Trans. Nonferrous Met. Soc. China 25(2015) 1847−
1855
Evaluation of wear and corrosion resistance of pure Mg wire produced by friction stir extrusion
Mohammad SHARIFZADEH 1
罗技摄像头, Mohammad ali ANSARI 1, Morteza NARV AN 1, Reza ABDI BEHNAGH 1,2, Alireza ARAEE 1, Mohammad Kazem BESHARATI GIVI 1
地府加点1. School of Mechanical Engineering, College of Engineering, University of Tehran, Tehran, Iran;
2. Faculty of Mechanical engineering, Urmia University of Technology, Urmia, Iran
去粉刺印Received 16 July 2014; accepted 6 May 2015
Abstract: A solid-state process —friction stir extrusion (FSE) was applied to produce wires from Mg chips. The FSE process was performed using tool rotational speed of 150, 250 and 355 r/min at a constant plunge rate of 20 mm/min. The microstructural evolution, tribological behavior and corrosion resistance of the reference specimen and the friction stir extruded specimens were investigated. Microstructural characteristics of the specimens were investigated by optical microscopy (OM) and scanning electron microscopy (SEM). The evaluations of mechanical properties include microhardness and dry sliding wear test. The corrosion resistance of the extruded specimens was characterized by potentiodynamic polarization test. The results show that the extruded specimens posss good surface quality and the process is beneficial for the improvement of hardness and wear resistance of the first machined chips. The produced wires are also found to h
ave adequate corrosion resistance. The results demonstrate that FSE is an effective strategy for converting the machined Mg chips into the usable wires. Key words: friction stir extrusion; Mg wire; friction; wear; hardness; corrosion
1 Introduction
Mg and its alloys are widely ud in automobile
industry, aerospace and structural components becau of
their excellent specific strength, low density, great mechanical properties, good machinability, high specific
stiffness, cold formability, good castability, and thermal
conductivity [1]. Since Mg has great machinability, large
quantities of Mg chips are produced at various machining process. Recycling process of Mg involves
traditional and non-traditional methods. In traditional
recycling, large amounts of scraps and chips are re-
melted consuming a large amount of energy. Due to the
susceptibility of Mg to oxidation, and the consumption of
a lot of energy and labor cost, this method is not
affordable. In non-traditional recycling of Mg, propod
by CHINO et al [2], a direct approach without condary
operation is implemented [3]. By this method, Mg chips
are compresd and the plastic deformation occurs during the consolidation process [4,5]. Non-traditional
methods are more efficient than traditional ones in terms
of air pollution (green recycling) and energy consumption. Furthermore, the specimens produced by solid-state recycling process show high strength becau of small grain size and homogeneous ox
ide dispersion. Some works on solid-state recycling of Mg alloys accompanied by AZ31 [1,3,5−7], AZ91 [4,8−10], ZK60 [11] and pure Mg [12], resulting in better
mechanical properties and grain refinement compared with the primary ingot. The combination of solid-state recycling and friction stir welding was developed by friction stir extrusion (FSE) in The Welding Institute
(TWI) in 1993 [13]. The FSE process follows the principles of the friction stir welding/processing (FSW-FSP), in which a rotating tool caus local softening of material through heat generated by friction and plastic work [14]. FSE is a solid-state material synthesizing process that produces extruded products by direct conversion of scraps into advanced bulk materials through mechanical and thermo-mechanical processing in a single step. Solid-state recycling eliminates the energy consumption pertinent to re-melting which is
necessary in the conventional synthesis process. Different materials such as Mg, Al, Ti and steel have potentials to be ud in this method [15−17]. Recently,
Corresponding author: Reza ABDI BEHNAGH; Tel: +98-0443-3728180; Fax: +98-0443-1980251; E-mail: r.abdibehnagh@mee.uut.ac.ir DOI: 10.1016/S1003-6326(15)63791-8
Mohammad SHARIFZADEH, et al/Trans. Nonferrous Met. Soc. China 25(2015) 1847−1855 1848
TANG and REYNOLDS [17] have investigated the synthesis process of AA2050 and AA2195 Al by FSE. They indicated that the microstructure of the extruded wires consisted of fully equiaxed and recrystallized grains. They concluded that the limits on the process appeared to be related with the extrusion temperature [17]. Recently, the FSE process of AA7277 Al alloy chips has been studied by ABDI-BEHNAGH et al [18] experimentally. They showed the formation of various zones in the cross-ction of the extruded wires, and the wire microstructure was characterized by equiaxed and recrystallized grains in the center of specimen. The tensile tests revealed that the mechanical properties of the extruded wire were comparable with the parent material in particular rotational speed.
Even though the investigations thoroughly demonstrate the potential of this process to emerge as an important recycling technique, however, not much is known about the wear and corrosion characteristics. As mentioned before, Mg offers a high potential for u as a lightweight structural material in transport applications such as automotive and motorbike. Since the may be subjected to rious environments, their corrosion behavior is a major concern. Mg is a naturally passive metal. Pitting corrosion will occur at free corrosion potential of Mg when expod to chloride ions in a
non-oxidizing medium [19]. It is generally obrved that corrosion pits initiate at flaws adjacent to a fraction of the condary pha particles such as Mg17Al12 as a result of the breakdown of passivity [19]. Moreover, Mg alloys have not traditionally been ud for high performance applications due to their low hardness and wear resistance.
Therefore, rearches in this direction are much needed to enhance the suitability of FSE for a wide range of industrial applications. Hence, the aim of this study is to investigate the corrosion and wear characteristics of the fully-consolidate Mg wires fabricated by FSE.
2 Experimental
The clean and dry Mg chips ud in this work was produced through machining Mg ingot without any lubricant fluid. The machining chip was prepared by a planning machine. The chip was 6−10 mm in length, 1−4 mm in width and about 0.2 mm in thickness. The chemical composition of the pure Mg ingot is <0.01% Al, 0.006% Cu, 0.03% Mn, 0.005% Zn, 0.005% Ca, <0.002% Sn and balanced Mg (mass fraction). The main components ud in the process including a container and a rotating plunge die with scrolled face, which were made of H13 tool steel. The container cavity is 21 mm in diameter and 50 mm in height. The rotating plunge die has a 20 mm outer diameter with a 5 mm cen
tral hole which defines the diameter of the extruded wire. As shown in Fig. 1, the rotating plunge die has a scrolled face pattern with zero tilt angle. The scrolled face pattern is similar to the FSW tool shoulder design [17]. The container was fixed on the table of computer numerical control (CNC) milling machine. The rotating plunge die rotates in a clockwi direction and moves toward the container which is charged with Mg chips. The rotation and movement of the plunge die relative to the container cau the mixing and stirring of Mg chips, during which the contact and pressure between the rotating plunge die and Mg chips lead to the conversion of mechanical energy to thermal energy. The wire length is limited by the volume of the container. The extrusion axis is Z-axis of the machine, therefore, the extrusion force is equal to Z-force. Three rotational speeds of 180, 250 and 355 r/min under constant plunge rate of 120 mm/min were implemented. An infrared thermometer was ud to monitor the temperature changes during the process. The temperature measurement is needed for the thermal analysis of the process. The schematic reprentation of the FSE process is shown in Fig. 2.
The surface roughness (R a) of the fabricated wires
Fig. 1 Components ud in FSE process
Fig. 2 Schematic reprentation of FSE process花园设计效果图
Mohammad SHARIFZADEH, et al/Trans. Nonferrous Met. Soc. China 25(2015) 1847−1855 1849
in the axial (extrusion) and traver directions was performed using a WYKO NT1100 optical profiling system. R a is ud as a roughness parameter.
The microstructures of the FSE samples were investigated by optical microscopy (OM). The microstructures of the cross-ctions of the wire were investigated perpendicular to the extrusion direction. The metal micro etch and specimen preparation for OM test were done bad on ASTM E407-07 [20] and ASTM E3-11 [21], respectively. The polish was performed with a 1000 grit Al2O3 p
aper, and then with a 1 mm diamond paste. The final polish was done by colloidal silica with 40 nm in diameter. No scratch was obrved on the polished surface. After polishing, the specimens were etched by acetic picral (5 mL acetic acid, 6 g picric acid, 10 mL water and 100 mL ethanol) at room temperature. The Vickers microhardness test was performed perpendicularly to the extrusion direction on the cross-ction of the wires, with a load of 0.25 N holding for 15 s. The sliding wear behavior of the wire was evaluated with a reciprocating wear tester. The dry wear test was carried out at room temperature by applying a normal load of 20 N to the surface of the sample with hardened alloy steel (13 mm in diameter). Along the test, the sliding speed of the cylinder on the surface of the sample was 0.25 m/s for a total sliding distance of 1000 m.
The localized electrochemical measurement was performed on the cross-ction of the samples in order to combine the microstructural analysis and the changes in corrosion susceptibility between the parent material and the FSE specimens. Potentiodynamic polarization is a test responsible for pitting, which can investigate the material behavior in various environments with different pH levels. It is a suitable and fast method to obtain potential pitting, corrosion rates and can give reproducible results. The electrochemical method ud was Tafel polarization test. The samples were mechanically polished with a 1 μm diamond paste to a mirror finish. The electrochemical potentiodyn
amic measurement was performed in a three-electrode cell arrangement. A platinum mesh with the geometric area of about 20 cm2 was ud as the counter electrode, while all potentials were measured with respect to a commercial saturated calomel electrode (SCE). The electrochemical experiment was carried out using a potentiostat/ galvanostat (Autolab 302N) run by PowerSuite software. The potentiodynamic curves were recorded at a scan rate of 0.3 mV/s in the potential range of −250 mV (vs OCP) to +700 mV (vs SCE).
3 Results and discussion
3.1 Thermal analysis
The temperature variation during the FSE process was monitored by an infrared thermometer pointed at the outer surface of the container. The purpo of this monitoring is to investigate the effect of temperature variation on the mechanical and metallurgical properties of the extruded wires. The temperature versus time curves of the FSE process during extrusion are shown in Fig. 3. The thermal energy in the FSE process is generated by the contact friction between the chips, and also the chips and die components by the inrtion of vertical load and the die rotation. The amount of heat affects the average grain size and the surface quality of the produced wires. The temperature m
easurement indicates that the maximum temperature during the process is 0.8T m with the rotation speed of 355 r/min, where T m is the melting temperature.
Fig. 3 Temperature variation vs time during FSE process
In the FSE process, the tool rotation results in the stirring of the material in the container which in turn increas the temperature of the metal. The incread temperature as a result of increasing rotational speed leads to the softening of the material and amends the plastic deformation and conquently eas the extrusion. However, excessive temperature ri paves the way for grain growth, leading to the aggravating of the mechanical properties of the specimen, and facilitating the formation of cracks [18]. On the other hand, insufficient temperature input results in the formation of voids, cold cracks, and twisting in the obtained wire [18]. So, a proper lection of the tool rotational speed as the input parameter is very important to obtain defect-free wires.
3.2 Microstructural analysis
The surface appearances of the wires produced by the FSE process are shown in Fig. 4. Good and poor surface quality of wires are obrved. The most important parameter affecting the process is the rotational speed [18]. Low rotational speed leads to
Mohammad SHARIFZADEH, et al/Trans. Nonferrous Met. Soc. China 25(2015) 1847−1855 1850
insufficient heat generation during the process which increas the possibility of the extrusion defects. As shown in Fig. 4, there are twisting and cold tearing along with the wire surface at the rota
tional speed of 180 r/min. However, at the rotational speeds of 250 and 355 r/min, the entire lengths of the wires are with smooth surface and without any cracks. Moreover, the results show that the surface roughness is minimum at the rotational speed of 355 r/min, with the R a values in the extrusion and traver directions are 0.23 and 1.41 µm, respectively.
The optical image of the parent material is shown in Fig. 5. It is apparent that large grains are distributed in the parent material with the size of veral millimeters, whereas the extruded specimens show relatively fine grains. The comparison of the microstructure views
Fig. 4 Surface appearances of FSE samples between the parent material and the wires in Figs. 5 and 6 proves that the grain refinement happens during the FSE process. In the FSE process, dynamic recrystallization that occurs during the deformation leads to the generation of fine grains. However, the average grain size is probably affected by the heat amount factor which is the direct result of the die rotation speed. The average grain size in the center of each specimen is
Fig. 5 Optical micrograph of Mg ingot
打电话的英文
Fig. 6 Optical micrographs of wires produced at different rotational speeds: (a, b) 180 r/min; (c, d) 250 r/min; (e, f) 355 r/min
Mohammad SHARIFZADEH, et al/Trans. Nonferrous Met. Soc. China 25(2015) 1847−1855 1851 shown in Table 1, which was measured by the mean
linear intercept method. The average grain size in the
FSE specimen ranges from 9 µm to 27 µm, which is
much smaller than that of the ba material (BM).
Moreover, the two optical microscopy images for each
鸡西大冷面sample show that the microstructure is not uniform along
the wire. As shown in Fig. 6, in all specimens, the grains
in the center are larger than tho in the boundary of the
wires. This may be attributed to the higher strain rate in
the center regions.
Table 1 Average grain size of FSE samples
Sample No. Rotational speed/
(r·min−1)
Plunge rate/
(mm·min−1)
Average grain
size/µm
1 180 20 15
2 250 20 9
3 355 20 27 3.3 Hardness and wear performance
For the BM, the average hardness value is approximately HV 25. A fine-grained material (one that has small grains) is harder and stronger than the coar- grained one, since the former has a greater total grain boundary area to impede the dislocation motion [22]. The Vickers microhardness profiles of the cross-ction of the wires perpendicular to the extrusion direction are shown in Fig. 7. It can be en that the hardness value along the diameter of the wire is higher than that of the BM. The relationship between the room-temperature hardness H and the average grain size D is reprented by Hall–Petch equation [22]:
H=H0+kD−1/2 (1) where D is the average grain diameter, H0 and k are the constants for a particular material.
It is suggested from Eq. (1) that the FSE specimens, when D is reduced to around or less than 5 µm, posss higher performance of the room-temperature hardness
表格法
Fig. 7 Microhardness profiles of BM and produced wires compared with the first machined chips.
Figure 8 shows the variation of friction coefficient with sliding distance (SD) for the BM and the FSE samples. The average friction coefficient of the BM is found to be 0.411 as indicated by high fluctuations in the friction curves. The minimum value of friction coefficient is about 0.106 that is achieved at the rotational speed of 355 r/min, which is clearly much lower than that of the BM. From Fig. 8, it is found that the average friction coefficient of 180 r/min sample is significantly higher than tho of 250 and 355 r/min samples. This result was not unexpected due to the lower hardness values of 180 r/min sample shown in Fig. 7.
门字开头的成语
Fig. 8 Variation of friction coefficient of BM (a) and wires produced at rotational speed of 180 r/min (b), 250 r/min (c) and 355 r/min (d)
