of H13 dipping tests was carried using AZ91D magnesium alloy at 680 ◦C. Samples were sectioned and analyzed with optical microscope, Scanning Electron Microscope (SEM), Energy Dispersive X-ray Analysis (EDX) and x-ray micro-diffraction system. The die casting trials were also carried out to conﬁrm the soldering development in high pressure die casting conditions. The results showed that the formation of intermetallic compound was started with the nucleation of the η-Fe2Al5 phase. Later, manganese substituted some of iron and this phase became (Fe,Mn)2Al5. Then, a metastable F-phase Mn23Al77 was formed at the outer layer.

Keywords: High pressure die casting, soldering, magnesium alloy, immersion, intermetallic layer

INTRODUCTION Magnesium alloys are the lightest structural metal material. The use of magnesium die cast products has been increasing over the last decade. With many advantages of the magnesium alloys, the magnesium high pressure die casting is becoming one of the most attractive and prospective industries. Magnesium alloys are not only 33% lighter than aluminium alloys, but also its manufacturing cost is comparatively low [1], despite the higher price of magnesium material. Perfect casting property makes it suitable for very thin wall casting with high quality surface finish. Unlike aluminium high pressure die casting in which soldering has been received a great deal of attention [1, 2, 3, 4, 5, 6, 7, 8, 9], little is known about soldering of magnesium alloys. The general believe is that soldering in magnesium die casting does not occur because of the low affinity between magnesium and iron. However, data from industry suggests that soldering in magnesium die casting is a problem. As good surface quality and thin wall strength are two distinct advantages of magnesium die cast products [10, 11, 12], even slight soldering may degrade magnesium castings qualities. Soldering in the aluminium high pressure die casting process has been investigated and two soldering mechanisms have been reported. The first one suggests that soldering is a series of steps involving erosive wear, corrosive wear, dissolution of die materials, and development of intermetallic phases [5]. The erosive wear is the mechanical interaction between the die surface and the alloy melt which contains some solidified particles. However the research by Chen and Jahedi [3]showed that core pins subjected to the high Investigation of the Soldering Reaction in Magnesium High Pressure Die Casting Dies163

Table 1. Compositions of H13 (wt.%) Cr Si Mo C Mn V Others Fe 4,9 1,01 1,35 0,38 0,42 0,92 <0,4 Balance

melt impingement exhibit no erosive wear before the development of the soldered layer. The second mechanism proposed that soldering is caused by corrosive wear due to the strong affinity of aluminium for iron in the die steel. During the die filling and solidification processes, the casting alloy reacts with the die steel and forms complex intermetallic compounds [13]. This reaction is similar to that of hot-dip aluminising [9] although measurements showed that the temperature in die cavity is about 100 ◦Clower than that in the hot- dipping process [6]. There is not much information in the literature about interactions between iron and magnesium alloys. However, magnesium alloys contain consider- able amount of aluminium. This research work started with dipping tests to understand the initial stages of development of intermetallic phases. Then, semi–industrial high pressure die casting experiments were carried out.

EXPERIMENTAL PROCEDURE MATERIALS The materials selected for the experiments were H13 tool steel and AZ91D magnesium alloy. H13 samples were machined into 25×25×2 mm square shapes and then heat treated. After heat treatment, the samples were polished with 1200 SiC paper and cleaned in a supersonic bath. A hole was drilled at the corner of the square samples to be held stable when immersed into the alloy melt. Table 1 gives the compositions of H13 steel. AZ91D is a widely used magnesium alloy for HPDC because of its good castability and high strength combined with moderate ductility [14]. Hence, it was selected as the casting alloy for these experiments. The chemical composition of the AZ91D alloy is presented in Table 2. 164 6TH INTERNATIONAL TOOLING CONFERENCE

Table 2. Compositions of AZ91D Alloy (wt.%) Al Mn Zn Si Others (Cu, Ni, Fe) Mg 8,18 0,104 0,90 0,011 <0,005 Balance

DIPPING TEST The experiments were carried out using a MiniMag furnace located at CSIRO Division of Manufacturing Science and Technology. It is designed for melting magnesium alloys. To prevent the exothermic reactions, AZ91D ingots were preheated before melting in the furnace. Preheating of the ingots removes moisture and makes it safe during melting. A mixture of SF6 (0.18 %) and nitrogen was used as a cover gas. Tem- perature of the furnace was kept constant at 680 ◦C. The schematic of the immersion set-up is shown in Fig. 1. H13 samples were suspended from

Figure 1. Schematic diagram of the immersion set-up. the holding bar and dipped into the melt for periods, from 5 minutes up to 3 hours. To make sure that the samples had direct contact with the melt, the slag layer on the surface of the melt was pierced. When the tests were completed, the samples were removed from the melt and buried into a dry sand to cool down. This prevented the burning of magnesium alloy Investigation of the Soldering Reaction in Magnesium High Pressure Die Casting Dies165 HIGH PRESSURE DIE CASTING OF AZ91D The semi-industrial machine for HPDC research was a Toshiba 250 ton HPDC machine housed in the CSIRO Division of manufacturing Science & Technology was used. Specially designed die with removable core pins was used [3]. The magnesium alloy used was AZ91D. The alloy temperature ◦ was maintained at 680 C. A mixture of 0.18% SF6 +N2 was used as cover gas. After certain shots the core pins were removed and sectioned and their microstructure were studied using optical microscopy and SEM. The composition and identiﬁcation of the phases in intermetallic layers were carried out using EDX and X-ray Micro-diffraction techniques.

RESULTS AND DISCUSSION IMMERSION TEST The results from immersion tests indicated that the surfaces of H13 steel samples were covered with silvery adherent coatings as illustrated in Fig. 2. The samples were cut, mounted and polished with a great care and then exam-

Figure 2. H13 dipped samples after removing from the AZ91D melt. ined under SEM. The structure is shown in Fig. 3. Three distinct areas were identiﬁed on the soldered area of unetched sample. The major composition 166 6TH INTERNATIONAL TOOLING CONFERENCE

Figure 3. SEM image of soldered area (unetched).

Table 3. Averaged composition (at% ) of elements in the intermetallic layer Al Fe Mn Others (Mg, Cr, Si, etc) Area 1 66,9 13,8 17,09 Balance Area 2 71,42 15,88 11,55 Balance Area 3 63,08 13,08 21,76 Balance of the intermetallic layer was iron, manganese and aluminium. These three elements made 98 atomic% of the composition of the intermatallic layer and the magnesium content was very low. The composition of the soldered layer at three different areas is shown in Table 3. A line scan was performed using EDX as shown in Fig. 4 to present the qualitative distributions of these elements. Although the areas 1 and 2 in Fig. 3 had different compositions, the X-Ray micro-diffraction results showed that they have the same structure as manganese rich η-Fe2Al5. The area 1 with higher manganese content had Investigation of the Soldering Reaction in Magnesium High Pressure Die Casting Dies167

(a) (b)

Figure 4. The results of line scan through intermetallic layer. Immersion time 2 hours and the sample was unetched. a lighter appearance and the area 2 with higher Fe content showed a darker appearance. The area 3 at Fig. 3 appeared as a crust on top of the inner (Fe,Mn)2Al5 with a much higher manganese content. The identification of intermetallic phases using conventional X-Ray Diffrac- tion (XRD) method was difficult because the layers were very thin and identification of the positions of certain phases was difficult. To overcome this problem, the X-Ray micro-diffraction technique was used [15]. The sample for X-Ray micro-diffraction was prepared as shown in Fig. 5. The points 1 and 3 were interfaces between intermetallic layer-AZ91D and intermetallic layer -H13 respectively. At the point 2, only orthorhombic η-Fe2Al5 phase

Figure 5. Schematic of polishing of sample for X-Ray micro-diffraction. was detected. The high content of manganese in this phase, could be (Fe, Mn)2Al5 where iron was substituted by manganese. At point 3, both (Fe, 168 6TH INTERNATIONAL TOOLING CONFERENCE

Mn)2Al5 and iron were detected. AZ91D has more than % 8 aluminium. Due to strong afﬁnity of aluminium to iron η-Fe2Al5 phase forms preferen- tially at the interface with H13 steel [7]. At point 1, the interface between intermetallic and of AZ91D alloy build- up η-Fe2Al5 phase, hexagonal Mg and a metastable structure F-phase Mn23 Al77. So the Mn and Al rich crust is likely to be the metastable F-phase Mn23Al77. Further work is in progress to understand the mechanism of the formation of F-phase Mn23Al77 phase.

The intermetallic layer The thickness of the intermetallic layer was ex- pected to vary with dipping time, as in the case of aluminium alloys [5, 7, 16, 17]. The relationship between dipping time and intermetallic layer thickness is presented in Fig. 6. It is clear that this relationship is linear. This is different from the results of aluminium immersion tests, which is parabolic [7]. It is believed that the presence of Si in aluminium alloys may reduce the intermetallic layer growth rate [18]. This may occur because Si might reduce the iron dissolution in the intermetallic layer or increasing the diffusion of iron the melt. As the immersion time increases, the Si accu- mulates along the intermetallic layer interface with the melt, which impedes the diffusion process and the growth of the intermetallic layer. On the other hand in AZ91D the amount of Si is very low and has no impending effect as magnesium alloys. The further work is in progress to fully understand the mechanism of the growth of the intermetallic layer in magnesium alloys. At this stage, it appears that the soldering in AZ91D alloy occurs as the

Figure 6. Thickness of the intermetallic Layer. Investigation of the Soldering Reaction in Magnesium High Pressure Die Casting Dies169

Table 4. Chemical composition of intermetallic layer on the core pin. Mg Al Mn Fe Others Inner layer 2,79 74, 1 12,17 9,54 Balance Outer layer (Crust) 3,27 63,06 32,28 1,30 balance following steps:

Nucleation of the Fe2Al5 compound due to the high afﬁnity of Fe to Al. Replacement of some of Fe atoms with manganese atoms. Diffusion of Al atoms to the H13 substrate surface causing Al super saturation at the interface of Mg-alloy and the substrate. The formation of the high manganese intermetallic phase at the top of Fe2Al5 phase occurs.

SOLDERINGLAYERFORMATIONINHIGHPRESSURE DIE CASTING OF AZ91D During HPDC of AZ91D experiment, the core pins initially were coated with a thin silvery layer as shown in Fig. 7. The EDX analysis of this layer showed that it was magnesium. After 300 shots soldering layer appeared on the core pin. The microstructure of the soldered layer on the core pin is presented in Fig. 8 and the EDX analysis of this layer is presented in Table 4. It appears that the soldering layer consist intermetallic phases observed in immersion test.

CONCLUSION The investigation of the mechanism of soldering in high pressure die casting of magnesium alloys were carried out by dipping H13 die steel into the AZ91D magnesium alloy. The intermetallic phases in the soldered layer were identiﬁed by the microstrure, EDX and XRD analyses. High pressure die casting trails were also carried out to compare the soldered layer formed during HPDC with the soldered layers from dipping tests. The following conclusions were made: 170 6TH INTERNATIONAL TOOLING CONFERENCE

Figure 7. Initial magnesium build-up on the core pin.

Figure 8. Microstructure of soldered layer on the core pin.

It appears that the soldering layer forms by nucleation of η-Fe2Al5 phase at the interface of the AZ91D melt with H13 die steel. Some of Investigation of the Soldering Reaction in Magnesium High Pressure Die Casting Dies171

the iron atoms replaced by manganese and this phase is orthorhombic η-(Fe,Mn)2Al5.

It appears that the outer layer of the soldered layer is F-phase, Mn23 Al77. The growth of intermetallic layer in AZ91D magnesium alloy is linear.

ACKNOWLEDGMENTS The authors express their gratitude to the CRC for Cast Metals Manufac- turing (CAST) for ﬁnancial support for this PhD project.

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