Research of Materials Science June 2014, Volume 3, Issue 2, PP.25-29
Research on Interface Behavior of the High Strength Copper/Steel Bimetal Composite Guide Plate Yuanhao Zhang 1, Ping Zhang 1,2, Yan Xu2#, Qingming Chang 2 1. Department of Materials Engineering, Hubei University of Automotive Technology, Shiyan 442002, China 2. College of Material and Metallurgy, Wuhan University of Science and Technology, Wuhan 430081, China #
Email: melody0803@foxmail.com
Abstract High strength copper/steel bimetal composite guide plate was produced by using solid-liquid casting cladding after brush plating and dipping infiltration pretreatment. The interface characteristics of the composite guide plate were analyzed by observing the microstructures, element distribution, microhardness and combination strength, etc. It has been found that metallurgical bond can be obtained on the copper/steel interface by the solid-liquid casting. And the combination strength of the interface can reach up to 250 MPa. The interface molding behavior in composite process was discussed in the end. Keywords: Bimetal; Composite Guide Plate; Metallurgical Bond; Interface Behavior
1 INTRODUCTION At present, Copper alloy, powder sintered copper/steel bimetal are adopted to make the guide plate of large and medium-sized stamping die guide parts in most domestic and foreign auto mould industry [1]. However, with the increasing demands of the high precision die for guide plate material properties, the current guide plate materials and forming methods can not meet the production needs. The method of liquid-solid molding process to prepare high strength copper/steel bimetal composite guide plate, not only can improve the comprehensive performance of guide to extend its service life, but also can reduce the use of the precious metal so as to cut down the production cost [2]. Therefore, the research of copper/steel bimetallic interface behavior is of great significance to guide the industrial production and improve the quality of our products.
2 EXPERIMENTAL 45 carbon steel was chose as the substrate material, and the composite high strength copper was homemade, and its chemical compositions were showed in Tab2.1: TAB 2.1 CHEMICAL COMPOSITION OF HOMEMADE HIGH STRENGTH COPPER ALLOY Alloy elements
Cu
Zn
Al
Fe
Mn
Contents (wt, % )
60~65
22~28
5~8
2~4
2~5
Firstly, the surface of base steel (100 mm in length,100 mm in width and 20 mm in thickness) was pretreated by mechanical polishing, pickling, water washing, copper brush plating, antioxidants dipping infiltration, drying, heating and holding at the temperature of 200~300℃. Secondly, configuring the copper alloy according to the composition, then making the base steel plate infiltrate into the liquid copper alloy (1100~1150℃) for 60~120 s before casting. Then, immediately putting the dipped steel in the preheated metal mold (250℃) , and casting rapidly. High strength copper/steel bimetal composite guide plate is prepared after cooling and solidification.
3 RESULTS AND ANALYSIS - 25 http://www.ivypub.org/rms
The sample of high strength copper/steel bimetal composite guide plate is showed in Fig3.1, and the thickness of Copper alloy is 3~5 mm in composite sheet.
FIG 3.1 HIGH STRENGTH COPPER/STEEL BIMETAL COMPOSITE GUIDE PLATE SAMPLE
3.1 Microstructure The metallograph near bimetal interface of high strength copper alloy/45 steel is described in Fig3.2. As shown in the picture, matrix structure of 45 steel is pearlite, and matrix structure of the high strength copper alloy is single phase of α Cu-Zn-Al-Fe-Mn solid solution which has high strength and good wear resistant with homogeneous microstructure and regular shape , and the bright white-colored area in high strength copper is Cu-rich structure, the gray area is Zn-rich structure. Fig3.3 shows the microstructure of the bimetal interface. As can be seen from this figure, interface transition layer is continuous and complete, no inclusions and cracks, and both sides are relatively flat. The average thickness of the transition layer is 35～50 μm. 45 steel
High strength copper
The transition layer
1 5 s t e e l
FIG 3.2 MICROSTRUCTURE NEAR THE BIMETAL INTERFACE 100
2 5 s t e e l
FIG 3.3 MICROSTRUCTURE IN THE BIMETAL INTERFACE
FIG 3.4 DISTRIBUTION OF ELEMENT NEAR THE BIMETAL INTERFACE
3.2 Elements distribution EDS composition line scanning results was showed in Fig3.4. It can be easily found that both iron and copper - 26 http://www.ivypub.org/rms
element are continuously distributed at the interfacial joint. Iron content has little change from base steel to transition layer, while it declines sharply at copper alloy side, and the distribution of copper is opposite. As we known, activation energy of solid metal atoms diffusion in the liquid metal is smaller than that of liquid metal atoms diffusion in the solid metal [3], so the iron element diffusion is much easier than copper element and its diffusion amount is more too. What’s more, the diffusion depths of the two elements have reached more than 450 μm, which further demonstrates that transition interface bonding between high strength copper alloy and 45 steel is good.
3.3 Microhardness The mechanical behavior of the high strength copper alloys/45 steel bimetal is evaluated by Vickers hardness measurements, with a 200 g loading force, 15 s holding time. Test results are showed in Fig3.5, which are average of three times measurement of the specimens. According to the curve, we can find that microhardness near the bimetal interface changes smoothly and continuously, which proves that the combination of the bimetal interface structure is evenly distributed. And microhardness of the interface at a distance about 250 μm on both sides is close to their matrix, which illustrates that mutual diffusion of Cu, Fe and other elements has occurred at the bimetal interface. The diffusion is beneficial to bonding of high strength copper alloy and 45 steel.
FIG 3.5 MICROHARDNESS OF THE BIMETAL INTERFACE
3.4 Combination strength Combination strength refers to binding force between the cladding material and base material, and it is an important index to evaluate the quality of composite encasing material. Shearing strength is used to assess the interface binding property in this study. Test samples size of 20*2 cm are adopted as specimens with a 5 mm/s loading speed measured on multifunctional material testing machine. The test results indicate that the average bonding strength can reach up to 250 MPa. Macroscopic surface and fracture morphology of shear specimen are showed in Fig3.6. From the diagram, we can find that the characteristics of tearing fracture are mainly dimples distribution and section of the fracture with a few avulsions. It can be defined as a typical feature of toughness. So there is no doubt that high strength copper and based steel has formed strong metallurgical bond. a 5
b 5
s t e e l
s t e e l
FIG 3.6 SHEAR SPECIMEN MACROSCOPIC SURFACE (A) AND SHEAR FRACTURE MORPHOLOGY (B) OF BIMETAL COMPOSITE STEEL - 27 http://www.ivypub.org/rms
4 DISCUSSIONS There are three main kinds of techniques in the process of high strength copper/steel bimetallic compounds: copper brush plating, hot-dipped copper and copper alloy casting. The interface behavior during compounding process is as follows:
4.1 Mechanical combination In copper brush plating process, a large number of pits and micro rough are emerged on the surface of substrate 45 steel after activation treatment, which play a certain clinching role on cladding material, so that copper ion can easily deposit on the surface of substrate steel .This kind of combination belongs to mechanical combination.
4.2 Fusion combination During hot-dipping copper procedure, the brush plating layer remelt in the function of high temperature, and the liquid copper overcome self-surface tension so as to spread along the steel plate surface. Copper liquid can achieve a good wetting state on solid steel encased in hot liquid due to activation of substrate steel, and the hotter the infiltration copper, the lower the interfacial tension, and the better wettability the cladding material has [4,5]. A thin chilled layer forms on the steel surface when substrate plate contact with hot liquid copper and it remelt under the following heat source. At the same time, skin layer of the steel starts to dissolve, so atom bonding in solid lattice is destroyed, and a thin mixed liquid metal layer clad at the interface in a short period. When the infiltration finish, the hot-dipped steel solidify outside-in to complete the fusion combination. In the subsequent casting process, the similar remelting and recrystallization reoccur. And the effect becomes more intense because of hotter mixed liquid phase and slower cooling speed, so that combination of the bimetal is better.
4.3 Diffusion combination Diffusion bonding is the major composite way in high strength copper/steel compound and it exists in both hotdipping copper and casting process. The elements in liquid and solid phase diffuse sufficiently and free until solidification under the coaction of concentration and temperature gradients. Element distribution status at the transition layer of bimetal interface is showed in Fig3.7. It proves that the elements proliferate amply during the compound process, and iron, manganese, copper and other elements distributed evenly. The diffusion is more obvious at the grain boundary and surface. This is mainly because there are a large number of crystal defects exist on grain boundary and surface, and energy of the defects is higher than other area so that it provides the bigger proliferation driving to diffuse faster and easily. In some way diffusion combination plays an important part in composite molding. 1 5
2 5
s t e e l
s t e e l
FIG 3.7 EDS SPECTRUM OF THE POINTS 1 AND 2 OF FIG3.3 ON INTERFACE
As we known, an oxide film is easily formed on the steel surface especially in high-temperature environment, and high-temperature oxidation is the key problem need to be solved firstly. So we brush copper on the base steel as a result of lower activity of copper atom than iron atom. Meanwhile, activating treatment in Brush plating process promotes the mechanical combination, and it also has a positive effect in succedent hot-dipping and casting process. - 28 http://www.ivypub.org/rms
5 CONCLUSIONS The High strength copper / steel bimetal composite plate has a good macro morphology, and microstructure of its transition layer is continuous and smooth with no inclusions, cracks and 35 ～ 50 μm average thickness. The microhardness of the bimetal interface changes smoothly and continuously, and it is close to their matrix at a distance about 250 μm on both cooper and steel sides. Shearing strength of the interface can reach up to 250 MPa, and characteristics of tearing fracture are mainly dimples distribution. It is a typical feature of toughness, so the interface has formed strong metallurgical bond. The interface behavior during compounding process mainly involves mechanical combination, fusion combination and diffusion combination.
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AUTHORS 1
Yuanhao Zhang (1971-), male, the Han
engineering in Wuhan University of Science and Technology
nationality, Master of engineering, adjunct
from 2009.9 to 2011.6; Studying Material forming and control
professor,” Research direction: material
engineering in Hubei University of Automotive Technology
processing
from 2005.9 to 2009.6. Email: 361813855@163.com
engineering,
Learning
experience: Studying materials processing engineering in Wuhan University of Science and Metallurgy Technology from 1995.9 to 1998.12; Studying casting in Shandong industrial university from 1991.9 to 1995.7. Email: 1020646746@qq.com.
3
Yan Xu (1988-), female, the Han nationality, engineering
master graduate, Research direction: material processing engineering. Learning experience: Studying materials processing engineering in Wu Han University of Science and Technology from 2011.9 to 2014.6; Studying Material Science and
2
Engineering in Hubei University of Automotive Technology
engineering, Research direction: composite material processing
from 2007.9 to 20011.6. Email: melody0803@foxmail.com.
Ping Zhang (1987-), male, the Han nationality, Master of
engineering. Learning experience: Studying materials processing
- 29 http://www.ivypub.org/rms