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Interface formation in magnesium–magnesium bimetal composites fabricated by insert molding method
Materials and Design 91 (2016) 122–131
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Interface formation in magnesium–magnesium bimetal composites fabricated by insert molding method K.N. Zhao a, J.C. Liu a, X.Y. Nie a, Y. Li a, H.X. Li a,⁎, Q. Du b, L.Z. Zhuang a, J.S. Zhang a a b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China SINTEF Materials and Chemistry, Trondheim, Norway
a r t i c l e
i n f o
Article history: Received 5 October 2015 Received in revised form 24 November 2015 Accepted 25 November 2015 Available online 1 December 2015 Keywords: Interface bonding mechanism Liquid–solid bonding Diffusion Insert molding method Interface mechanical behavior
a b s t r a c t Magnesium–magnesium bimetal composites were prepared by insert molding method in this study. To achieve good metallurgical bonding between AZ31 insert and AZ91 matrix, different fabrication process parameters such as volume ratio (V91:V31 = 1:1 or 2:1) and insert temperature (650 °C, 675 °C, 700 °C), were tested. It was shown that high-quality metallurgical interface bonding could be achieved when the volume ratio (VAZ91:VAZ31) is 2:1 and the insert temperature is 675 °C. The average interface tensile strength of the bimetal fabricated by insert molding could approach 100 MPa, which is close to the reported as-cast AZ91 tensile strength value. The fracture analysis also indicated that during the tensile test of bimetal composite, the crack initiation began in the weak zone of casting AZ91 alloy. Moreover, the bonding mechanism of the interface were also investigated in details combined the temperature measurement experiments and the corresponding numerical simulation work in this study. The results presented in this paper indicated that the insert molding method is a promising and effective approach to develop advanced Mg/Mg bimetal composites for widening the application of magnesium alloys. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium alloy is an important engineering light metallic material due to its high specific strength, high specific stiffness, better casting properties and good machining properties [1–3]. The research and application of Mg alloys have been extended from navigation and military fields to those civil products with high additional value such as automobile, computer and communication equipments [4–7]. In many cases, one single light metal material does not satisfy all the requirements of high performance at a low cost [8–10]. Therefore, the bimetal composites such as Mg/Al, Al/Al and Mg/Mg are attracting more and more attention in recent years due to the combination of their outstanding properties from the bonding metal components [11–17]. Compared with Al/Al and Mg/Al composites, the study on Mg/Mg bimetal composites are insufficient and so far only few papers could be found. Papis et al. [14] fabricated Mg/Mg bimetal composites in a laboratory-scale by dropping magnesium melt (pure Mg or AJ62) onto a solid magnesium alloy substrate (AZ31). Defect-free interfaces could be attained by operation in an Ar atmosphere. However, in this joining process the Zn pre-treatment process on the solid substrate must be ⁎ Corresponding author. E-mail address: [email protected] (H.X. Li).
http://dx.doi.org/10.1016/j.matdes.2015.11.095 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
applied before realizing bimetallic bonding in order to alleviate the unbeneficial influence on interface bonding caused from the oxides on the surface of magnesium alloys. As a result, the complexity of fabrication process and production cost were undoubtedly increased. Moreover, the bonding mechanism of the joint interface was not well investigated and the mechanical properties of the joint interface were unknown. In another paper, Liu et al. [18] reported the fabrication of AZ31/AZ31 laminated metal composites via warm roll bonding. Though a relatively higher tensile strength could be attained for the AZ31/AZ31 laminated metal composites compared with that of single AZ31 Mg alloy, they didn't investigate the bonding mechanism between AZ31/ AZ31 alloys. Moreover, during the warm roll bonding the oxides on the surface of magnesium alloys would influence the bonding strength, which had not been discussed. Recently, a new compound casting method, named insert molding method, has been utilized to fabricate Ti/Al and Mg/Al bimetal composites [19–22]. This method exhibits the excellent industrial application prospect due to its outstanding advantages such as low production cost, low energy consumption, simple production procedure, high interface bonding strength and protection from the oxidation on the surface of the alloys. Nie et al. [20,21] and Dezellus et al. [19] have fabricated Ti/ Al bimetal composites with good metallurgical bonding using this method and the average shear strength of the interface layer is much
K.N. Zhao et al. / Materials and Design 91 (2016) 122–131 Table 1 Chemical compositions (wt.%) of the AZ31 and AZ91 alloys used in this study. Material
Al
Si
Fe
Cu
Mn
Mg
Zn
AZ31 AZ91
3.06 9.1
0.016 0.09
0.002 0.003
0.002 0.002
0.47 0.15
Bal. Bal
0.7 1.18
higher than that of the aluminum matrix. Liu et al. [22] also fabricated Mg/Al bimetal composites and the good interface mechanical behavior could be achieved. The natural oxide layer on the surface of the substrate can be alleviated and the continuous metallic bonding interface can be formed. However, fabricating Mg/Mg bimetal composites using the insert molding method has not been reported so far though it can be considered as a new insight for the fabrication of Mg/Mg bimetal composites. In this paper, the above-mentioned corresponding study will be carried out. Moreover, the influence of different process parameters on the bonding characterization and mechanical behavior at the interface will also be investigated. The emphasis is on understanding the bonding mechanism of Mg/Mg bimetal interface using insert molding method.
2. Experimental and simulation procedures 2.1. Specimen preparation Commercially AZ31 and AZ91 magnesium alloys were used to prepare the Mg/Mg bimetal composites in this study and their chemical compositions are illustrated in Table 1. In order to fabricate the Mg/ Mg bimetal composites by the insert molding method, cylindrical inserts with 30 mm in diameter and 100 mm in height were machined from AZ31 magnesium ingots. Their surfaces were ground with silicon carbide papers up to 1500 grit before the experiments and then treated by alkaline cleaning, acid pickling and ultrasonically degreased with acetone. Similarly, AZ91 ingots were cut into pieces and pre-treated followed by the same procedure except for mechanical polishing. Then the selected AZ91 magnesium alloy ingots were melted in a steel crucible located in an electrical resistance furnace. During the melting process, a protective gas mixture containing 0.2% by volume SF6 and remainder CO2 were used for preventing oxides formation. Moreover, AZ91 melt was regularly stirred and the dross floating on the surface was removed.
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To achieve good metallurgical bonding between AZ31 insert and AZ91 matrix, different fabrication process parameters such as different volume ratio (V91:V31 = 1:1 or 2:1) and different insert temperature (650 °C, 675 °C, 700 °C) were tested in this study. Insert temperature indicates the stabilized AZ91 melt initial temperature when AZ31 rods were immersed into AZ91 melt. When the insert temperature is arrived, the AZ31 rods were immersed into AZ91 melt and kept at the AZ91 melt for 2 min. After that the entire assembly (containing mold, AZ31 insert and AZ91 melt) protected by a protective gas, was taken out of the furnace and cooled to room temperature. During the insert molding experiments, the temperature data was measured with a K-type thermocouple. All the information were displayed and recorded by a YOKOGAWA data acquisition system. The schematic sketch of fabricating Mg/Mg composite samples using the insert molding method is shown in Fig. 1(A). It should be noted that a customized graphitic cover was set on the top of the crucible when the insert molding experiment was carried out so that the AZ31 insert could be kept vertical and centered.
2.2. Interface characterization and mechanical tests To analyze the interface formation, the specimens were cut from the middle part of the samples perpendicular to the cylindrical insert with the thickness of 5 mm. The specimens for microstructure observation were successively ground using SiC papers, polished using diamond suspensions and MgO particles solution, and etched in a solution of HNO3 (4% in volume fraction) and alcohol. The microstructures of the specimens were examined using a Carl. ZEISS Axio Imager A2m and a ZEISS EVO-18 scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The elemental distributions were analyzed by JEOL JXA-8230 electron probe micro-analyzer (EPMA). To test the mechanical performance, AZ31/AZ91 bimetal composite specimens were taken from the middle part of the cylindrical samples, and then cut into cylindrical tensile specimens according to the ASTM E8/E8M Standard. Tensile tests were carried out on a DDL universal material testing machine at a strain rate of 5.0 × 10−3 s−1 at an ambient temperature. For comparison, parent metals (AZ31 and AZ91) were also tested at the same condition. Since no ASTM standard exists for these joint materials, a self-defined test was carried out to investigate the shear strengths of the joint interface. The size of tested specimen was shown in Fig 1(B). A universal testing machine was used to conduct this shear experiment by setting the joint sample in a special clamping
Fig. 1. The schematic sketch of the experimental set-up preparing Mg/Mg bimetal composites using insert molding method (A); the size of the specimens for shear strength test (B); the experimental set-up of shear strength tests (C).
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Fig. 2. Optical micrographs of the interfaces formed in specimens prepared by different casting temperatures: (A/B) 650 °C, (C/D) 675 °C, (E/F) 700 °C and different volume ratios V91:V31 = 1:1 (A, C, E), V91:V31 = 2:1 (B, D, F).
fixture shown in Fig. 1(C). The initial strain rate was 5.0 × 10−3 s−1 and the tests were done at room temperature. The average shear strength (σ) was calculated using the following equation [23]:
The tests were performed using a nano-indenter under the indentation load of 50 g for 10 s. 2.3. Numerical simulation for the interface solidification
σ¼
F max ld
ð1Þ
where Fmax, l and d is the maximum load, the width of the samples and their length, respectively. To ensure the repeatability, at least three samples were tested at each testing condition. The Vickers hardness across the interface regions were also measured by the micro-hardness tester.
Table 2 Experimental process parameters and interface bonding results in this study. Joint no.
Casting temperature (°C)
Volume ratio (V91:V31)
A B C D E F
650 650 675 675 700 700
1:1 2:1 1:1 2:1 1:1 2:1
Bonding (yes or no or partly) No No Partly Yes Yes Yes
To explain the interface formation mechanism, it is necessary to investigate the solidification behavior of the alloys at the interface. For the AZ31 and AZ91 alloy, the solidus temperature and liquidus temper), 470 °C (TAZ91 ) and 630 °C (TAZ31 ), 595 °C ature are 605 °C (TAZ31 S S L AZ91 ), respectively [24]. The calculation model used in this simulation (TL is as same as the insert molding setup shown in Fig. 1(A). Here the different insert temperature and the different melt/insert volume ratio can be simulated to explain the influence of process parameters on the interface bonding state using ANSYS software. In the numerical simulation, only temperature field was calculated without considering fluid flow and mechanical deformation. 3. Results 3.1. Interface formation The optical micrograph images at the interface of AZ31/AZ91 bimetal composites fabricated at different insert temperature and different volume ratio are illustrated in Fig. 2. The corresponding experimental parameters and interface bonding results are also presented in Table 2. From Fig. 2 and Table 2, it can be seen that no interface bonding
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Fig. 3. SEM image of the interfaces formed in specimens prepared at a temperature of 675 °C and volume ration of (V91:V31) = 2:1.
can be observed when the insert temperature is below 650 °C. Moreover, a clear interface separation between AZ31 magnesium alloy insert and AZ91 magnesium alloy melt can be observed due to the lower insert temperature. When the insert temperature is larger than 700 °C, the AZ31/AZ91 composites interface could get faint since more areas are melted on the insert surface. Besides the insert temperature, volume ratio was also found to influence the interface bonding. Only little metallic bonding points could be seen when the volume ratio (VAZ91:VAZ31) is 1:1 (T = 675 °C). However, when the volume ratio (VAZ91:VAZ31) is 2:1, the good metallurgical interface can be formed at 675 °C. One may attribute this difference, caused by the different volume ratio, to the difference of contact and solidification time between the AZ91 melt and AZ31 solid insert. To further characterize the interface bonding behavior, SEM analysis was also carried out by the authors in this study. Fig. 3 exhibits the corresponding SEM micrographs of the local bonding interface of AZ31/ AZ91 bimetal composites prepared at the insert temperature of 675 °C and volume ratio of 2:1. It is clearly shown that a good metallurgical bonding between AZ31 and AZ91 alloys has been achieved and a continuous interface transition zone between AZ31 and AZ91 alloys can be observed. However, no intermetallic compounds can be formed at the transition zone of the interface, which is in agreement with the other studies from the same type of metal bonding [10,12,14]. If analyzing the phases marked using number 1, 2, and 3 in Fig. 3, one can conclude that β-Al17Mg12, α-Mg and Mn-rich phases can be formed at the side of AZ91 alloy (Table 3), which is much different from the deformed microstructures at the side of AZ31 alloy. The samples were also analyzed using EDS through line scan spectrum (Fig. 4) and area scan spectrum (Fig. 5). As shown in Fig. 4, the element Mg tends to decrease at the interface of the composites when approaching AZ91 side. Moreover, at the interface between AZ31 insert and AZ91 matrix, the elements such as Al and Zn vary obviously, exhibiting the increasing tendency when approaching AZ91 side. One also notices that the variation tendencies of the elements such as Mg, Al and Zn are consistent with the chemical composition analysis of the bonding alloy components shown in Table 1. The similar results are also shown in Fig. 5. Therefore, it can be speculated that the diffusion of Al and Zn elements maybe play an important role during the interface bonding of AZ31/AZ91 bimetal composites, which will be discussed later.
3.2. Mechanical behaviors The mechanical behaviors of the AZ31/AZ91 bimetal composites were evaluated by measuring their microhardness, tensile strength and shear strength. From the microhardness measurements (HV0.05), a continuous hardness alteration zone between the AZ91 matrix and the insert AZ31 can be observed as shown in Fig. 6. At AZ31 side, the hardness values are initially constant (between HV63 to 69), and then continuously increase at the interface due to the increased supersaturation of the aluminum atoms when approaching AZ91 side. Finally, within a few tens of micrometers, the hardness is changed to be the value of the as-cast AZ91 alloy (between HV72 to 81). This gradient change indicates the existence of “fuse zone”, i.e. interface transition zone, which is about 100 μm in width.
Table 3 Results of spot concentration analysis corresponding to the points indicated in Fig. 3. Position
Mg (at.%)
Al (at.%)
Zn (at.%)
Mn (at.%)
Phases
1 2 3
61.94 97.19 6.97
36.45 2.81 53.61
1.60 – –
– – 39.42
β-Mg17Al12 α-Mg Mn rich
Fig. 4. Line scan spectrum of the interface of the sample prepared at a temperature of 675 °C and volume ration of (V91:V31) = 2:1.
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Fig. 7(A) illustrates the tensile strength value for the samples fabricated at 675 °C and 700 °C with different volume ratio of 1: 1 and 2:1, respectively. From Fig. 7 (A), the lowest tensile strength (41 MPa) can be attained for the samples fabricated at 675 °C with a volume ratio of 1:1 due to the incomplete metallurgical bonding between AZ31 insert and AZ91 melt. Moreover, compared with the samples fabricated at 700 °C, it seems that the highest tensile strength value (98 MPa) can be achieved for the samples fabricated at 675 °C with a volume ratio of 2:1, indicating the good metallurgical bonding at this process parameter and relatively finer grain size of AZ91 alloy due to lower insert temperature. The corresponding tensile stress–strain curve for the samples fabricated at 675 °C with a volume ratio of 2:1 was shown in Fig. 7(B). As can be seen in Fig. 7(B), besides the high tensile strength of 98 MPa, the true strain is also approaching 1.3% for the AZ31/AZ91 bimetal composites. For comparison, the tensile strengths and stress–strain curves of two parent metals, i.e., AZ31 and AZ91 alloys were also exhibited in Fig. 7(C) and (D), respectively. Compared with Fig. 7(B) and (C–D), it can be noticed that the tensile properties of the current AZ31/AZ91 bimetal composites are comparable to the weak AZ91 as-cast parent metal, which is obviously lower than that of AZ31 wrought alloys. The shear strength is another important indicator to evaluate the bonding quality at the bonding interface. Fig. 8 exhibits the shear strength values and the corresponding load–displacement curves for the samples fabricated at 675 °C with volume ratio of 2:1 and parent metals such as AZ31 and AZ91. It can also be seen that the current AZ31/AZ91 bimetal composites can still attain the higher shear strength value although it is not superior to the two parent metals. Fig. 9 exhibits the typical fracture cross section view and the fracture surface morphology for AZ31/AZ91 samples fabricated at 675 °C with a volume ratio of 2:1. From Fig. 9 (A), it indicates that indeed, the fracture can occur at the side of AZ91 alloy rather than at the side of AZ31 alloy, which has been proved by Fig. 7 and Fig. 8. It is well known that AZ31 alloy is the wrought alloys with the relatively higher tensile strength than as-cast AZ91 alloy and the fracture can easily occur at the position with the weak tensile strength. Since AZ91 alloy is typical as-cast microstructure with a small amount of pores and unexpected casting defects (Fig. 9(C)), which make it become the typical weak zone and the fracture is easily induced at the side of AZ91 alloys. From Fig. 9(B) and the
Fig. 6. The measured microhardness measurements (HV0.05) for the sample prepared at a temperature of 675 °C and volume ration of (V91:V31) = 2:1.
locally magnified figure shown at Fig. 9(C) and (D), it is also seen that the fracture is typical cleavage fracture for this AZ31/AZ91 bimetal composite, further indicating that the fracture indeed occurred at the side of AZ91 as-cast alloys. It indicates that, in order to improve the integrated performance of Mg/Mg bimetal composites, the casting defects should be avoided and the further grain refinement should be carried out by the addition of grain refiner or the proper heat treatment and deformation processes in the future. 4. Discussion Insert molding method can be used as a powerful and economical joining method to fabricate bimetal composites. During the insert molding experiments, it mainly includes three types of interface bonding state, surface layer of the insert melted and welded with the casting (type 1), only a mechanical joint established (type 2), or insert fully
Fig. 5. Area scan spectrum of the interface of the sample prepared at a temperature of 675 °C and volume ration of (V91:V31) = 2:1.
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Fig. 7. (A) Tensile strengths for the samples fabricated at 675 °C and 700 °C with different volume ratio of 1: 1and 2:1, respectively; (B) the tensile stress–strain curves for the samples fabricated at 675 °C with a volume ratio of 2:1; (C) Tensile strengths for parent metals such as AZ31 and AZ91 alloys; (D) load–displacement curves for the parent metals such as AZ31 and AZ91 alloys.
melt and lost its shape (type 3) [25]. Fig. 10 shows the temperature history of the melt and the insert (top figures) and the solidification progress of the interface melt (bottom figures). In Fig. 10 (A), the melt maintains the liquid state for some time. Metallurgical bonding is possible with this type of solidification where the interface melt is in contact with the high temperature insert for a sufficient duration (type 1).
However, if the contact is longer, further melting of the insert may occur. If the time exceeds a certain period or keeps at very high temperature, the insert may melt and lose its thickness or shape (type 3). In the case of Fig. 10 (B), the alloy melt solidifies rapidly at the interface and does not melt again, making it impossible to achieve metallurgical bonding (type 2). From this study, it can be found that two major
Fig. 8. (A) Shear strength for the samples fabricated at 675 °C with volume ratio of 2:1 and parent metals such as AZ31 and AZ91 alloys; (B) typical load–displacement curves obtained from the shear tests of AZ91, AZ31 and AZ31/AZ91 bimetal composites.
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Fig. 9. (A) The typical fracture cross section view for AZ31/AZ91 samples fabricated at 675 °C with a volume ratio of 2:1; (B) the typical fracture surface with the lower magnification times; (C–D) the typical fracture surface with the higher magnification times (C: casting pores; D: cleavage fracture surface).
factors, which can determine the progress of solidification, are the volume ratio between the melt and insert and the insert temperature. Here the actual temperature measurement and numerical simulation calculation have been carried out to estimate the interface bonding state. The corresponding point for the temperature measurement and simulation calculation at the interface has also been shown in Fig. 1. Fig. 11(A) and (B) exhibits the simulated results of the interface temperature at three insert temperatures (650 °C, 675 °C, 700 °C) with a volume radio 2:1 and 1:1, respectively. From Fig. 11, it can be seen that when insert temperature and volume ratio are 650 °C and 2:1 or
675 °C, 650 °C and 1:1, the interface temperature is obviously below 605 °C during the insert molding experiments. As described previously, the parameter that determines the bonding state is the solidus temperature of insert. When the contact temperature is lower than solidus = 605 °C), a metallurgical bonding is imtemperature of AZ31 (TAZ31 S possible (type 2). However, when the insert temperature and volume ratio are 675 °C, 700 °C and 2:1 or 700 °C and 1:1, the interface temperature is obviously larger than 605 °C. Thus it is possible to attain the good interfacial metallurgical bonding if keeping that temperature for an optimized period. Moreover, considering the grain coarsening at
Fig. 10. Three types of temperature and solidification progress at the interface during insert molding experiments: (A) slow solidification; (B) direct solidification; (C) insert melt totally.
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Fig. 11. The simulated results of three insert temperatures (650 °C, 675 °C, 700 °C) with a volume radio 2:1 (A) and 1:1 (B), respectively.
the higher insert temperature such as 700 °C and control complexity, 675 °C and 2:1 seems to be the optimized process parameters in this study. Meanwhile, it was found that the contact period for AZ31 insert and AZ91 melt to achieve alloying and metallurgical bonding is 90– 120 s. With the contact time is 90–120 s, partial to 80% or better of bonding with partial melting of the insert (type 1) can be expected, depending on the surface conditions of the insert. However, if the insert temperature is larger than 700 °C or the contact period exceeds 300 s, the AZ31 insert can melt and lost its shape. In addition to the numerical simulation, we also measured the temperature variation near the interface. Fig. 12 exhibits the experimental temperatures data acquired by Data Acquisition System at AZ31 side adjacent to interface. As a comparison, the numerical simulation data for the same point was also given in Fig. 12. It can be seen from Fig. 12 that when the AZ31 insert was immersed into AZ91 melt, the temperature on the insert surface would increase rapidly and exceed the solidus temperature of AZ31 alloys in 52 s (point a). That is, the AZ31 insert surface could partially be melted, which can also be proved from the decrease of heating rate in the temperature curve due to the existence of endothermic reaction. Then the temperature experienced a slow increase until reaching the maximum value of 616 °C in 167 s (point m). After that since the whole assembly would be removed out of the
Fig. 12. The experimental results of temperatures measured at 675 °C at the interface (a, b: = 605 °C in theory; c: Mg–Al eutectic composition = 437 °C in theory; m: the maxTAZ31 S imum temperature value).
furnace, the melted AZ31 surface would be solidified with the decrease of temperature. When the temperature was 609 °C (point b), the solidification of AZ31 surface basically tends to terminate if not considering the mixing of AZ31 and AZ91 melt. With the temperature is continuously decreased to be the liquidus temperature of AZ91 alloy, the AZ91 melt can start up its solidification progress (point c) until finally the nonequilibrium solidification process ends (point d). Fig. 13 (A) and (B) exhibits the schematic diagram of the interface solidification of AZ31/AZ91 bimetal composites at 616 °C and 599 °C, respectively. As shown in Fig. 13 (A), it can be seen that with the solidification of the AZ31 melting surface proceeds, solid solution of Mg can be formed with solute elements Al and Zn rejected into the remaining liquid. Similarly, the solidification of AZ91alloy also began when the temperature is close to 599 °C with the formation of primary dendrites. As the dendrites become thick, the solidification temperature is reduced constantly. The elements Al and Zn were also rejected into the Mg matrix melt and the eutectic solidification occurred (β-Al17 Mg12 , α-Mg) when its composition reached XE (Mg-Al eutectic composition point) from Mg-Al phase diagram [26]. The phases formed from the eutectic solidification of AZ91 melt have also been proved in Fig. 3. From Fig. 12, it was also seen that the melting and solidification process lasted for nearly 1000s. Therefore, it should be noted that the elements Al and Zn had sufficient time to be diffused because the liquid phase can act as diffusion channels at the interface, which also explained the composition variation of Al and Zn elements when approaching AZ91 side as shown in Fig. 4. That is, from the above truth it seems that the elements diffusion can play a crucial role in achieving interface metallurgical bonding during the insert experiments. Besides the above-mentioned proper temperature condition, another important factor to form a good metallurgical bonding is to eliminate the formation of the Mg oxides during the insert experiments if considering the wetting behavior between the AZ31 insert and AZ91 melt. By precisely controlling the process parameters such as the insert temperature and volume radio, it is possible to alleviate the oxidation on the interface bonding of two different magnesium alloys. Fig. 14 exhibits the concentration maps of element O at the interface for the unbonding samples (675 °C, 1:1, mechanical joint) and the bonding samples (675 °C, 2:1, metallurgical joint), respectively. It is very clear that there exists an obvious interface separation between the two magnesium alloys for the unbonding samples. Moreover, element O tends to be aggregated along the interface (Fig. 14A), indicating the formation of Mg oxides. However, for the bonding samples with the optimized process parameters, the surface layer of AZ31 insert could be melted and so the existed oxides thin layer on the surface could enter into the Mg melt or aggregate on the surface of Mg melt with the stirring of AZ31 insert.
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Fig. 13. Solidification process at the interface of AZ31/AZ91 bimetal composites for two critical temperatures: (A) 616 °C, (B) 599 °C.
Therefore, element O could distribute uniformly at the bonding samples, which has been shown in Fig. 14(B). Therefore, by this study it can be proved that insert molding is an effective method to alleviate the oxidation formation, which is very beneficial for attaining good metallurgical interface bonding during the fabrication of bimetal composites.
5. Conclusions In this study, the AZ31/AZ91bimetal composites have been prepared by inserting AZ31 alloy rod into AZ91magnesium alloy melt. Based on the observation and analysis in this study, it can be summarized as follows:
(1) High-quality Mg/Mg bimetal composites with good metallurgical bonding have been successfully prepared without the formation of the distinct intermetallic compounds and oxides at the interface by optimizing the process parameters such as insert temperature and volume ratio. (2) The Mg/Mg bonding process via insert molding is considered to be a rapid solidification process since high diffusion rates of the elements such as Al and Zn in liquid phase of the interface. Therefore, the interface structure can be tailored by the variations of process parameters. (3) The Mg/Mg bimetal composites fabricated in this study also exhibit good tensile properties, i.e., tensile strength of 98 MPa and tensile strain of 1.3%. Moreover, the fracture could occur at the side of as-cast AZ91 alloys, which can provide the further
Fig. 14. SEM micrographs of the interface region along with the corresponding concentration maps of element O for unbonding (A, only mechanical joint) and bonding samples (B, metallurgical bonding).
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optimization room for the improvement of mechanical performance in the future. (4) Current insert molding method provides a new insight for the fabrication of Mg/Mg bimetal composites.
Acknowledgements This research was supported by Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, Specialized Research Fund for the Doctoral Program of Higher Education (No. 20120006110019) and the Opening Research Fund of State Key Laboratory for Advanced Metals and Materials (No. 2012Z-13, 2014ZD-02). References [1] L.J. Polmear, Magnesium alloys and applications, Mater. Sci. Tech. Lond. 10 (1994) 1–16. [2] B.L. Mordike, T. Ebert, Magnesium: properties — applications — potential, Mater. Sci. Eng. A 302 (2001) 37–45. [3] S. Shen, S. Cai, G. Xu, Y. Li, T. Zhang, M. Zhang, Bond strength and corrosion resistance of bioglass coated magnesium alloy fabricated by uniaxial pressing and microwave hybrid heating, Mater. Des. 86 (2015) 610–615. [4] C. Blawert, N. Hort, K. Kainer, Automotive applications of magnesium and its alloys, Trans. Indian Inst. Metals 57 (2004) 397–408. [5] A.A. Luo, Magnesium casting technology for structural applications, J. Magnes. Alloy 1 (2013) 2–22. [6] M.K. Kulekci, Magnesium and its alloys applications in automotive industry, Int. J. Adv. Manuf. Technol. 39 (2007) 851–865. [7] H. Asgari, A.G. Odeshi, J.A. Szpunar, On dynamic deformation behavior of WE43 magnesium alloy sheet under shock loading conditions, Mater. Des. 63 (2014) 552–564. [8] X.R. Li, W. Liang, X.G. Zhao, Y. Zhang, X.P. Fu, F.C. Liu, Bonding of Mg and Al with Mg– Al eutectic alloy and its application in aluminum coating on magnesium, J. Alloys Compd. 471 (2009) 408–411. [9] M. Strzelecka, J. Iwaszko, M. Malik, S. Tomczyński, Surface modification of the AZ91 magnesium alloy, Archives of Civil and Mechanical Engineering (2015). [10] T. Liu, Q. Wang, Y. Sui, Q. Wang, W. Ding, An investigation into aluminum– aluminum bimetal fabrication by squeeze casting, Mater. Des. 68 (2015) 8–17.
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[11] J. Volder, An overview of a number of casting processes, Aluminum 4 (1993) 11–16. [12] K.J.M. Papis, B. Hallstedt, J.F. Löffler, P.J. Uggowitzer, Interface formation in aluminium–aluminium compound casting, Acta Mater. 56 (2008) 3036–3043. [13] K.J.M. Papis, J.F. Loeffler, P.J. Uggowitzer, Light metal compound casting, Science in China Series E: Technological Sciences 52 (2009) 46–51. [14] K.J.M. Papis, J.F. Löffler, P.J. Uggowitzer, Interface formation between liquid and solid Mg alloys—an approach to continuously metallurgic joining of magnesium parts, Mater. Sci. Eng. A 527 (2010) 2274–2279. [15] E. Hajjari, M. Divandari, S.H. Razavi, T. Homma, S. Kamado, Intermetallic compounds and antiphase domains in Al/Mg compound casting, Intermetallics 23 (2012) 182–186. [16] E. Hajjari, M. Divandari, S.H. Razavi, S.M. Emami, T. Homma, S. Kamado, Dissimilar joining of Al/Mg light metals by compound casting process, J. Mater. Sci. 46 (2011) 6491–6499. [17] X.Y. Song, J.B. Sun, D.S. Zhong, Y.S. Yu, T.M. Wang, Z.Q. Cao, et al., Study on Al/Mg compound materials by solid–liquid bonding method, Mater. Res. Innov. 16 (2012) 51–55. [18] C.Y. Liu, Q. Wang, Y.Z. Jia, R. Jing, B. Zhang, M.Z. Ma, et al., Microstructures and mechanical properties of Mg/Mg and Mg/Al/Mg laminated composites prepared via warm roll bonding, Mater. Sci. Eng. A 556 (2012) 1–8. [19] O. Dezellus, L. Milani, F. Bosselet, M. Sacerdote-Peronnet, D. Rouby, J.-C. Viala, Mechanical testing of titanium/aluminium–silicon interfaces by push-out, J. Mater. Sci. 43 (2008) 1749–1756. [20] X.Y. Nie, J.C. Liu, H.X. Li, Q. Du, J.S. Zhang, L.Z. Zhuang, An investigation on bonding mechanism and mechanical properties of Al/Ti compound materials prepared by insert moulding, Mater. Des. 63 (2014) 142–150. [21] X.Y. Nie, K.N. Zhao, H.X. Li, Q. Du, J.S. Zhang, L.Z. Zhuang, Comparisons of interface microstructure and mechanical behavior between Ti/Al and Ti-6Al-4 V/Al bimetallic composites, China Foundry 12 (2015) 1–8. [22] J.C. Liu, J. Hu, X.Y. Nie, H.X. Li, Q. Du, J.S. Zhang, et al., The interface bonding mechanism and related mechanical properties of Mg/Al compound materials fabricated by insert molding, Mater. Sci. Eng. A 635 (2015) 70–76. [23] S.M. Emami, M. Divandari, H. Arabi, E. Hajjari, Effect of melt-to-solid insert volume ratio on Mg/Al dissimilar metals bonding, J. Mater. Eng. Perform. 22 (2012) 123–130. [24] H. Baker, Nonferrous alloys and special-purpose materials, ASM Handbook, 2, 1993. [25] T. Noguchi, N. Horikawa, H. Nagate, T. Nakamura, K. Sato, Application of flow and solidification simulation in cast-in insertion processing, International Journal of Cast Metals Research 18 (2005) 214–220. [26] H. Baker, Alloy phase diagrams, ASM Handbook, 3 1992, p. 48.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Interface formation in magnesium–magnesium bimetal composites fabricated by insert molding method K.N. Zhao a, J.C. Liu a, X.Y. Nie a, Y. Li a, H.X. Li a,⁎, Q. Du b, L.Z. Zhuang a, J.S. Zhang a a b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China SINTEF Materials and Chemistry, Trondheim, Norway
a r t i c l e
i n f o
Article history: Received 5 October 2015 Received in revised form 24 November 2015 Accepted 25 November 2015 Available online 1 December 2015 Keywords: Interface bonding mechanism Liquid–solid bonding Diffusion Insert molding method Interface mechanical behavior
a b s t r a c t Magnesium–magnesium bimetal composites were prepared by insert molding method in this study. To achieve good metallurgical bonding between AZ31 insert and AZ91 matrix, different fabrication process parameters such as volume ratio (V91:V31 = 1:1 or 2:1) and insert temperature (650 °C, 675 °C, 700 °C), were tested. It was shown that high-quality metallurgical interface bonding could be achieved when the volume ratio (VAZ91:VAZ31) is 2:1 and the insert temperature is 675 °C. The average interface tensile strength of the bimetal fabricated by insert molding could approach 100 MPa, which is close to the reported as-cast AZ91 tensile strength value. The fracture analysis also indicated that during the tensile test of bimetal composite, the crack initiation began in the weak zone of casting AZ91 alloy. Moreover, the bonding mechanism of the interface were also investigated in details combined the temperature measurement experiments and the corresponding numerical simulation work in this study. The results presented in this paper indicated that the insert molding method is a promising and effective approach to develop advanced Mg/Mg bimetal composites for widening the application of magnesium alloys. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction Magnesium alloy is an important engineering light metallic material due to its high specific strength, high specific stiffness, better casting properties and good machining properties [1–3]. The research and application of Mg alloys have been extended from navigation and military fields to those civil products with high additional value such as automobile, computer and communication equipments [4–7]. In many cases, one single light metal material does not satisfy all the requirements of high performance at a low cost [8–10]. Therefore, the bimetal composites such as Mg/Al, Al/Al and Mg/Mg are attracting more and more attention in recent years due to the combination of their outstanding properties from the bonding metal components [11–17]. Compared with Al/Al and Mg/Al composites, the study on Mg/Mg bimetal composites are insufficient and so far only few papers could be found. Papis et al. [14] fabricated Mg/Mg bimetal composites in a laboratory-scale by dropping magnesium melt (pure Mg or AJ62) onto a solid magnesium alloy substrate (AZ31). Defect-free interfaces could be attained by operation in an Ar atmosphere. However, in this joining process the Zn pre-treatment process on the solid substrate must be ⁎ Corresponding author. E-mail address: [email protected] (H.X. Li).
http://dx.doi.org/10.1016/j.matdes.2015.11.095 0264-1275/© 2015 Elsevier Ltd. All rights reserved.
applied before realizing bimetallic bonding in order to alleviate the unbeneficial influence on interface bonding caused from the oxides on the surface of magnesium alloys. As a result, the complexity of fabrication process and production cost were undoubtedly increased. Moreover, the bonding mechanism of the joint interface was not well investigated and the mechanical properties of the joint interface were unknown. In another paper, Liu et al. [18] reported the fabrication of AZ31/AZ31 laminated metal composites via warm roll bonding. Though a relatively higher tensile strength could be attained for the AZ31/AZ31 laminated metal composites compared with that of single AZ31 Mg alloy, they didn't investigate the bonding mechanism between AZ31/ AZ31 alloys. Moreover, during the warm roll bonding the oxides on the surface of magnesium alloys would influence the bonding strength, which had not been discussed. Recently, a new compound casting method, named insert molding method, has been utilized to fabricate Ti/Al and Mg/Al bimetal composites [19–22]. This method exhibits the excellent industrial application prospect due to its outstanding advantages such as low production cost, low energy consumption, simple production procedure, high interface bonding strength and protection from the oxidation on the surface of the alloys. Nie et al. [20,21] and Dezellus et al. [19] have fabricated Ti/ Al bimetal composites with good metallurgical bonding using this method and the average shear strength of the interface layer is much
K.N. Zhao et al. / Materials and Design 91 (2016) 122–131 Table 1 Chemical compositions (wt.%) of the AZ31 and AZ91 alloys used in this study. Material
Al
Si
Fe
Cu
Mn
Mg
Zn
AZ31 AZ91
3.06 9.1
0.016 0.09
0.002 0.003
0.002 0.002
0.47 0.15
Bal. Bal
0.7 1.18
higher than that of the aluminum matrix. Liu et al. [22] also fabricated Mg/Al bimetal composites and the good interface mechanical behavior could be achieved. The natural oxide layer on the surface of the substrate can be alleviated and the continuous metallic bonding interface can be formed. However, fabricating Mg/Mg bimetal composites using the insert molding method has not been reported so far though it can be considered as a new insight for the fabrication of Mg/Mg bimetal composites. In this paper, the above-mentioned corresponding study will be carried out. Moreover, the influence of different process parameters on the bonding characterization and mechanical behavior at the interface will also be investigated. The emphasis is on understanding the bonding mechanism of Mg/Mg bimetal interface using insert molding method.
2. Experimental and simulation procedures 2.1. Specimen preparation Commercially AZ31 and AZ91 magnesium alloys were used to prepare the Mg/Mg bimetal composites in this study and their chemical compositions are illustrated in Table 1. In order to fabricate the Mg/ Mg bimetal composites by the insert molding method, cylindrical inserts with 30 mm in diameter and 100 mm in height were machined from AZ31 magnesium ingots. Their surfaces were ground with silicon carbide papers up to 1500 grit before the experiments and then treated by alkaline cleaning, acid pickling and ultrasonically degreased with acetone. Similarly, AZ91 ingots were cut into pieces and pre-treated followed by the same procedure except for mechanical polishing. Then the selected AZ91 magnesium alloy ingots were melted in a steel crucible located in an electrical resistance furnace. During the melting process, a protective gas mixture containing 0.2% by volume SF6 and remainder CO2 were used for preventing oxides formation. Moreover, AZ91 melt was regularly stirred and the dross floating on the surface was removed.
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To achieve good metallurgical bonding between AZ31 insert and AZ91 matrix, different fabrication process parameters such as different volume ratio (V91:V31 = 1:1 or 2:1) and different insert temperature (650 °C, 675 °C, 700 °C) were tested in this study. Insert temperature indicates the stabilized AZ91 melt initial temperature when AZ31 rods were immersed into AZ91 melt. When the insert temperature is arrived, the AZ31 rods were immersed into AZ91 melt and kept at the AZ91 melt for 2 min. After that the entire assembly (containing mold, AZ31 insert and AZ91 melt) protected by a protective gas, was taken out of the furnace and cooled to room temperature. During the insert molding experiments, the temperature data was measured with a K-type thermocouple. All the information were displayed and recorded by a YOKOGAWA data acquisition system. The schematic sketch of fabricating Mg/Mg composite samples using the insert molding method is shown in Fig. 1(A). It should be noted that a customized graphitic cover was set on the top of the crucible when the insert molding experiment was carried out so that the AZ31 insert could be kept vertical and centered.
2.2. Interface characterization and mechanical tests To analyze the interface formation, the specimens were cut from the middle part of the samples perpendicular to the cylindrical insert with the thickness of 5 mm. The specimens for microstructure observation were successively ground using SiC papers, polished using diamond suspensions and MgO particles solution, and etched in a solution of HNO3 (4% in volume fraction) and alcohol. The microstructures of the specimens were examined using a Carl. ZEISS Axio Imager A2m and a ZEISS EVO-18 scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS). The elemental distributions were analyzed by JEOL JXA-8230 electron probe micro-analyzer (EPMA). To test the mechanical performance, AZ31/AZ91 bimetal composite specimens were taken from the middle part of the cylindrical samples, and then cut into cylindrical tensile specimens according to the ASTM E8/E8M Standard. Tensile tests were carried out on a DDL universal material testing machine at a strain rate of 5.0 × 10−3 s−1 at an ambient temperature. For comparison, parent metals (AZ31 and AZ91) were also tested at the same condition. Since no ASTM standard exists for these joint materials, a self-defined test was carried out to investigate the shear strengths of the joint interface. The size of tested specimen was shown in Fig 1(B). A universal testing machine was used to conduct this shear experiment by setting the joint sample in a special clamping
Fig. 1. The schematic sketch of the experimental set-up preparing Mg/Mg bimetal composites using insert molding method (A); the size of the specimens for shear strength test (B); the experimental set-up of shear strength tests (C).
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Fig. 2. Optical micrographs of the interfaces formed in specimens prepared by different casting temperatures: (A/B) 650 °C, (C/D) 675 °C, (E/F) 700 °C and different volume ratios V91:V31 = 1:1 (A, C, E), V91:V31 = 2:1 (B, D, F).
fixture shown in Fig. 1(C). The initial strain rate was 5.0 × 10−3 s−1 and the tests were done at room temperature. The average shear strength (σ) was calculated using the following equation [23]:
The tests were performed using a nano-indenter under the indentation load of 50 g for 10 s. 2.3. Numerical simulation for the interface solidification
σ¼
F max ld
ð1Þ
where Fmax, l and d is the maximum load, the width of the samples and their length, respectively. To ensure the repeatability, at least three samples were tested at each testing condition. The Vickers hardness across the interface regions were also measured by the micro-hardness tester.
Table 2 Experimental process parameters and interface bonding results in this study. Joint no.
Casting temperature (°C)
Volume ratio (V91:V31)
A B C D E F
650 650 675 675 700 700
1:1 2:1 1:1 2:1 1:1 2:1
Bonding (yes or no or partly) No No Partly Yes Yes Yes
To explain the interface formation mechanism, it is necessary to investigate the solidification behavior of the alloys at the interface. For the AZ31 and AZ91 alloy, the solidus temperature and liquidus temper), 470 °C (TAZ91 ) and 630 °C (TAZ31 ), 595 °C ature are 605 °C (TAZ31 S S L AZ91 ), respectively [24]. The calculation model used in this simulation (TL is as same as the insert molding setup shown in Fig. 1(A). Here the different insert temperature and the different melt/insert volume ratio can be simulated to explain the influence of process parameters on the interface bonding state using ANSYS software. In the numerical simulation, only temperature field was calculated without considering fluid flow and mechanical deformation. 3. Results 3.1. Interface formation The optical micrograph images at the interface of AZ31/AZ91 bimetal composites fabricated at different insert temperature and different volume ratio are illustrated in Fig. 2. The corresponding experimental parameters and interface bonding results are also presented in Table 2. From Fig. 2 and Table 2, it can be seen that no interface bonding
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Fig. 3. SEM image of the interfaces formed in specimens prepared at a temperature of 675 °C and volume ration of (V91:V31) = 2:1.
can be observed when the insert temperature is below 650 °C. Moreover, a clear interface separation between AZ31 magnesium alloy insert and AZ91 magnesium alloy melt can be observed due to the lower insert temperature. When the insert temperature is larger than 700 °C, the AZ31/AZ91 composites interface could get faint since more areas are melted on the insert surface. Besides the insert temperature, volume ratio was also found to influence the interface bonding. Only little metallic bonding points could be seen when the volume ratio (VAZ91:VAZ31) is 1:1 (T = 675 °C). However, when the volume ratio (VAZ91:VAZ31) is 2:1, the good metallurgical interface can be formed at 675 °C. One may attribute this difference, caused by the different volume ratio, to the difference of contact and solidification time between the AZ91 melt and AZ31 solid insert. To further characterize the interface bonding behavior, SEM analysis was also carried out by the authors in this study. Fig. 3 exhibits the corresponding SEM micrographs of the local bonding interface of AZ31/ AZ91 bimetal composites prepared at the insert temperature of 675 °C and volume ratio of 2:1. It is clearly shown that a good metallurgical bonding between AZ31 and AZ91 alloys has been achieved and a continuous interface transition zone between AZ31 and AZ91 alloys can be observed. However, no intermetallic compounds can be formed at the transition zone of the interface, which is in agreement with the other studies from the same type of metal bonding [10,12,14]. If analyzing the phases marked using number 1, 2, and 3 in Fig. 3, one can conclude that β-Al17Mg12, α-Mg and Mn-rich phases can be formed at the side of AZ91 alloy (Table 3), which is much different from the deformed microstructures at the side of AZ31 alloy. The samples were also analyzed using EDS through line scan spectrum (Fig. 4) and area scan spectrum (Fig. 5). As shown in Fig. 4, the element Mg tends to decrease at the interface of the composites when approaching AZ91 side. Moreover, at the interface between AZ31 insert and AZ91 matrix, the elements such as Al and Zn vary obviously, exhibiting the increasing tendency when approaching AZ91 side. One also notices that the variation tendencies of the elements such as Mg, Al and Zn are consistent with the chemical composition analysis of the bonding alloy components shown in Table 1. The similar results are also shown in Fig. 5. Therefore, it can be speculated that the diffusion of Al and Zn elements maybe play an important role during the interface bonding of AZ31/AZ91 bimetal composites, which will be discussed later.
3.2. Mechanical behaviors The mechanical behaviors of the AZ31/AZ91 bimetal composites were evaluated by measuring their microhardness, tensile strength and shear strength. From the microhardness measurements (HV0.05), a continuous hardness alteration zone between the AZ91 matrix and the insert AZ31 can be observed as shown in Fig. 6. At AZ31 side, the hardness values are initially constant (between HV63 to 69), and then continuously increase at the interface due to the increased supersaturation of the aluminum atoms when approaching AZ91 side. Finally, within a few tens of micrometers, the hardness is changed to be the value of the as-cast AZ91 alloy (between HV72 to 81). This gradient change indicates the existence of “fuse zone”, i.e. interface transition zone, which is about 100 μm in width.
Table 3 Results of spot concentration analysis corresponding to the points indicated in Fig. 3. Position
Mg (at.%)
Al (at.%)
Zn (at.%)
Mn (at.%)
Phases
1 2 3
61.94 97.19 6.97
36.45 2.81 53.61
1.60 – –
– – 39.42
β-Mg17Al12 α-Mg Mn rich
Fig. 4. Line scan spectrum of the interface of the sample prepared at a temperature of 675 °C and volume ration of (V91:V31) = 2:1.
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Fig. 7(A) illustrates the tensile strength value for the samples fabricated at 675 °C and 700 °C with different volume ratio of 1: 1 and 2:1, respectively. From Fig. 7 (A), the lowest tensile strength (41 MPa) can be attained for the samples fabricated at 675 °C with a volume ratio of 1:1 due to the incomplete metallurgical bonding between AZ31 insert and AZ91 melt. Moreover, compared with the samples fabricated at 700 °C, it seems that the highest tensile strength value (98 MPa) can be achieved for the samples fabricated at 675 °C with a volume ratio of 2:1, indicating the good metallurgical bonding at this process parameter and relatively finer grain size of AZ91 alloy due to lower insert temperature. The corresponding tensile stress–strain curve for the samples fabricated at 675 °C with a volume ratio of 2:1 was shown in Fig. 7(B). As can be seen in Fig. 7(B), besides the high tensile strength of 98 MPa, the true strain is also approaching 1.3% for the AZ31/AZ91 bimetal composites. For comparison, the tensile strengths and stress–strain curves of two parent metals, i.e., AZ31 and AZ91 alloys were also exhibited in Fig. 7(C) and (D), respectively. Compared with Fig. 7(B) and (C–D), it can be noticed that the tensile properties of the current AZ31/AZ91 bimetal composites are comparable to the weak AZ91 as-cast parent metal, which is obviously lower than that of AZ31 wrought alloys. The shear strength is another important indicator to evaluate the bonding quality at the bonding interface. Fig. 8 exhibits the shear strength values and the corresponding load–displacement curves for the samples fabricated at 675 °C with volume ratio of 2:1 and parent metals such as AZ31 and AZ91. It can also be seen that the current AZ31/AZ91 bimetal composites can still attain the higher shear strength value although it is not superior to the two parent metals. Fig. 9 exhibits the typical fracture cross section view and the fracture surface morphology for AZ31/AZ91 samples fabricated at 675 °C with a volume ratio of 2:1. From Fig. 9 (A), it indicates that indeed, the fracture can occur at the side of AZ91 alloy rather than at the side of AZ31 alloy, which has been proved by Fig. 7 and Fig. 8. It is well known that AZ31 alloy is the wrought alloys with the relatively higher tensile strength than as-cast AZ91 alloy and the fracture can easily occur at the position with the weak tensile strength. Since AZ91 alloy is typical as-cast microstructure with a small amount of pores and unexpected casting defects (Fig. 9(C)), which make it become the typical weak zone and the fracture is easily induced at the side of AZ91 alloys. From Fig. 9(B) and the
Fig. 6. The measured microhardness measurements (HV0.05) for the sample prepared at a temperature of 675 °C and volume ration of (V91:V31) = 2:1.
locally magnified figure shown at Fig. 9(C) and (D), it is also seen that the fracture is typical cleavage fracture for this AZ31/AZ91 bimetal composite, further indicating that the fracture indeed occurred at the side of AZ91 as-cast alloys. It indicates that, in order to improve the integrated performance of Mg/Mg bimetal composites, the casting defects should be avoided and the further grain refinement should be carried out by the addition of grain refiner or the proper heat treatment and deformation processes in the future. 4. Discussion Insert molding method can be used as a powerful and economical joining method to fabricate bimetal composites. During the insert molding experiments, it mainly includes three types of interface bonding state, surface layer of the insert melted and welded with the casting (type 1), only a mechanical joint established (type 2), or insert fully
Fig. 5. Area scan spectrum of the interface of the sample prepared at a temperature of 675 °C and volume ration of (V91:V31) = 2:1.
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Fig. 7. (A) Tensile strengths for the samples fabricated at 675 °C and 700 °C with different volume ratio of 1: 1and 2:1, respectively; (B) the tensile stress–strain curves for the samples fabricated at 675 °C with a volume ratio of 2:1; (C) Tensile strengths for parent metals such as AZ31 and AZ91 alloys; (D) load–displacement curves for the parent metals such as AZ31 and AZ91 alloys.
melt and lost its shape (type 3) [25]. Fig. 10 shows the temperature history of the melt and the insert (top figures) and the solidification progress of the interface melt (bottom figures). In Fig. 10 (A), the melt maintains the liquid state for some time. Metallurgical bonding is possible with this type of solidification where the interface melt is in contact with the high temperature insert for a sufficient duration (type 1).
However, if the contact is longer, further melting of the insert may occur. If the time exceeds a certain period or keeps at very high temperature, the insert may melt and lose its thickness or shape (type 3). In the case of Fig. 10 (B), the alloy melt solidifies rapidly at the interface and does not melt again, making it impossible to achieve metallurgical bonding (type 2). From this study, it can be found that two major
Fig. 8. (A) Shear strength for the samples fabricated at 675 °C with volume ratio of 2:1 and parent metals such as AZ31 and AZ91 alloys; (B) typical load–displacement curves obtained from the shear tests of AZ91, AZ31 and AZ31/AZ91 bimetal composites.
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Fig. 9. (A) The typical fracture cross section view for AZ31/AZ91 samples fabricated at 675 °C with a volume ratio of 2:1; (B) the typical fracture surface with the lower magnification times; (C–D) the typical fracture surface with the higher magnification times (C: casting pores; D: cleavage fracture surface).
factors, which can determine the progress of solidification, are the volume ratio between the melt and insert and the insert temperature. Here the actual temperature measurement and numerical simulation calculation have been carried out to estimate the interface bonding state. The corresponding point for the temperature measurement and simulation calculation at the interface has also been shown in Fig. 1. Fig. 11(A) and (B) exhibits the simulated results of the interface temperature at three insert temperatures (650 °C, 675 °C, 700 °C) with a volume radio 2:1 and 1:1, respectively. From Fig. 11, it can be seen that when insert temperature and volume ratio are 650 °C and 2:1 or
675 °C, 650 °C and 1:1, the interface temperature is obviously below 605 °C during the insert molding experiments. As described previously, the parameter that determines the bonding state is the solidus temperature of insert. When the contact temperature is lower than solidus = 605 °C), a metallurgical bonding is imtemperature of AZ31 (TAZ31 S possible (type 2). However, when the insert temperature and volume ratio are 675 °C, 700 °C and 2:1 or 700 °C and 1:1, the interface temperature is obviously larger than 605 °C. Thus it is possible to attain the good interfacial metallurgical bonding if keeping that temperature for an optimized period. Moreover, considering the grain coarsening at
Fig. 10. Three types of temperature and solidification progress at the interface during insert molding experiments: (A) slow solidification; (B) direct solidification; (C) insert melt totally.
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Fig. 11. The simulated results of three insert temperatures (650 °C, 675 °C, 700 °C) with a volume radio 2:1 (A) and 1:1 (B), respectively.
the higher insert temperature such as 700 °C and control complexity, 675 °C and 2:1 seems to be the optimized process parameters in this study. Meanwhile, it was found that the contact period for AZ31 insert and AZ91 melt to achieve alloying and metallurgical bonding is 90– 120 s. With the contact time is 90–120 s, partial to 80% or better of bonding with partial melting of the insert (type 1) can be expected, depending on the surface conditions of the insert. However, if the insert temperature is larger than 700 °C or the contact period exceeds 300 s, the AZ31 insert can melt and lost its shape. In addition to the numerical simulation, we also measured the temperature variation near the interface. Fig. 12 exhibits the experimental temperatures data acquired by Data Acquisition System at AZ31 side adjacent to interface. As a comparison, the numerical simulation data for the same point was also given in Fig. 12. It can be seen from Fig. 12 that when the AZ31 insert was immersed into AZ91 melt, the temperature on the insert surface would increase rapidly and exceed the solidus temperature of AZ31 alloys in 52 s (point a). That is, the AZ31 insert surface could partially be melted, which can also be proved from the decrease of heating rate in the temperature curve due to the existence of endothermic reaction. Then the temperature experienced a slow increase until reaching the maximum value of 616 °C in 167 s (point m). After that since the whole assembly would be removed out of the
Fig. 12. The experimental results of temperatures measured at 675 °C at the interface (a, b: = 605 °C in theory; c: Mg–Al eutectic composition = 437 °C in theory; m: the maxTAZ31 S imum temperature value).
furnace, the melted AZ31 surface would be solidified with the decrease of temperature. When the temperature was 609 °C (point b), the solidification of AZ31 surface basically tends to terminate if not considering the mixing of AZ31 and AZ91 melt. With the temperature is continuously decreased to be the liquidus temperature of AZ91 alloy, the AZ91 melt can start up its solidification progress (point c) until finally the nonequilibrium solidification process ends (point d). Fig. 13 (A) and (B) exhibits the schematic diagram of the interface solidification of AZ31/AZ91 bimetal composites at 616 °C and 599 °C, respectively. As shown in Fig. 13 (A), it can be seen that with the solidification of the AZ31 melting surface proceeds, solid solution of Mg can be formed with solute elements Al and Zn rejected into the remaining liquid. Similarly, the solidification of AZ91alloy also began when the temperature is close to 599 °C with the formation of primary dendrites. As the dendrites become thick, the solidification temperature is reduced constantly. The elements Al and Zn were also rejected into the Mg matrix melt and the eutectic solidification occurred (β-Al17 Mg12 , α-Mg) when its composition reached XE (Mg-Al eutectic composition point) from Mg-Al phase diagram [26]. The phases formed from the eutectic solidification of AZ91 melt have also been proved in Fig. 3. From Fig. 12, it was also seen that the melting and solidification process lasted for nearly 1000s. Therefore, it should be noted that the elements Al and Zn had sufficient time to be diffused because the liquid phase can act as diffusion channels at the interface, which also explained the composition variation of Al and Zn elements when approaching AZ91 side as shown in Fig. 4. That is, from the above truth it seems that the elements diffusion can play a crucial role in achieving interface metallurgical bonding during the insert experiments. Besides the above-mentioned proper temperature condition, another important factor to form a good metallurgical bonding is to eliminate the formation of the Mg oxides during the insert experiments if considering the wetting behavior between the AZ31 insert and AZ91 melt. By precisely controlling the process parameters such as the insert temperature and volume radio, it is possible to alleviate the oxidation on the interface bonding of two different magnesium alloys. Fig. 14 exhibits the concentration maps of element O at the interface for the unbonding samples (675 °C, 1:1, mechanical joint) and the bonding samples (675 °C, 2:1, metallurgical joint), respectively. It is very clear that there exists an obvious interface separation between the two magnesium alloys for the unbonding samples. Moreover, element O tends to be aggregated along the interface (Fig. 14A), indicating the formation of Mg oxides. However, for the bonding samples with the optimized process parameters, the surface layer of AZ31 insert could be melted and so the existed oxides thin layer on the surface could enter into the Mg melt or aggregate on the surface of Mg melt with the stirring of AZ31 insert.
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Fig. 13. Solidification process at the interface of AZ31/AZ91 bimetal composites for two critical temperatures: (A) 616 °C, (B) 599 °C.
Therefore, element O could distribute uniformly at the bonding samples, which has been shown in Fig. 14(B). Therefore, by this study it can be proved that insert molding is an effective method to alleviate the oxidation formation, which is very beneficial for attaining good metallurgical interface bonding during the fabrication of bimetal composites.
5. Conclusions In this study, the AZ31/AZ91bimetal composites have been prepared by inserting AZ31 alloy rod into AZ91magnesium alloy melt. Based on the observation and analysis in this study, it can be summarized as follows:
(1) High-quality Mg/Mg bimetal composites with good metallurgical bonding have been successfully prepared without the formation of the distinct intermetallic compounds and oxides at the interface by optimizing the process parameters such as insert temperature and volume ratio. (2) The Mg/Mg bonding process via insert molding is considered to be a rapid solidification process since high diffusion rates of the elements such as Al and Zn in liquid phase of the interface. Therefore, the interface structure can be tailored by the variations of process parameters. (3) The Mg/Mg bimetal composites fabricated in this study also exhibit good tensile properties, i.e., tensile strength of 98 MPa and tensile strain of 1.3%. Moreover, the fracture could occur at the side of as-cast AZ91 alloys, which can provide the further
Fig. 14. SEM micrographs of the interface region along with the corresponding concentration maps of element O for unbonding (A, only mechanical joint) and bonding samples (B, metallurgical bonding).
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optimization room for the improvement of mechanical performance in the future. (4) Current insert molding method provides a new insight for the fabrication of Mg/Mg bimetal composites.
Acknowledgements This research was supported by Beijing Laboratory of Metallic Materials and Processing for Modern Transportation, Specialized Research Fund for the Doctoral Program of Higher Education (No. 20120006110019) and the Opening Research Fund of State Key Laboratory for Advanced Metals and Materials (No. 2012Z-13, 2014ZD-02). References [1] L.J. Polmear, Magnesium alloys and applications, Mater. Sci. Tech. Lond. 10 (1994) 1–16. [2] B.L. Mordike, T. Ebert, Magnesium: properties — applications — potential, Mater. Sci. Eng. A 302 (2001) 37–45. [3] S. Shen, S. Cai, G. Xu, Y. Li, T. Zhang, M. Zhang, Bond strength and corrosion resistance of bioglass coated magnesium alloy fabricated by uniaxial pressing and microwave hybrid heating, Mater. Des. 86 (2015) 610–615. [4] C. Blawert, N. Hort, K. Kainer, Automotive applications of magnesium and its alloys, Trans. Indian Inst. Metals 57 (2004) 397–408. [5] A.A. Luo, Magnesium casting technology for structural applications, J. Magnes. Alloy 1 (2013) 2–22. [6] M.K. Kulekci, Magnesium and its alloys applications in automotive industry, Int. J. Adv. Manuf. Technol. 39 (2007) 851–865. [7] H. Asgari, A.G. Odeshi, J.A. Szpunar, On dynamic deformation behavior of WE43 magnesium alloy sheet under shock loading conditions, Mater. Des. 63 (2014) 552–564. [8] X.R. Li, W. Liang, X.G. Zhao, Y. Zhang, X.P. Fu, F.C. Liu, Bonding of Mg and Al with Mg– Al eutectic alloy and its application in aluminum coating on magnesium, J. Alloys Compd. 471 (2009) 408–411. [9] M. Strzelecka, J. Iwaszko, M. Malik, S. Tomczyński, Surface modification of the AZ91 magnesium alloy, Archives of Civil and Mechanical Engineering (2015). [10] T. Liu, Q. Wang, Y. Sui, Q. Wang, W. Ding, An investigation into aluminum– aluminum bimetal fabrication by squeeze casting, Mater. Des. 68 (2015) 8–17.
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