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steel bimetallic composites
Accepted Manuscript Effects of zinc coating on interfacial microstructures and mechanical properties of aluminum/steel bimetallic composites Wenming Jiang, Zitian Fan, Guangyu Li, Chi Li PII:
S0925-8388(16)30931-8
DOI:
10.1016/j.jallcom.2016.03.276
Reference:
JALCOM 37164
To appear in:
Journal of Alloys and Compounds
Received Date: 20 January 2016 Revised Date:
28 March 2016
Accepted Date: 31 March 2016
Please cite this article as: W. Jiang, Z. Fan, G. Li, C. Li, Effects of zinc coating on interfacial microstructures and mechanical properties of aluminum/steel bimetallic composites, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.03.276. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effects of zinc coating on interfacial microstructures and
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mechanical properties of aluminum/steel bimetallic composites
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Wenming Jiang *, Zitian Fan *, Guangyu Li, Chi Li State Key Lab of Materials Processing and Die & Mould Technology, Huazhong
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University of Science and Technology, Wuhan 430074, PR China
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ABSTRACT
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In the present work, the effects of zinc coating on interfacial microstructures and
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mechanical properties of the aluminum/steel bimetallic composites prepared by a
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solid-liquid diffusion method were investigated, and the formation mechanism of
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intermetallic compounds at the interface between the aluminum and the steel were
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also discussed. The results show that a relatively uniform and compact interface
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between the aluminum and the steel was formed with the application of the zinc
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coating. The reaction layer that was mainly composed of τ6-Al4.5FeSi, α-Al rich, α +
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η eutectoid, η-Zn and eutectic silicon phases, between the aluminum and the steel,
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had an average thickness of approximately 650 µm. The microhardnesses at the
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interface between the aluminum and the steel gradually decreased from the steel
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insert side to the aluminum base side, where the microhardnesses were obviously
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higher than those of the aluminum base metal. Moreover, the shear stress of the
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aluminum/steel bimetallic composite with the zinc coating increased by 71 %
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compared to that of the one without the zinc coating. The zinc coating played
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important roles in the removal of oxidation of the steel insert and the improvement
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* Corresponding authors. Tel./fax: +86 27 87540094. E-mail addresses: [email protected] (M. Jiang), [email protected] (Z. Fan).
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of the wettability between the aluminum and the steel, which promoted the
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metallurgical reaction of the aluminum with steel, resulting in a significant
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improvement of the bonding between the aluminum and the steel.
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Key words: Zinc coating; Bimetallic composites; Intermetallic compounds;
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Microstructure; Mechanical properties
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1. Introduction
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Aluminum and steel alloys are regarded as the most important engineering
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materials, because of the many benefits of aluminum alloys such as low density, good
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corrosion resistance, superior thermal and electrical conductivity as well as high
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strength to weight ratio [1, 2]. In addition, the steel alloys possess some advantages
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including high strength, superior toughness, good creep resistance and low cost [3, 4].
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However, one of these alloys is difficult to meet the practical requirements of
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industrial applications in some cases. Using combined structures consisting of
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aluminum and steel alloys as the bimetallic composites may be the most effective way
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to meet the demands for engineering applications, especially in the automotive
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industry, because they combine several promising properties that cannot be provided
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by monolithic materials [5]. It is well known that the aluminum/steel bimetallic
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composites are constituted by an assembly of different layers including aluminum and
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steel sections, namely, multi-layer composites [6], in which each layer can provide
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different properties. However, it always remains difficult to prepare the
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aluminum/steel
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thermal-physical properties between the aluminum and the steel, such as melting
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points, thermal conductivities and thermal expansion coefficients [7-9], which leads to
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the poor wettability and metallurgical bonding between the aluminum and the steel.
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bimetallic
composites,
due
to
larger
differences
in
the
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An excellent metallurgical bonding between the aluminum and the steel is of great
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importance to guarantee the sealing and heat transfer and to ensure a perfect
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metallurgical connection for the aluminum/steel bimetallic composites [10]. In
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addition, the knowledge about the control of oxide scale of the steel surface as well as
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the improvement of the wettability between the aluminum and the steel are still
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incomplete in the production of the aluminum/steel bimetallic composites [11], and
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how to obtain an excellent metallurgical bonding between the aluminum and the steel
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is also a challenging concept.
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In the present work, the structural carbon steel inserts were coated using a zinc
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alloy in order to protect the surface of the steel insert from the oxidation and to
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improve the wettability between the aluminum and the steel, and the ZL114A
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aluminum/structural carbon steel bimetallic composites were then prepared by a
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solid-liquid diffusion method. The effects of zinc coating on the macro-characteristics,
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interfacial microstructures and mechanical properties of the aluminum/steel bimetallic
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composites were investigated, and the formation mechanism of intermetallic phases at
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the interface between the aluminum and the steel were also discussed.
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2. Experimental procedures
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2.1. Materials
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Table 1
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Chemical composition of the carbon steel (wt.%). Element
C
Mn
Si
P
S
Ni
Cr
Fe
Content
0.24
0.65
0.37
0.03
0.02
0.25
0.25
Balance
Table 2 Chemical composition of the ZL114A aluminum alloy (wt.%). Element
Si
Mg
Ti
Fe
Al
Content
6.75
0.63
0.14
0.084
Balance
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In this study, the structural carbon steel and ZL114A aluminum alloy were used as
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a substrate material and a molten aluminum bath, respectively, whose chemical
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compositions are respectively listed in Table 1 and 2. A zinc alloy containing 0.1 wt.%
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Ni was employed as a coating material for the carbon steel inserts. The cylindrical steel inserts with a 42 mm diameter, a 3 mm wall thickness and a
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65 mm height were machined from a carbon steel tube. The surfaces of the cylindrical
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steel inserts were ground with silicon carbide papers up to 1200 grit before
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respectively rinsed using a 0.5 mol/l hydrochloric acid and a ethanol.
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2.2. Fabrication of composites and wetting experiments
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Fig. 1. Schematic illustration of the experimental setting.
Fig. 1 shows a schematic illustration of the experimental setting. A stainless steel
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crucible was first preheated at 300 oC in an electrical resistance furnace, and a
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preheated ZL114A aluminum ingot was then placed inside the stainless steel crucible;
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meanwhile, the zinc alloy was melted in the other stainless steel crucible, with a
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melting temperature of 450 ºC. When the temperature of the molten ZL114A
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aluminum metal reached 740 ºC, the melt was refined using the argon gas, and the
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slag was then skimmed. Prior to the pouring process, the steel inserts with a
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preheating temperature of 300 ºC were immersed into the molten metal of the zinc
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alloy at 450 ºC for 5 min. Next, the zinc-coated steel inserts were rapidly placed
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inside a metal mold with a preheating temperature of 300 ºC to pouring, and the
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molten ZL114A aluminum metal was then poured into the metal mold at 730 ºC. The
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aluminum/zinc-coated steel bimetallic composites were finally obtained after pouring
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and solidification. Additionally, the steel inserts without the zinc coating were also
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used to fabricate the aluminum/steel bimetallic composites using an experimental
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condition being the same as the aluminum/zinc-coated steel bimetallic composites for
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comparison.
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Wetting tests were performed under a high purity argon gas atmosphere using the
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carbon steel platelets of 60×60×2 mm3 as a substrate, with and without zinc coating,
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using the ZL114A aluminum alloy as a melt. The carbon steel substrate was first
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preheated to 300 ºC, and a ZL114A aluminum alloy drop with a temperature of 730 ºC
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was then placed on the carbon steel substrate for a controlled time of 90 s. The
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wetting behavior between the aluminum melt and the steel substrate was monitored by
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a video system. The contact angle was finally measured by a custom-made analysis
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program to evaluate the wettability between the aluminum and the steel.
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2.3. Microstructural characterizations
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In order to investigate interfacial microstructures of the aluminum/steel bimetallic
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composites, metallographic samples were cut from the bimetallic composites using an
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electrical discharge machine. Subsequently, the metallographic cross-sections were
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etched using a 0.5 % hydrofluoric acid solution after grinding and polishing. The
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interfacial microstructures of the metallographic samples were observed using an Me
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F-3 metallographic microscope or a Quanta 400 scanning electron microscope (SEM)
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equipped with an energy-dispersive X-ray spectroscopy (EDS). The compositional
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variations of the reaction layer between the aluminum and the steel were identified by
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the EDS analysis. In order to display gaps at the interface between the aluminum and
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the steel, the cross-sections of the bimetallic composites were stained to red using an
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imaging agent after grinding, and then observed using a stereomicroscope.
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2.4. Mechanical properties An HV-1000 hardness tester was used to examine the microhardness distributions
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at the interface zone of the aluminum/steel bimetallic composites using a load of 300
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g for a dwell time of 15 s.
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Fig. 2. Schematic sketch of the setup for the push out tests (unit: mm).
Push out tests were conducted using a ZwickZ100 universal testing machine to
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determine the bonding strength of the aluminum/steel bimetallic composites [12-14].
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Fig. 2 presents a schematic sketch of the setup for the push out tests. The bimetallic
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samples were first put on a steel supporting surface with a hole of 46 mm diameter
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and then pushed through a steel cylinder stub punch at a cross-head displacement rate
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of 0.5 mm/min, to obtain a maximum load as the bonding strength of the bimetallic
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composites. At least three samples under each experimental condition were performed
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for the push out tests in order to minimize errors.
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3. Results
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3.1. Macro-characteristics
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To illustrate the gaps at the interface between the aluminum and the steel, the
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marco-characteristics of the aluminum/steel bimetallic composites with and without
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zinc coating are shown in Fig. 3. It is evident that a large number of gaps exhibiting
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red are observed at the interface of the aluminum/steel bimetallic composite without
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the zinc coating, indicating that a poor bonding between the aluminum and the steel is
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formed, as shown in Fig. 3a. As can be seen in Fig. 3b, the interface of the
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aluminum/steel bimetallic composite with the zinc coating is almost absent from the
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gaps, and shows a better bonding between the aluminum and the steel in comparison
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with that of the one without the zinc coating, which suggests that the zinc coating has
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a significant effect on the elimination of the gaps at the interface between the
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aluminum and the steel.
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Fig. 3. Marco-characteristics of the aluminum/steel bimetallic composites with and without zinc coating: (a)
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without zinc coating; (b) with zinc coating.
3.2. Interfacial microstructures
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Fig. 4 exhibits optical micrographs of the interfacial microstructures of the
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aluminum/steel bimetallic composites with and without zinc coating. As can be seen
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in Fig. 4a and 4b, in the interfacial microstructures from the aluminum/steel bimetallic
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composite without the zinc coating, the gaps are almost present in the whole interface
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between the aluminum and the steel, and a large and continuous gap with a maximum
20
size of about 40 µm is observed at the interface, revealing a poor integrity between the
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aluminum and the steel. In this case, it means that the reaction of the aluminum with
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steel has not occurred, thereby forming a poor mechanical bonding without a reaction
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layer between the aluminum and the steel. In contrast, with the application of the zinc
4
coating, it is obvious that a relatively uniform and compact interface that consists of
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different reaction zones is formed between the aluminum and the steel, the average
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thickness of which reaches approximately 650 µm, as shown in Fig. 4c. Fig. 4d shows
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a high magnification optical micrograph related to Fig. 4c. As can be seen, the
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interface between the aluminum and the steel is free from pores and gaps. Besides, the
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interfacial microstructure exhibits different microstructure morphologies in the
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different zones of the interface, which seems that the metallurgical interface formed
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between the aluminum and the steel consists of different intermetallic phases.
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Fig. 4. Optical micrographs of interfacial microstructures of the aluminum/steel bimetallic composites with and without zinc coating: (a) low magnification without zinc coating; (b) high magnification without zinc coating; (c) low magnification with zinc coating; (d) high magnification with zinc coating.
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Fig. 5. SEM micrographs and EDS analysis of the interface of the aluminum/steel bimetallic composites with the zinc coating: (a) SEM micrograph; (b) EDS line scan.
Fig. 5 shows SEM micrograph and EDS analysis of the interface of the
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aluminum/steel bimetallic composites with the zinc coating. The SEM micrograph
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also reveals from Fig. 5a that a relatively uniform and compact interface is formed
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between the aluminum and the steel, in accordance with the findings of the Fig. 4.
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According to the EDS line scan analysis of the interface shown in Fig. 5b, it can be
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seen that the Al, Si, Fe and Zn elements clearly diffuse at the interface between the
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aluminum and the steel, where the content of the Al element gradually decreases
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across the interface from the aluminum base side toward the steel insert side, while
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the diffusion of the Fe element shows a contrary law.
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In order to further demonstrate the interfacial microstructures of the
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aluminum/steel bimetallic composites with the zinc coating, more detailed
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observations and compositional analysis with respect to the interface were performed
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using the SEM and EDS analysis. Fig. 6 presents the SEM micrographs of interfacial
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microstructures of the aluminum/steel bimetallic composites with the zinc coating,
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taken from different areas A-F marked in Fig. 5a. The quantitative analysis results of
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the distributions of the Al, Fe, Si and Zn elements taking the EDS analysis method at
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18 different points indicated in Fig. 6 are listed in Table 3. Adjacent to the steel insert,
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the microstructure primarily consists of elongated crystals, as shown in Fig. 6a. In
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light of the results of the EDS analysis and the Fe-Al-Si system [15, 16], the elongated
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crystals are confirmed to be the τ6-Al4.5FeSi phase. In the middle of the interface
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shown in Fig. 6b-e, some white and grey phases are observed, which is suggested that
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they are respectively α + η eutectoid and α-Al rich phases in accordance with the
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results of the EDS analysis and the Al-Zn phase diagram (as shown in Fig. 7) [17],
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also showing that the α-Al rich phase is surrounded by a laminated α + η eutectoid
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phase [18]. As can be seen in Fig. 6b, an η-Zn phase is observed in the inner of the α
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+ η eutectoid intermetallic compound. What is more, the elongated τ6-Al4.5FeSi phase
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and plate-like eutectic silicon particles are also detected in the middle of the interface.
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Adjacent to the aluminum base, the τ6-Al4.5FeSi intermetallic compound that exhibits
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a fine particle morphology is still present, as shown in Fig. 6f. The results above
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reveal that complex reactions occur among the aluminum, steel and zinc alloys,
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thereby forming the metallurgical interface which is composed of τ6-Al4.5FeSi, α-Al
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rich, α + η eutectoid, η-Zn and eutectic silicon phases.
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Fig. 6. SEM micrographs of interfacial microstructures of the aluminum/steel bimetallic composites with the zinc coating taken from different areas shown in Fig. 5a: (a)-(f) corresponding to areas A-F, respectively. Table 3 Results of EDS analysis corresponding to the points indicated in Fig. 6.
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Element compositions (at.%) Number
Inference
Fe
Si
Zn
component
100.00
-
-
Fe
-
2
65.89
12.73
14.98
06.40
τ6-Al4.5FeSi
3
64.24
10.28
13.74
11.74
τ6-Al4.5FeSi
4
63.04
00.97
00.18
35.80
α-Al rich
78.52
00.55
01.25
19.68
α-Al rich
81.23
-
-
18.77
α-Al rich
08.94
-
-
91.06
η-Zn
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42.80
-
-
57.20
α + η eutectoid
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84.35
-
-
15.65
α-Al rich
10
65.63
14.79
17.41
02.17
τ6-Al4.5FeSi
11
11.31
-
84.81
03.88
eutectic silicon
12
80.37
-
-
19.63
α-Al rich
13
45.80
-
-
54.20
α + η eutectoid
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65.26
15.61
19.13
-
τ6-Al4.5FeSi
15
08.12
-
87.42
04.46
eutectic silicon
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19.72
-
80.28
-
eutectic silicon
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73.80
13.25
11.28
01.67
τ6-Al4.5FeSi
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100.00
-
-
-
Al
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Fig. 7. Al-Zn phase diagram.
3.3. Mechanical properties
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Fig. 8. Microhardness distributions at the interface zone of the aluminum/steel bimetallic composites with the zinc coating.
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Fig. 8 plots the microhardness distributions at the interface zone of the
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aluminum/steel bimetallic composites with the zinc coating. As can be seen, the
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microhardnesses at the interface between the aluminum and the steel gradually
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decrease from the steel insert side to the aluminum base side, where the
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microhardnesses are obviously higher than those of the aluminum base metal,
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implying that the interface reaction between the aluminum and the steel has occurred.
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In order to better reveal the microhardness variations at the interface between the
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aluminum and the steel, the indentation sizes at different reaction zones obtained by
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microhardness testing are shown in Fig. 9. It can be found that the indentation sizes
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of the aluminum base metal are much larger than those of the intermetallic
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compounds and steel substrate, particularly in the steel substrate.
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Fig. 9. Optical micrographs of interfacial microstructures of the aluminum/steel bimetallic composites with the zinc coating after microhardness testing: (a) low magnification optical micrograph; (b) high magnification optical micrograph.
Fig. 10 illustrates the shear stresses of the aluminum/steel bimetallic composites
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with and without zinc coating to exhibit the bonding strength of the bimetallic
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composites. As shown in Fig. 10, the shear stress of the aluminum/steel bimetallic
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composite with the zinc coating is higher 71 % compared to that of the one without
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the zinc coating, due to the compact interface and good metallurgical bonding
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between the aluminum and the steel, indicating that the zinc coating is beneficial to
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improve the bonding strength of the aluminum/steel bimetallic composites.
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Fig. 10. Shear stresses of the aluminum/steel bimetallic composites with and without zinc coating.
4. Discussion
The solid-liquid diffusion method is a process via which two metallic materials-
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one in solid state and the other liquid state- are brought into contact with each other in
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such a manner that a diffusion reaction zone forms between the two materials, and
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thus a continuous metallic transition occurs from one metal to the other [19, 20]. In
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the present work, the solid state steel and liquid state aluminum are brought into
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contact with each other, thereby forming a continuous metallic transition from the
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aluminum to the steel. However, the aluminum and steel are incompatible so that the
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aluminum melt is difficult to spread on the surface of the steel insert, which is largely
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due to the oxidation of steel surface occurring in the preheating stage, as well as the
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larger differences in thermal-physical properties between the aluminum and the steel.
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The oxide scale would prevent the direct contact between aluminum melt and steel
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insert, thereby preventing the diffusion and chemical reaction processes of the
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aluminum with steel. On the other hand, the molten aluminum cannot well wet the
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steel substrate without the zinc coating, as shown in Fig.11a. As a result, in the case of
3
without zinc coating, a large number of gaps at the interface between the aluminum
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and the steel are formed; meanwhile, the reaction layer between the aluminum and the
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steel has not been formed, as shown in Figs. 3a and 4a, leading to a poor bonding
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strength.
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Fig. 11. Interfacial wetting between ZL114A aluminum alloy and carbon steel: (a) without zinc coating; (b) with zinc coating.
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With the application of the zinc coating, a good contact between zinc and steel is
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first produced during zinc coating as the oxide scale formed on the steel in the
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preheating stage can be dissolved during this process. Secondly, because the pouring
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temperature used (730 ºC) is far higher than the melting temperature of zinc (420 ºC),
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the zinc coating is likely to melt during aluminum pouring process and, as a
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consequence, the liquid aluminum can spread more easily on the zinc coated steel
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surface. Furthermore, the zinc coating also obviously improves the wettability of
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liquid aluminum on the steel, and changes the contact angle from 116° of without zinc
19
coating to 72°, as shown in Fig.11b, which is in good agreement with other reports [7,
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21]. Thus a combination of these three effects results in a significant improved contact
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between the aluminum and the steel. In this case, a relatively uniform and compact
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interface is obtained between the aluminum and the steel, resulting a superior bonding
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strength, as shown in Figs. 3b, 4c and 5. The formation mechanism of the intermetallic phases at the interface between the
4
aluminum and the steel can be explained as follows. As the molten aluminum metal is
5
poured into the metal mold, the zinc coating of surface of the steel insert melts and
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then dissolves in the aluminum melt as a result of the heat capacity of the aluminum
7
melt, forming an Al-Zn-Si mixed liquid. After contacting with the steel insert, the
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complex metallurgical reactions among the Al, Fe, Zn and Si elements occur at the
9
interface because Fe atoms from the steel insert diffuse into the Al-Zn-Si liquid. It is
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pointed out that the electron affinity of Al-Fe is larger than that of Al-Zn, thus the
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formations of the Al-Fe-Si intermetallic compounds in the interface zone during
12
solidification would preferentially take place through the following reactions [16, 22,
13
23].
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L →θ
15
L +θ → τ 5 +(Al)
16
L +τ 5 → (Al) + τ 6
(3)
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L +τ 4 → τ 6 +(Si )
(4)
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L → ( Al ) + ( Si )+τ 6
(5)
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where, θ is Al13Fe4, τ4 is Al3FeSi2, τ5 is Al7.4Fe2Si, and τ6 is Al4.5FeSi [7, 24]. The
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τ6-Al4.5FeSi intermetallic compound formed with Al and Si according to the above
21
reactions is a stable ternary phase in the Al-Fe-Si system and has a monoclinic crystal
22
structure, and shows an elongated plate shape [25, 26], as shown in Fig. 6a. Adjacent
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to the aluminum base, because the diffusion of the Fe atoms is limited, the
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τ6-Al4.5FeSi phase exhibits a fine particle morphology, as shown in Fig. 6f. Besides, there is also a metallurgical reaction between the aluminum and the zinc,
4
forming the α-Al rich, α + η eutectoid and η-Zn phases. The α-Al rich phase that is
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surrounded by a laminated eutectoid α + η structure rich in zinc presents a dendritic
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structure consisting of primary dendrites rich in aluminum, as shown in Fig. 6b. The α
7
+ η eutectoid derives from a transformation of the α dendrites and β peritectic at 275
8
ºC and then forms during the final stage of solidification [17, 18]. However, many
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literatures suggested that the Fe2Al5 and FeAl3 intermetallic compounds are probably
10
formed between the aluminum and the steel [10, 27]; the reaction between the steel
11
and the zinc may also generate the FeZn13 and FeZn10 intermetallic compounds [28,
12
29]. Interestingly, these intermetallic compounds have not been detected in the present
13
work, which can be explained by the fact that the formations of the intermetallic
14
phases depend on many factors when dissimilar metals come into contact, such as the
15
chemical potentials, nucleation conditions at the beginning of the interdiffusion
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process, mobilities of the constituent elements, and alloys compositions [7].
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Fig. 12. SEM micrographs and EDS analysis of the interface of the steel insert with the zinc coating: (a) SEM micrograph; (b) EDS line scan; (c-f) EDS analysis corresponding to 1-4 points marked in Fig.11a, respectively.
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The SEM micrographs and EDS analysis of the interface of the steel insert with
7
zinc coating are shown in Fig. 12. As can be seen, the Fe and Zn elements do not
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obviously diffuse at the interface between the steel and the zinc coating; meanwhile,
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the Fe-Zn intermetallic compounds have not been formed, which can also prove that
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the metallurgical reaction between the steel and the zinc has not occurred during the
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hot-dipping zinc process of the steel insert. It is worth indicating that the nucleation
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and growth of the Fe-Zn intermetallic compounds are influenced by many factors
13
such as the steel composition, zinc bath temperature, immersion time, and surface
14
roughness of the steel [30-32]. According to these investigations, it is essential to note
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that the zinc coating plays important roles in the removal of oxidation of the steel
16
insert and the improvement of the wettability between the aluminum and the steel,
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promoting a metallurgical bonding of the aluminum and the steel. Our future works
2
will further investigate the formation mechanism of the intermetallic phases at the
3
interface among the aluminum, steel and zinc, and focus on the preparation of the
4
aluminum/steel bimetallic castings with a complex shape based on the previous
5
fundamental investigations.
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5. Conclusions
8
(1) With the application of the zinc coating, a relatively uniform and compact
9
interface was formed between the aluminum and the steel. The reaction layer
10
which was mainly composed of τ6-Al4.5FeSi, α-Al rich, α + η eutectoid, η-Zn and
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eutectic silicon phases had an average thickness of about 650 µm.
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(2) The microhardnesses at the interface between the aluminum and the steel
13
gradually decreased from the steel insert side to the aluminum base side, where
14
the microhardnesses were obviously higher than those of the aluminum base
15
metal.
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(3) The shear stress of the aluminum/steel bimetallic composite with the zinc coating
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was higher 71 % than that of the one without the zinc coating, which is attributed
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to the compact interface and good metallurgical bonding between the aluminum and the steel.
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(4) The zinc coating played important roles in the removal of oxidation of the steel
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insert and the improvement of the wettability between the aluminum and the
22
steel, promoting the metallurgical reaction of the aluminum with steel and
23
resulting in a remarkable improvement of the bonding between the aluminum
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Acknowledgments This work was funded by Project 51204124 supported by the National Natural
5
Science Foundation of China, Project P2015-09 supported by State Key Laboratory
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of Materials Processing and Die & Mould Technology, HUST, Project 2015MS053
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supported by the Fundamental Research Funds for the Central Universities. The
8
authors would also like to thank the support of the Research Project of State Key
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Laboratory of Materials Processing and Die & Mould Technology and the Analytical
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and Testing Center, HUST.
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[31] J.D. Culcasi, P.R. Sere, C.I. Elsner, A.R. Di Sarli, Control of the growth of
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coatings on dual phase steel, Acta Mater. 60 (2012) 2973-2981.
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Figure captions:
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Fig. 1. Schematic illustration of the experimental setting.
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Fig. 2. Schematic sketch of the setup for the push out tests (unit: mm).
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Fig. 3. Marco-characteristics of the aluminum/steel bimetallic composites with and
5
without zinc coating: (a) without zinc coating; (b) with zinc coating.
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Fig. 4. Optical micrographs of interfacial microstructures of the aluminum/steel
7
bimetallic composites with and without zinc coating: (a) low magnification without
8
zinc coating; (b) high magnification without zinc coating; (c) low magnification with
9
zinc coating; (d) high magnification with zinc coating.
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Fig. 5. SEM micrographs and EDS analysis of the interface of the aluminum/steel
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bimetallic composites with the zinc coating: (a) SEM micrograph; (b) EDS line scan.
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Fig. 6. SEM micrographs of interfacial microstructures of the aluminum/steel
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bimetallic composites with the zinc coating taken from different areas shown in Fig.
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5a: (a)-(f) corresponding to areas A-F, respectively.
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Fig. 7. Al-Zn phase diagram.
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Fig. 8. Microhardness distributions at the interface zone of the aluminum/steel
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bimetallic composites with the zinc coating.
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Fig. 9. Optical micrographs of interfacial microstructures of the aluminum/steel
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bimetallic composites with the zinc coating after microhardness testing: (a) low
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magnification optical micrograph; (b) high magnification optical micrograph.
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Fig. 10. Shear stresses of the aluminum/steel bimetallic composites with and without
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zinc coating.
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Fig. 11. Interfacial wetting between ZL114A aluminum alloy and carbon steel: (a)
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without zinc coating; (b) with zinc coating
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Fig. 12. SEM micrographs and EDS analysis of the interface of the steel insert with
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the zinc coating: (a) SEM micrograph; (b) EDS line scan; (c-f) EDS analysis
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corresponding to 1-4 points marked in Fig.11a, respectively.
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ACCEPTED MANUSCRIPT Highlights: Aluminum/zinc-coated steel bimetallic composites were successfully prepared.
Formation mechanism of intermetallic phases at the interface were discussed.
A uniform and compact interface with an average thickness of 650 µm was
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formed.
Interface consisted of τ6-Al4.5FeSi, α-Al rich, α + η eutectoid, η-Zn and Si phases.
Zinc coatings increased shear stress of the composites by 71 %.
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S0925-8388(16)30931-8
DOI:
10.1016/j.jallcom.2016.03.276
Reference:
JALCOM 37164
To appear in:
Journal of Alloys and Compounds
Received Date: 20 January 2016 Revised Date:
28 March 2016
Accepted Date: 31 March 2016
Please cite this article as: W. Jiang, Z. Fan, G. Li, C. Li, Effects of zinc coating on interfacial microstructures and mechanical properties of aluminum/steel bimetallic composites, Journal of Alloys and Compounds (2016), doi: 10.1016/j.jallcom.2016.03.276. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effects of zinc coating on interfacial microstructures and
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mechanical properties of aluminum/steel bimetallic composites
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Wenming Jiang *, Zitian Fan *, Guangyu Li, Chi Li State Key Lab of Materials Processing and Die & Mould Technology, Huazhong
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University of Science and Technology, Wuhan 430074, PR China
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ABSTRACT
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In the present work, the effects of zinc coating on interfacial microstructures and
8
mechanical properties of the aluminum/steel bimetallic composites prepared by a
9
solid-liquid diffusion method were investigated, and the formation mechanism of
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intermetallic compounds at the interface between the aluminum and the steel were
11
also discussed. The results show that a relatively uniform and compact interface
12
between the aluminum and the steel was formed with the application of the zinc
13
coating. The reaction layer that was mainly composed of τ6-Al4.5FeSi, α-Al rich, α +
14
η eutectoid, η-Zn and eutectic silicon phases, between the aluminum and the steel,
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had an average thickness of approximately 650 µm. The microhardnesses at the
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interface between the aluminum and the steel gradually decreased from the steel
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insert side to the aluminum base side, where the microhardnesses were obviously
18
higher than those of the aluminum base metal. Moreover, the shear stress of the
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aluminum/steel bimetallic composite with the zinc coating increased by 71 %
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compared to that of the one without the zinc coating. The zinc coating played
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important roles in the removal of oxidation of the steel insert and the improvement
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* Corresponding authors. Tel./fax: +86 27 87540094. E-mail addresses: [email protected] (M. Jiang), [email protected] (Z. Fan).
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of the wettability between the aluminum and the steel, which promoted the
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metallurgical reaction of the aluminum with steel, resulting in a significant
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improvement of the bonding between the aluminum and the steel.
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Key words: Zinc coating; Bimetallic composites; Intermetallic compounds;
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Microstructure; Mechanical properties
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1. Introduction
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Aluminum and steel alloys are regarded as the most important engineering
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materials, because of the many benefits of aluminum alloys such as low density, good
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corrosion resistance, superior thermal and electrical conductivity as well as high
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strength to weight ratio [1, 2]. In addition, the steel alloys possess some advantages
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including high strength, superior toughness, good creep resistance and low cost [3, 4].
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However, one of these alloys is difficult to meet the practical requirements of
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industrial applications in some cases. Using combined structures consisting of
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aluminum and steel alloys as the bimetallic composites may be the most effective way
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to meet the demands for engineering applications, especially in the automotive
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industry, because they combine several promising properties that cannot be provided
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by monolithic materials [5]. It is well known that the aluminum/steel bimetallic
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composites are constituted by an assembly of different layers including aluminum and
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steel sections, namely, multi-layer composites [6], in which each layer can provide
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different properties. However, it always remains difficult to prepare the
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aluminum/steel
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thermal-physical properties between the aluminum and the steel, such as melting
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points, thermal conductivities and thermal expansion coefficients [7-9], which leads to
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the poor wettability and metallurgical bonding between the aluminum and the steel.
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bimetallic
composites,
due
to
larger
differences
in
the
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An excellent metallurgical bonding between the aluminum and the steel is of great
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importance to guarantee the sealing and heat transfer and to ensure a perfect
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metallurgical connection for the aluminum/steel bimetallic composites [10]. In
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addition, the knowledge about the control of oxide scale of the steel surface as well as
5
the improvement of the wettability between the aluminum and the steel are still
6
incomplete in the production of the aluminum/steel bimetallic composites [11], and
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how to obtain an excellent metallurgical bonding between the aluminum and the steel
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is also a challenging concept.
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In the present work, the structural carbon steel inserts were coated using a zinc
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alloy in order to protect the surface of the steel insert from the oxidation and to
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improve the wettability between the aluminum and the steel, and the ZL114A
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aluminum/structural carbon steel bimetallic composites were then prepared by a
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solid-liquid diffusion method. The effects of zinc coating on the macro-characteristics,
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interfacial microstructures and mechanical properties of the aluminum/steel bimetallic
15
composites were investigated, and the formation mechanism of intermetallic phases at
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the interface between the aluminum and the steel were also discussed.
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2. Experimental procedures
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2.1. Materials
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Table 1
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Chemical composition of the carbon steel (wt.%). Element
C
Mn
Si
P
S
Ni
Cr
Fe
Content
0.24
0.65
0.37
0.03
0.02
0.25
0.25
Balance
Table 2 Chemical composition of the ZL114A aluminum alloy (wt.%). Element
Si
Mg
Ti
Fe
Al
Content
6.75
0.63
0.14
0.084
Balance
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In this study, the structural carbon steel and ZL114A aluminum alloy were used as
2
a substrate material and a molten aluminum bath, respectively, whose chemical
3
compositions are respectively listed in Table 1 and 2. A zinc alloy containing 0.1 wt.%
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Ni was employed as a coating material for the carbon steel inserts. The cylindrical steel inserts with a 42 mm diameter, a 3 mm wall thickness and a
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65 mm height were machined from a carbon steel tube. The surfaces of the cylindrical
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steel inserts were ground with silicon carbide papers up to 1200 grit before
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respectively rinsed using a 0.5 mol/l hydrochloric acid and a ethanol.
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2.2. Fabrication of composites and wetting experiments
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Fig. 1. Schematic illustration of the experimental setting.
Fig. 1 shows a schematic illustration of the experimental setting. A stainless steel
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crucible was first preheated at 300 oC in an electrical resistance furnace, and a
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preheated ZL114A aluminum ingot was then placed inside the stainless steel crucible;
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meanwhile, the zinc alloy was melted in the other stainless steel crucible, with a
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melting temperature of 450 ºC. When the temperature of the molten ZL114A
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aluminum metal reached 740 ºC, the melt was refined using the argon gas, and the
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slag was then skimmed. Prior to the pouring process, the steel inserts with a
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preheating temperature of 300 ºC were immersed into the molten metal of the zinc
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alloy at 450 ºC for 5 min. Next, the zinc-coated steel inserts were rapidly placed
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inside a metal mold with a preheating temperature of 300 ºC to pouring, and the
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molten ZL114A aluminum metal was then poured into the metal mold at 730 ºC. The
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aluminum/zinc-coated steel bimetallic composites were finally obtained after pouring
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and solidification. Additionally, the steel inserts without the zinc coating were also
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used to fabricate the aluminum/steel bimetallic composites using an experimental
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condition being the same as the aluminum/zinc-coated steel bimetallic composites for
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comparison.
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Wetting tests were performed under a high purity argon gas atmosphere using the
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carbon steel platelets of 60×60×2 mm3 as a substrate, with and without zinc coating,
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using the ZL114A aluminum alloy as a melt. The carbon steel substrate was first
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preheated to 300 ºC, and a ZL114A aluminum alloy drop with a temperature of 730 ºC
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was then placed on the carbon steel substrate for a controlled time of 90 s. The
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wetting behavior between the aluminum melt and the steel substrate was monitored by
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a video system. The contact angle was finally measured by a custom-made analysis
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program to evaluate the wettability between the aluminum and the steel.
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2.3. Microstructural characterizations
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In order to investigate interfacial microstructures of the aluminum/steel bimetallic
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composites, metallographic samples were cut from the bimetallic composites using an
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electrical discharge machine. Subsequently, the metallographic cross-sections were
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etched using a 0.5 % hydrofluoric acid solution after grinding and polishing. The
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interfacial microstructures of the metallographic samples were observed using an Me
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F-3 metallographic microscope or a Quanta 400 scanning electron microscope (SEM)
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equipped with an energy-dispersive X-ray spectroscopy (EDS). The compositional
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variations of the reaction layer between the aluminum and the steel were identified by
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the EDS analysis. In order to display gaps at the interface between the aluminum and
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the steel, the cross-sections of the bimetallic composites were stained to red using an
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imaging agent after grinding, and then observed using a stereomicroscope.
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2.4. Mechanical properties An HV-1000 hardness tester was used to examine the microhardness distributions
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at the interface zone of the aluminum/steel bimetallic composites using a load of 300
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g for a dwell time of 15 s.
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Fig. 2. Schematic sketch of the setup for the push out tests (unit: mm).
Push out tests were conducted using a ZwickZ100 universal testing machine to
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determine the bonding strength of the aluminum/steel bimetallic composites [12-14].
11
Fig. 2 presents a schematic sketch of the setup for the push out tests. The bimetallic
12
samples were first put on a steel supporting surface with a hole of 46 mm diameter
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and then pushed through a steel cylinder stub punch at a cross-head displacement rate
14
of 0.5 mm/min, to obtain a maximum load as the bonding strength of the bimetallic
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composites. At least three samples under each experimental condition were performed
16
for the push out tests in order to minimize errors.
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3. Results
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3.1. Macro-characteristics
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To illustrate the gaps at the interface between the aluminum and the steel, the
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marco-characteristics of the aluminum/steel bimetallic composites with and without
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zinc coating are shown in Fig. 3. It is evident that a large number of gaps exhibiting
2
red are observed at the interface of the aluminum/steel bimetallic composite without
3
the zinc coating, indicating that a poor bonding between the aluminum and the steel is
4
formed, as shown in Fig. 3a. As can be seen in Fig. 3b, the interface of the
5
aluminum/steel bimetallic composite with the zinc coating is almost absent from the
6
gaps, and shows a better bonding between the aluminum and the steel in comparison
7
with that of the one without the zinc coating, which suggests that the zinc coating has
8
a significant effect on the elimination of the gaps at the interface between the
9
aluminum and the steel.
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Fig. 3. Marco-characteristics of the aluminum/steel bimetallic composites with and without zinc coating: (a)
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3.2. Interfacial microstructures
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Fig. 4 exhibits optical micrographs of the interfacial microstructures of the
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aluminum/steel bimetallic composites with and without zinc coating. As can be seen
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in Fig. 4a and 4b, in the interfacial microstructures from the aluminum/steel bimetallic
18
composite without the zinc coating, the gaps are almost present in the whole interface
19
between the aluminum and the steel, and a large and continuous gap with a maximum
20
size of about 40 µm is observed at the interface, revealing a poor integrity between the
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aluminum and the steel. In this case, it means that the reaction of the aluminum with
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steel has not occurred, thereby forming a poor mechanical bonding without a reaction
3
layer between the aluminum and the steel. In contrast, with the application of the zinc
4
coating, it is obvious that a relatively uniform and compact interface that consists of
5
different reaction zones is formed between the aluminum and the steel, the average
6
thickness of which reaches approximately 650 µm, as shown in Fig. 4c. Fig. 4d shows
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a high magnification optical micrograph related to Fig. 4c. As can be seen, the
8
interface between the aluminum and the steel is free from pores and gaps. Besides, the
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interfacial microstructure exhibits different microstructure morphologies in the
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different zones of the interface, which seems that the metallurgical interface formed
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between the aluminum and the steel consists of different intermetallic phases.
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Fig. 4. Optical micrographs of interfacial microstructures of the aluminum/steel bimetallic composites with and without zinc coating: (a) low magnification without zinc coating; (b) high magnification without zinc coating; (c) low magnification with zinc coating; (d) high magnification with zinc coating.
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Fig. 5. SEM micrographs and EDS analysis of the interface of the aluminum/steel bimetallic composites with the zinc coating: (a) SEM micrograph; (b) EDS line scan.
Fig. 5 shows SEM micrograph and EDS analysis of the interface of the
6
aluminum/steel bimetallic composites with the zinc coating. The SEM micrograph
7
also reveals from Fig. 5a that a relatively uniform and compact interface is formed
8
between the aluminum and the steel, in accordance with the findings of the Fig. 4.
9
According to the EDS line scan analysis of the interface shown in Fig. 5b, it can be
10
seen that the Al, Si, Fe and Zn elements clearly diffuse at the interface between the
11
aluminum and the steel, where the content of the Al element gradually decreases
12
across the interface from the aluminum base side toward the steel insert side, while
13
the diffusion of the Fe element shows a contrary law.
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In order to further demonstrate the interfacial microstructures of the
15
aluminum/steel bimetallic composites with the zinc coating, more detailed
16
observations and compositional analysis with respect to the interface were performed
17
using the SEM and EDS analysis. Fig. 6 presents the SEM micrographs of interfacial
18
microstructures of the aluminum/steel bimetallic composites with the zinc coating,
19
taken from different areas A-F marked in Fig. 5a. The quantitative analysis results of
20
the distributions of the Al, Fe, Si and Zn elements taking the EDS analysis method at
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18 different points indicated in Fig. 6 are listed in Table 3. Adjacent to the steel insert,
2
the microstructure primarily consists of elongated crystals, as shown in Fig. 6a. In
3
light of the results of the EDS analysis and the Fe-Al-Si system [15, 16], the elongated
4
crystals are confirmed to be the τ6-Al4.5FeSi phase. In the middle of the interface
5
shown in Fig. 6b-e, some white and grey phases are observed, which is suggested that
6
they are respectively α + η eutectoid and α-Al rich phases in accordance with the
7
results of the EDS analysis and the Al-Zn phase diagram (as shown in Fig. 7) [17],
8
also showing that the α-Al rich phase is surrounded by a laminated α + η eutectoid
9
phase [18]. As can be seen in Fig. 6b, an η-Zn phase is observed in the inner of the α
10
+ η eutectoid intermetallic compound. What is more, the elongated τ6-Al4.5FeSi phase
11
and plate-like eutectic silicon particles are also detected in the middle of the interface.
12
Adjacent to the aluminum base, the τ6-Al4.5FeSi intermetallic compound that exhibits
13
a fine particle morphology is still present, as shown in Fig. 6f. The results above
14
reveal that complex reactions occur among the aluminum, steel and zinc alloys,
15
thereby forming the metallurgical interface which is composed of τ6-Al4.5FeSi, α-Al
16
rich, α + η eutectoid, η-Zn and eutectic silicon phases.
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Fig. 6. SEM micrographs of interfacial microstructures of the aluminum/steel bimetallic composites with the zinc coating taken from different areas shown in Fig. 5a: (a)-(f) corresponding to areas A-F, respectively. Table 3 Results of EDS analysis corresponding to the points indicated in Fig. 6.
Al
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Element compositions (at.%) Number
Inference
Fe
Si
Zn
component
100.00
-
-
Fe
-
2
65.89
12.73
14.98
06.40
τ6-Al4.5FeSi
3
64.24
10.28
13.74
11.74
τ6-Al4.5FeSi
4
63.04
00.97
00.18
35.80
α-Al rich
78.52
00.55
01.25
19.68
α-Al rich
81.23
-
-
18.77
α-Al rich
08.94
-
-
91.06
η-Zn
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42.80
-
-
57.20
α + η eutectoid
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84.35
-
-
15.65
α-Al rich
10
65.63
14.79
17.41
02.17
τ6-Al4.5FeSi
11
11.31
-
84.81
03.88
eutectic silicon
12
80.37
-
-
19.63
α-Al rich
13
45.80
-
-
54.20
α + η eutectoid
14
65.26
15.61
19.13
-
τ6-Al4.5FeSi
15
08.12
-
87.42
04.46
eutectic silicon
16
19.72
-
80.28
-
eutectic silicon
17
73.80
13.25
11.28
01.67
τ6-Al4.5FeSi
18
100.00
-
-
-
Al
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3.3. Mechanical properties
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Fig. 8. Microhardness distributions at the interface zone of the aluminum/steel bimetallic composites with the zinc coating.
9
Fig. 8 plots the microhardness distributions at the interface zone of the
10
aluminum/steel bimetallic composites with the zinc coating. As can be seen, the
11
microhardnesses at the interface between the aluminum and the steel gradually
12
decrease from the steel insert side to the aluminum base side, where the
13
microhardnesses are obviously higher than those of the aluminum base metal,
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implying that the interface reaction between the aluminum and the steel has occurred.
2
In order to better reveal the microhardness variations at the interface between the
3
aluminum and the steel, the indentation sizes at different reaction zones obtained by
4
microhardness testing are shown in Fig. 9. It can be found that the indentation sizes
5
of the aluminum base metal are much larger than those of the intermetallic
6
compounds and steel substrate, particularly in the steel substrate.
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Fig. 9. Optical micrographs of interfacial microstructures of the aluminum/steel bimetallic composites with the zinc coating after microhardness testing: (a) low magnification optical micrograph; (b) high magnification optical micrograph.
Fig. 10 illustrates the shear stresses of the aluminum/steel bimetallic composites
14
with and without zinc coating to exhibit the bonding strength of the bimetallic
15
composites. As shown in Fig. 10, the shear stress of the aluminum/steel bimetallic
16
composite with the zinc coating is higher 71 % compared to that of the one without
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the zinc coating, due to the compact interface and good metallurgical bonding
2
between the aluminum and the steel, indicating that the zinc coating is beneficial to
3
improve the bonding strength of the aluminum/steel bimetallic composites.
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Fig. 10. Shear stresses of the aluminum/steel bimetallic composites with and without zinc coating.
4. Discussion
The solid-liquid diffusion method is a process via which two metallic materials-
9
one in solid state and the other liquid state- are brought into contact with each other in
10
such a manner that a diffusion reaction zone forms between the two materials, and
11
thus a continuous metallic transition occurs from one metal to the other [19, 20]. In
12
the present work, the solid state steel and liquid state aluminum are brought into
13
contact with each other, thereby forming a continuous metallic transition from the
14
aluminum to the steel. However, the aluminum and steel are incompatible so that the
15
aluminum melt is difficult to spread on the surface of the steel insert, which is largely
16
due to the oxidation of steel surface occurring in the preheating stage, as well as the
17
larger differences in thermal-physical properties between the aluminum and the steel.
18
The oxide scale would prevent the direct contact between aluminum melt and steel
19
insert, thereby preventing the diffusion and chemical reaction processes of the
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aluminum with steel. On the other hand, the molten aluminum cannot well wet the
2
steel substrate without the zinc coating, as shown in Fig.11a. As a result, in the case of
3
without zinc coating, a large number of gaps at the interface between the aluminum
4
and the steel are formed; meanwhile, the reaction layer between the aluminum and the
5
steel has not been formed, as shown in Figs. 3a and 4a, leading to a poor bonding
6
strength.
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Fig. 11. Interfacial wetting between ZL114A aluminum alloy and carbon steel: (a) without zinc coating; (b) with zinc coating.
11
With the application of the zinc coating, a good contact between zinc and steel is
12
first produced during zinc coating as the oxide scale formed on the steel in the
13
preheating stage can be dissolved during this process. Secondly, because the pouring
14
temperature used (730 ºC) is far higher than the melting temperature of zinc (420 ºC),
15
the zinc coating is likely to melt during aluminum pouring process and, as a
16
consequence, the liquid aluminum can spread more easily on the zinc coated steel
17
surface. Furthermore, the zinc coating also obviously improves the wettability of
18
liquid aluminum on the steel, and changes the contact angle from 116° of without zinc
19
coating to 72°, as shown in Fig.11b, which is in good agreement with other reports [7,
20
21]. Thus a combination of these three effects results in a significant improved contact
21
between the aluminum and the steel. In this case, a relatively uniform and compact
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interface is obtained between the aluminum and the steel, resulting a superior bonding
2
strength, as shown in Figs. 3b, 4c and 5. The formation mechanism of the intermetallic phases at the interface between the
4
aluminum and the steel can be explained as follows. As the molten aluminum metal is
5
poured into the metal mold, the zinc coating of surface of the steel insert melts and
6
then dissolves in the aluminum melt as a result of the heat capacity of the aluminum
7
melt, forming an Al-Zn-Si mixed liquid. After contacting with the steel insert, the
8
complex metallurgical reactions among the Al, Fe, Zn and Si elements occur at the
9
interface because Fe atoms from the steel insert diffuse into the Al-Zn-Si liquid. It is
10
pointed out that the electron affinity of Al-Fe is larger than that of Al-Zn, thus the
11
formations of the Al-Fe-Si intermetallic compounds in the interface zone during
12
solidification would preferentially take place through the following reactions [16, 22,
13
23].
14
L →θ
15
L +θ → τ 5 +(Al)
16
L +τ 5 → (Al) + τ 6
(3)
17
L +τ 4 → τ 6 +(Si )
(4)
18
L → ( Al ) + ( Si )+τ 6
(5)
19
where, θ is Al13Fe4, τ4 is Al3FeSi2, τ5 is Al7.4Fe2Si, and τ6 is Al4.5FeSi [7, 24]. The
20
τ6-Al4.5FeSi intermetallic compound formed with Al and Si according to the above
21
reactions is a stable ternary phase in the Al-Fe-Si system and has a monoclinic crystal
22
structure, and shows an elongated plate shape [25, 26], as shown in Fig. 6a. Adjacent
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to the aluminum base, because the diffusion of the Fe atoms is limited, the
2
τ6-Al4.5FeSi phase exhibits a fine particle morphology, as shown in Fig. 6f. Besides, there is also a metallurgical reaction between the aluminum and the zinc,
4
forming the α-Al rich, α + η eutectoid and η-Zn phases. The α-Al rich phase that is
5
surrounded by a laminated eutectoid α + η structure rich in zinc presents a dendritic
6
structure consisting of primary dendrites rich in aluminum, as shown in Fig. 6b. The α
7
+ η eutectoid derives from a transformation of the α dendrites and β peritectic at 275
8
ºC and then forms during the final stage of solidification [17, 18]. However, many
9
literatures suggested that the Fe2Al5 and FeAl3 intermetallic compounds are probably
10
formed between the aluminum and the steel [10, 27]; the reaction between the steel
11
and the zinc may also generate the FeZn13 and FeZn10 intermetallic compounds [28,
12
29]. Interestingly, these intermetallic compounds have not been detected in the present
13
work, which can be explained by the fact that the formations of the intermetallic
14
phases depend on many factors when dissimilar metals come into contact, such as the
15
chemical potentials, nucleation conditions at the beginning of the interdiffusion
16
process, mobilities of the constituent elements, and alloys compositions [7].
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Fig. 12. SEM micrographs and EDS analysis of the interface of the steel insert with the zinc coating: (a) SEM micrograph; (b) EDS line scan; (c-f) EDS analysis corresponding to 1-4 points marked in Fig.11a, respectively.
6
The SEM micrographs and EDS analysis of the interface of the steel insert with
7
zinc coating are shown in Fig. 12. As can be seen, the Fe and Zn elements do not
8
obviously diffuse at the interface between the steel and the zinc coating; meanwhile,
9
the Fe-Zn intermetallic compounds have not been formed, which can also prove that
10
the metallurgical reaction between the steel and the zinc has not occurred during the
11
hot-dipping zinc process of the steel insert. It is worth indicating that the nucleation
12
and growth of the Fe-Zn intermetallic compounds are influenced by many factors
13
such as the steel composition, zinc bath temperature, immersion time, and surface
14
roughness of the steel [30-32]. According to these investigations, it is essential to note
15
that the zinc coating plays important roles in the removal of oxidation of the steel
16
insert and the improvement of the wettability between the aluminum and the steel,
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promoting a metallurgical bonding of the aluminum and the steel. Our future works
2
will further investigate the formation mechanism of the intermetallic phases at the
3
interface among the aluminum, steel and zinc, and focus on the preparation of the
4
aluminum/steel bimetallic castings with a complex shape based on the previous
5
fundamental investigations.
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5. Conclusions
8
(1) With the application of the zinc coating, a relatively uniform and compact
9
interface was formed between the aluminum and the steel. The reaction layer
10
which was mainly composed of τ6-Al4.5FeSi, α-Al rich, α + η eutectoid, η-Zn and
11
eutectic silicon phases had an average thickness of about 650 µm.
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(2) The microhardnesses at the interface between the aluminum and the steel
13
gradually decreased from the steel insert side to the aluminum base side, where
14
the microhardnesses were obviously higher than those of the aluminum base
15
metal.
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(3) The shear stress of the aluminum/steel bimetallic composite with the zinc coating
17
was higher 71 % than that of the one without the zinc coating, which is attributed
18 19
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to the compact interface and good metallurgical bonding between the aluminum and the steel.
20
(4) The zinc coating played important roles in the removal of oxidation of the steel
21
insert and the improvement of the wettability between the aluminum and the
22
steel, promoting the metallurgical reaction of the aluminum with steel and
23
resulting in a remarkable improvement of the bonding between the aluminum
20
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2 3
Acknowledgments This work was funded by Project 51204124 supported by the National Natural
5
Science Foundation of China, Project P2015-09 supported by State Key Laboratory
6
of Materials Processing and Die & Mould Technology, HUST, Project 2015MS053
7
supported by the Fundamental Research Funds for the Central Universities. The
8
authors would also like to thank the support of the Research Project of State Key
9
Laboratory of Materials Processing and Die & Mould Technology and the Analytical
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and Testing Center, HUST.
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Figure captions:
2
Fig. 1. Schematic illustration of the experimental setting.
3
Fig. 2. Schematic sketch of the setup for the push out tests (unit: mm).
4
Fig. 3. Marco-characteristics of the aluminum/steel bimetallic composites with and
5
without zinc coating: (a) without zinc coating; (b) with zinc coating.
6
Fig. 4. Optical micrographs of interfacial microstructures of the aluminum/steel
7
bimetallic composites with and without zinc coating: (a) low magnification without
8
zinc coating; (b) high magnification without zinc coating; (c) low magnification with
9
zinc coating; (d) high magnification with zinc coating.
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Fig. 5. SEM micrographs and EDS analysis of the interface of the aluminum/steel
11
bimetallic composites with the zinc coating: (a) SEM micrograph; (b) EDS line scan.
12
Fig. 6. SEM micrographs of interfacial microstructures of the aluminum/steel
13
bimetallic composites with the zinc coating taken from different areas shown in Fig.
14
5a: (a)-(f) corresponding to areas A-F, respectively.
15
Fig. 7. Al-Zn phase diagram.
16
Fig. 8. Microhardness distributions at the interface zone of the aluminum/steel
17
bimetallic composites with the zinc coating.
18
Fig. 9. Optical micrographs of interfacial microstructures of the aluminum/steel
19
bimetallic composites with the zinc coating after microhardness testing: (a) low
20
magnification optical micrograph; (b) high magnification optical micrograph.
21
Fig. 10. Shear stresses of the aluminum/steel bimetallic composites with and without
22
zinc coating.
23
Fig. 11. Interfacial wetting between ZL114A aluminum alloy and carbon steel: (a)
24
without zinc coating; (b) with zinc coating
25
Fig. 12. SEM micrographs and EDS analysis of the interface of the steel insert with
26
the zinc coating: (a) SEM micrograph; (b) EDS line scan; (c-f) EDS analysis
27
corresponding to 1-4 points marked in Fig.11a, respectively.
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ACCEPTED MANUSCRIPT Highlights: Aluminum/zinc-coated steel bimetallic composites were successfully prepared.
Formation mechanism of intermetallic phases at the interface were discussed.
A uniform and compact interface with an average thickness of 650 µm was
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Interface consisted of τ6-Al4.5FeSi, α-Al rich, α + η eutectoid, η-Zn and Si phases.
Zinc coatings increased shear stress of the composites by 71 %.
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