STUDIES ON TENSILE PROPERTIES AT VARIOUS STRAIN RATES AND CORROSION BEHAVIOR OF PEAK-AGED Al-6 Si-0 . 5 Mg (-0 . 5 Cu ) ALLOYS

This paper focuses on the effect of Cu additions on the tensile properties of the Al-6Si-0.5Mg alloy at various strain rates and electrochemical corrosion behavior. The addition of Cu resulted in an increase in the tensile strength and showed higher strength all over the experimental strain rates. Evaluations of tensile properties at the three different strain rates (10-4, 10-3 and 102 s-1) showed that they affected the tensile properties significantly. The strength was better at higher strain rate but ductility was poor. Electrochemical Impedance Spectroscopy (EIS) and potentiodynamic polarisation have been used to evaluate the corrosion resistance of Cu-free and 0.5 wt% Cu content Al-6Si-0.5Mg alloy in a 0.1 M NaCl solution. The 0.5 wt% Cu addition to the Al-6Si-0.5Mg alloy showed that Cu decreased susceptibility to corrosion compared to the Cu free Al-6Si-0.5Mg alloy. The magnitude of the open circuit potential (OCP), corrosion potential (Ecorr) and pitting corrosion potential (Epit) of the Al-6Si-0.5Mg alloy were shifted to the more noble direction due to the 0.5 wt% Cu addition and thermal modification. ABSTRAK: Kajian ini memberi tumpuan kepada kesan penambahan Cu pada sifat tegangan aloi Al-6Si-0.5Mg di dalam pelbagai kadar keterikan dan tingkah laku kakisan elektrokimia. Penambahan kepada Cu mengakibatkan peningkatan dalam kekuatan tegangan dan menunjukkan kekuatan yang lebih tinggi di seluruh kadar keterikan didalam eksperimen. Penilaian ke atas sifat tegangan pada tiga kadar keterikan yang berbeza (10-4, 10-3 dan 10-2 s-1) menunjukkan bahawa ia memberi kesan kepada sifat tegangan dengan ketara. Kekuatannya adalah lebih baik pada kadar terikan yang lebih tinggi tetapi ductility adalah kurang. Eelectrochemical impedance spektroskopi (EIS) dan polarisasi potentiodynamic telah digunakan bagi membuat penilaian terhadap rintangan kakisan bebas Cu dan 0.5 wt% kandungan Cu Al-6Si-0.5Mg aloi dalam 0.1 M larutan NaCl. Penambahan 0.5 wt% Cu kepada aloi Al-6Si-0.5Mg menunjukkan bahawa Cu mengurangkan kerentanan untuk kakisan berbanding dengan Cu bebas aloi Al-6Si-0.5Mg. Magnitud potensi litar terbuka (OCP), potensi kakisan (Ecorr) dan potensi hakisan pitting (Epit) daripada aloi Al-6Si-0.5Mg telah menunjukkan hasil yang lebih baik kerana penambahan 0.5 wt% Cu dan pengubahsuaian haba.


INTRODUCTION
An excellent aptitude to the casting and forging processes, machinability, corrosion resistance, and high strength-to-weight ratio, which increases performance and fuels economy, make heat treatable aluminium alloys suitable materials for various crucial applications in the automotive industry, such as engine blocks, pistons and cylinder heads [1,2].The high strength levels are achieved by the T6 heat treatment that provides strengthening through the precipitation hardening mechanism [3].T6 heat treatment consists of three fundamental steps: solution, quench, and artificial aging (carried out in furnace).In the solution treatment, the high temperatures promote the homogeneous diffusion of the hardening elements in the matrix.Through the water quench, and subsequent artificial aging, a supersaturated matrix and a precipitation of hardening phases are respectively obtained.In fact, after the quench, the supersaturated matrix forces the constitutional elements to form coherent precipitates reducing the total free energy of the system.These precipitates hinder the dislocation motion resulting in increased yield strength and hardness.However, the final hardness of the alloy results from the heat treatment parameters, such as the time and temperature of the solution treatment, the quenching conditions, and the artificial aging procedure.In the case of Al-Si-Mg casting alloys, the short storage at room temperature between quenching and aging phases, due to the industrial handling of the components, is also one of the crucial parameters affecting the tensile properties of the material [4].This time, known as "preaging", affects the precipitation sequence of metastable phases that occur during the artificial aging.This sequence, for Al-Si-Mg alloys, can be summarised as follows: α(sss) → zone_GP → β''→β' → β (Mg2Si) [5].In it, α (sss) is the supersaturated solid solution, GP zones are coherent Guiner-Preston zones, β'' is the needle shaped precipitates associated with the peak-aged condition, β' is the rod shaped precipitates that form after β'' precipitates in the aging sequence, and β is the equilibrium phase [3,6].For Al-Si-Mg-Cu alloys, the precipitation behaviors are rather complicated and several phases such as β(Mg2Si), θ(CuAl2), S(CuMgAl2) or Q(Cu2Mg8Si6Al5) in metastable situations may exist [7][8][9].
Results of tests on aluminium alloys at different strain-rate levels have been reported by a number of investigators.At room temperature, a very low, yet slightly positive, increase in flow stress with strain rate was reported by [10].Similar observations regarding rate sensitivity of aluminium alloys, now in tension, were reported by investigating the AA7003-T79 and AA7108-T6 alloys [11].On the other hand, the flow stress and fracture strain of AA6005-T6 were shown to have rather strong positive strain-rate sensitivity [12].
The alloys containing copper are the least resistant to corrosion; but this can be improved by coating each side of the copper-containing alloy with a thin layer of high purity aluminium, thus gaining a three-ply metal (Alclad).This cladding acts as a mechanical shield and offers sacrificial protection.When aluminium surfaces are exposed to the atmosphere, a thin invisible oxide (Al2O3) skin forms; this protects the metal from further corrosion in many environments.This film protects the metal from further oxidation unless this coating is destroyed, and the material remains fully protected against corrosion [13][14].A number of studies have been carried out to assess the effect of Cu content and the distribution of second phase intermetallic particles on the corrosion behavior of Al alloys.The distribution of Cu in the microstructure affects the susceptibility to localized corrosion.Intergranular corrosion (IGC) is generally believed to be associated with Cu-containing grain boundary precipitates and the Precipitates Free Zones (PFZ) along grain boundaries.In heat treatable Al-Si-Mg(-Cu) series alloys, the susceptibility to localized corrosion [pitting and/or intergranular (IGC)] and the extent of attack are mainly controlled by the type, amount, and distribution of the precipitates that form in the alloy during any thermal or thermomechanical treatment performed during manufacturing processes.The composition of an alloy and its thermal treatment are important to determine the susceptibility of the alloy to corrosion [15][16][17][18][19][20].Depending on the composition of the alloy and parameters of the heat treatment process, precipitates form in the bulk of the grain, or in the bulk as well as in the grain boundaries [14,21].
In this work, the authors present a study of the tensile properties at various strain rates and electrochemical corrosion behavior of an Al-6Si-0.5Mgalloy with low Cu content.

Materials
Aluminium and aluminium A356 alloy, contained in a clay-graphite crucible, were melted in a gas-fired pot furnace.Copper, in sheet form (99.98% purity), was then added by plunging.Magnesium (99.7% purity) in the form of ribbons, and packed in an Al-foil, was added to the melt.The final temperature of the melt was maintained at 900±15 o C. Before casting, the melt was degassed with solid hexachloroethane (C2H6) and homogenized by stirring at 700 o C. Casting was done in a metal mould measuring 15 mm × 150 mm × 300 mm and preheated to 200 o C. All the alloys were analyzed by wet chemical and spectrochemical methods simultaneously.

Heat Treatment
The cast samples were ground to remove the oxide layer from the surface and were homogenized for 24 hours at 500 o C. Samples for tension tests were prepared from the homogenized plates according to the ASTM standard (Sub-size standard ASTM E8 M-04).The tension test samples were solution-treated at 540 o C for 120 minutes and quenched in an ice-salt-water solution.The solutionized samples were finally aged at 225 o C for 1 hr (peak-aged).

Tension Tests
Tensile testing was carried out in an Instron testing machine at three different cross-head speeds: 0.15, 1.5, and 15 mm/minute, which are equal to the nominal strain rates of 10 -4 , 10 -3 , and 10 -2 s -1 respectively for each alloy.The averages of three consistent test results were accepted as the tensile value for the corresponding sample.Fractographic observations of the fractured surfaces of selected samples were carried out on a scanning electron microscope (SEM).

Potentiodynamic Polarization Measurements
The peak-aged (225 o C for 1 hr) rectangular samples (30 mm × 10 mm × 5 mm) were prepared for metallographic observation and subsequent electrochemical test.Deionized water and an analytical reagent grade sodium chloride (NaCl) were used for the preparation of a 0.1 M solution (simulated seawater).All measurements were carried out at room temperature.A computer-controlled Gamry Framework TM Series G 300™ and Series G 750™ Potentiostat/Galvanostat/ZRA were used for the electrochemical measurements.The potentiodynamic polarization studies were configured in cells, using three-electrode assembly: a saturated calomel reference electrode, a platinum counter electrode, and the sample in the form of coupons with an exposed area of 0.50 cm 2 or 10 mm × 5 mm as a working electrode.Only one 10 mm × 5 mm surface was exposed to the test solution, the other surfaces being covered with Teflon tape.The system was allowed to establish a steady-state open circuit potential (OCP).The potential range selected was -1 to +1 V and measurements were made at a scan rate of 0.50 mV/s.The corrosion current (Icorr), corrosion potential (Ecorr), pitting corrosion potential (Epit), and corrosion rate (mpy) were calculated from the Tafel curve.The tests were carried out at room temperature in solutions containing 0.1 M of NaCl at a fixed and neutral pH value.The corroded samples were cleaned in distilled water and examined under an optical light microscope (OLM) and a scanning electron microscope (SEM).

Electrochemical Impedance Measurements
As in the potentiodynamic polarization test, three electrode cell arrangements were also used in the electrochemical impedance measurements.Rectangular samples (10 mm × 5 mm) were connected with copper wire and adopted as a working electrode.EIS tests were performed in simulated seawater at room temperature over a frequency range of 100 kHz to 0.2 Hz using a 5 mV amplitude sinusoidal voltage.The 10 mm × 5 mm sample surface was immersed in simulated seawater (corrosion medium).All the measurements were performed at the open circuit potential (OCP).The test cells were maintained at room temperature and the NaCl solution was refreshed regularly during the whole test period.The impedance spectra were collected, fitting the experimental results to an equivalent circuit (EC) using the Echem Analyst TM data analysis software and evaluating the solution resistance (Rs), polarization resistance or charge transfer resistance (Rct) and double layer capacitance (Cp) of the thermal treated alloys.

Tensile Properties
The typical UTS-strain rate curves of the alloys at 225 o C are plotted in Fig. 1.Strong work hardening happens during the tensile testing.To compare with the tensile behaviors of the alloys, typical tensile test experiments are conducted at three different strain rates (10 -4 , 10 -3 and 10 -2 s -1 ).Enhancing strain rates results in an obvious increase in fracture strength.When the strain rates are below 10 -3 s -1 , work hardening decreases strongly.Work hardening decreases strongly during the plastic deformation of the sample at 10 -4 s -1 , and a necking phenomenon is observed at this strain rate before fracture.The tensile strength increases more pronouncedly with the increase of strain rates and Cu content.An alloy containing 0.5 wt% Cu, Al-6Si-0.5Mg-0.5Cushows higher ultimate tensile strength all over the strain rates.
Figure 2 indicates the yield strength (0.2% proof strength) vs. strain rates of the alloys.The increase in proof strengths with the strain rates of the alloys is very similar to the ultimate tensile strengths.With the increase of strain rate, the maximum yield strength being attained at 10 -2 s -1 .Cu addition increases the yield strength with strain rates.The intermetallic particles could contribute a reinforcement effect in the Al alloy matrix.The higher yield strength is due to the effect of precipitation hardening and higher strain hardening.
Figure 3 demonstrates the variation of % elongation with the strain rates of the alloys.It is observed that at the strain rate for which strength is maximum (10 -2 s -1 ), the ductility values of the alloys pass through a minima.The ductility value of the Al-6Si-0.5Mg-0.5Cuaged alloy is found to be higher than that of the Cu-free alloy over all strain rates.Fig. 1: Ultimate tensile strength-strain rate curves of investigated peak-aged alloys.
Fig. 2: Yield strength-strain rate curves of investigated peak-aged alloys.
Figures 4 and 5 show the SEM micrographs of the fracture surfaces of the Al-6Si-0.5Mgand Al-6Si-0.5Mg-0.5Cualloy at various strain rates.Cu addition (0.5 wt%) to Al-6Si-0.5Mgalloy increases the number of dimples and their sizes.Cu free Al-6Si-0.5Mgalloy shows the shallower dimples and many transcrystalline facet planes (Fig. 4a) but the 0.5 wt% Cu content alloy (Fig. 5a) increases the depth of the dimples and their number.The deeper dimples at lower strain rate indicate ductile tearing.At the lower strain rate (10 -4 s -1 ) the dimples are larger and deeper than the higher strain rates' (10 -3 s -1 , 10 -2 s -1 ) tensile testing samples.The cleavage facets (Fig. 5b) are mainly created at high strain hardening tensile testing.

Electrochemical Corrosion Behavior
Table 1 shows the Electrochemical Impedance Spectroscopy (EIS) test results.The open circuit potential (OCP) with exposure of aged Al-6Si-0.5Mg(-0.5Cu) alloys to simulated seawater is shown in Table 1.The steady state OCP of the Cu-free alloy is -0.8454V and it is the higher negative OCP value between the alloys under

Impedance Measurements
The data obtained was modeled and the equivalent circuit that best fitted the experimental data is shown in Fig. 6.Rs represents the ohmic solution resistance of the electrolyte.Rct and Cp are the charge transfer resistance and electrical double layer capacitance respectively, which correspond to the Faradaic process at the alloy/media interface.Figure 7 shows the Nyquist diagrams (suggested equivalent circuit model shown in Fig. 6) of the Al-6Si-0.5Mg(-0.5Cu) alloys in simulated seawater.In the Nyquist diagrams, the imaginary component of the impedance (Z") against real part (Z') is obtained in the form of a capacitive-resistive semicircle for each sample.The solution resistance (Rs) of the alloys varies from 40-44 Ω (Table 1) and there are insignificant changes of Rs values for the alloys during EIS testing.The Rs values are negligible with respect to Rct and the electrolyte behaves as a good ionic conductor.Impedance measurements showed that in simulated seawater, the addition of 0.5 wt% Cu in the Al-6Si-0.5Mgalloy increases the charge transfer resistance (Rct).For the Cu-free Al-6Si-0.5Mgalloy, the charge transfer resistance (Rct) value in simulated seawater is 15.57kΩ, and this is increased to 25.75 kΩ with the addition of 0.5 wt% Cu into the Al-6Si-0.5Mgalloy.The increase in the charge transfer resistance indicates an increase in the corrosion resistance of the Al-6Si-0.5Mgalloy with Cu addition.The double layer capacitance (Cp) of the Cu-free Al-6Si-0.5Mgalloy is 1.259 µF, which is the lower value between the alloys investigated.The double layer capacitance of Al-6Si-0.5Mgalloy increased to 1.793 µF with the addition of 0.5 wt% Cu.
Figure 8 shows the experimental EIS results in a Bode magnitude diagram for the Al-6Si-0.5Mg(-0.5Cu) alloys.Bode plots show the total impedance behavior against applied frequency.At high frequencies, only the very mobile ions in solutio n are excited so that the solution resistance (Rs) can be assessed.At lower-intermediate frequencies, capacitive charging of the solid-liquid interface occurs.The capacitive value Cp can provide very important information about oxide properties when passivation or thicker oxides are formed on the surface.At low frequency, the capacitive charging disappears because the charge transfer of electrochemical reaction can occur and this measured value of the resistance corresponds directly to the corrosion rate.For this reason, this low frequency impedance value is referred to as polarization or charge transfer resistance (Rct).Table 2 shows the potentiodynamic polarization test results obtained from the electrochemical tests.The potentiodynamic polarization curves for the Al-6Si-0.5Mg(-0.5Cu) alloys in simulated seawater are shown in Fig. 9.The anodic current density of the Al-6Si-0.5Mgalloy decreased with the Cu addition.This is caused by the slowing of the anodic reaction of the Al-6Si-0.5Mg-0.5Cualloy.The addition of Cu caused the formation of micro-galvanic cells in the α-aluminium matrix.The different intermetallic compounds (like Mg2Si, Al2Cu etc.) led to the formation of microgalvanic cells because of the difference of corrosion potential between the intermetallics and the α-aluminium matrix.It was well known that the addition of Cu increased the corrosion potential of a number of Al-Cu-Si alloys.For the Cu-free Al-6Si-0.5Mgalloy, the corrosion potential was -764 mV, which was the higher negative potential between the alloys investigated.With the addition of 0.5 wt% Cu, the corrosion potential of the Al-6Si-0.5Mgalloy shifted towards more positive values.Pitting potential (Epit) of the 0.5 wt% Cu content alloy also shifted towards more positive values (from -480 mV to -408 mV).Potentiodynamic tests showed that in simulated seawater, the addition of Cu into the Al-6Si-0.5Mgalloy decreased the corrosion current (Icorr).For the Cu-free Al-6Si-0.5Mgalloy, the corrosion current (Icorr) value in simulated seawater was 6.3 µA, and this decreased to 5.640 µA with the addition of the 0.5 wt% Cu to the Al-6Si-0.5Mgalloy.The corresponding corrosion rate decreased for the alloy (4.732 mpy) as well.
The microstructure of some selected as-corroded samples was observed under and scanning electron microscope (SEM).There was evidence of the corrosion products of the intermetallic compounds in all the samples examined.Besides, several pits were visible in all the samples examined.It is probable that the pits were formed by the intermetallics dropping out from the surface due to the dissolution of the surrounding matrix.However, it is also possible that the pits are caused by selective dissolution of the intermetallic particles of the second phase precipitates.The forms of the corrosion in the studied Al-6Si-0.5Mg(-0.5Cu) alloys are not completely uniform and are predominantly comprised of pitting corrosion as obtained by the SEM.Samples were characterized by SEM following the potentiodynamic polarization tests.The peak-aged Cu-free Al-6Si-0.5Mgalloy exhibited pits on their surface (Fig. 10), that had apparently nucleated randomly.Conversely, the exposed surface of the alloys exhibited a corrosion product covering the surface after polarization.There are more pits in the Cu-free Al-6Si-0.5Mgalloy compared to the Al-6Si-0.5Mg-0.5Cualloy.All the SEM micrographs (Fig. 10a and Fig. 10b) also showed that there was no corrosion in the fragmented and modified Al-Si eutectics.

CONCLUSIONS
The following conclusions could be drawn from the above results and discussions.
 The addition of 0.5 wt% Cu to the thermally treated Al-6Si-0.5Mgcast allo y resulted in improved tensile properties.The strain rate affects the ultimate tensile strength, yield strength, and ductility.The ultimate tensile strength and yield strength increased with decreasing strain rate but ductility decreased for the alloys.The Al-6Si-0.5Mg-0.5Cualloy shows better strength and improved ductility over all strain rates.
 The magnitude of the charge transfer resistance (corrosion resistance) value as an impedance parameter, increased with the addition of 0.5 wt% Cu into Al-6Si-0.5Mgalloy.The electrochemical parameters obtained from polarization curves showed that the corrosion current (Icorr) and corrosion rate (mpy) decreased with the addition of 0.5 wt% Cu into Al-6Si-0.5Mgalloy.The open circuit potential (OCP), corrosion potential (Ecorr) and pitting corrosion potential (Epit) were shifted in the more noble direction due to 0.5 wt% Cu addition into Al-6Si-0.5Mgalloy.The microstructures studied indicate the pitting corrosion in the investigated alloys.