Influence of heat treatment on the machinability and corrosion behavior of AZ91

Sweth Chowry V,Rvikumr Dumpl,Ann Kumr S,Konih VV,Rtn Sunil B,∗

aDepartment of Mechanical Engineering,Rajiv Gandhi University of Knowledge Technologies(AP-IIIT),Nuzvid,521202,India

bDepartment of Mechanical Engineering,Visvesvaraya National Institute of Technology,Nagpur,Maharashtra 440010,India

cDepartment of Mechanical Engineering,Dayananda Sagar University,Bengaluru,560068,India

dDepartment of Mechanical Engineering,Tirumala Engineering College,Narasaraopet,Andhra Pradesh 522601,India

1.Introduction

Magnesium(Mg)alloys are now becoming attractive alternate light metals for structural applications due to low density and high specific strength compared with the aluminum alloys[1,2].AZ(aluminium and zinc)series alloys are the most widely used group of Mg alloys in the manufacturing industry.Among them,AZ91 is a die cast Mg alloy which is widely used in automobile and aerospace industries.Basically,AZ91 Mg alloy contains two phases known asα-phase which is a solid solution of Mg and Al;andβ-phase(Mg17Al12)which is a compound made of Mg and Al[3,4].

Machining is one of crucial manufacturing processes which is widely carried out to produce structural members or components.Machining of magnesium is not difficult;however a careful attention must be exercised during the machining of Mg due to its inflammability nature.Unless the temperature reaches to high enough,where the chip catches fire,the inflammable nature of Mg does not create any complexity during machining.However,the formation of flank built up edge;higher cutting forces and surface roughness are the other critical issues,which require more attention[5-8].Birol Akyuz[9]reported lower cutting forces for AZ91 Mg alloy compared with AZ21 alloy during turning of AZ series Mg alloys.Lu et al.[10]reported the influence of cutting speed on the depth of affected layer thickness during turning of AZ31 Mg alloy.In our earlier study,a significant role of aluminium content on machining of AZ series Mg alloys during drilling was observed[11].In AZ series Mg alloys,the presence ofβ-phase influences the corrosion behavior to a great extent.The machinability of Mg alloys is also influenced by the amount and distribution of theβ-phase.In the materials engineering,heat treatment is a promising technique to alter the microstructure in order to achieve the desired phases,and its volume fraction to alter the bulk behavior.Through heat treatment technique,super saturated alloys with required phase distribution and properties can be obtained.Therefore,an attempt has been made in the present study,to tailor the microstructure in order to achieve the super saturated AZ91 Mg alloy through heat treatment technique.The effect of microstructure on machining characteristics during turning and also corrosion behavior were investigated.

2.Experimental details

AZ91 Mg alloy(8.6%Al,0.85%Zn,0.002%Fe,0.03%Mn and remaining being Mg by Wt.%)cast billet was obtained from Exclusive Magnesium,Hyderabad.Specimens of size 10mm×10mm×10mm and rods of diameter 25mm with a length of 150mm were cut from the billet.Heat treatment was carried out at 410°C for 6,12 and 24h.Initially,the specimens were placed in the furnace and slowly heated up to 410°C with a heating rate of 5°C/min.The samples after different intervals of time were taken out from the furnace and subsequently quenched in water.The specimens were coded as 0h,6h,12h and 24h for untreated,heat treated for 6,12 and 24h respectively.Microstructural observations were carried out(before and after heat treatment)using a polarized optical microscope(Leica DMI5000M,Germany).All the samples were metallographically polished by using different grades of emery sheets followed by alumina polishing and then finally cleaned with ethanol.Then the polished samples were etched with picric acid reagent for 20s.Average grain sizes were measured for all the samples by linear intercept method[12].Microhardness measurements were carried out by Vickers indentation method(MHT-Smart,Omnitech,India)by applying 100g load for 10s.Indents were placed across the cross section of a representative specimen cut from the rods before and after heat treatment.Measurements were obtained for every 1mm.

Machining experiments on the specimens were conducted using a lathe machine(HMT,India)mounted with tungsten carbide cutting tool.The workpiece was fixed in the three jaw chuck and the cutting tool was fixed on a dynamometer(Kistler 9403,Switzerland),which was fixed on the tool post of the lathe machine.Tailstock was used to give support to the workpiece with the help of dead centre.Fig.1(a)shows the photograph obtained during the machining and Fig.1(b)shows the photograph of the rods after machining.The cutting parameters(speed in rpm,feed rate in mm/r and depth of cut(DOC)in mm)were selected in such a way that one is lower value and the other is higher value.Turning operations were carried out at two different cutting speeds(325 and 715rpm),two different feed rates(0.16 and 0.04mm/r)and two DOCs(0.1 and 0.4mm).Turning experiments were done on dry condition and cutting forces were recorded with the help of the dynamometer.The measurement of cutting forces was started a few seconds before the cutting tool touched the workpiece and continued up to a few seconds of machining for all the samples.After machining,chips were carefully collected and morphology of the chips was observed for every set of cutting parameters.

Electrochemical tests were carried out using 3.5%NaCl solution as the electrolyte(IVIUM Soft,Netherlands).During the experiments,platinum rod was used as a counter electrode and a saturated calomel electrode(SCE)was used as a reference electrode with the specimen(area 1cm2)as working electrode.All the specimens were polished using emery papers up to 2000 grade to obtain the same level of surface roughness before the experiments.Open circuit potential(OCP)was established for 30min and then with the reference to OCP,the potential scanning range was fixed for all the specimens.The experiments were conducted at a scan rate of 5 mVs-1.Corrosion current density(icorr)and corrosion potential(Ecorr)were obtained from the polarization plots using Tafel extrapolation method[13].The corrosion rate(CR)was calculated using the Eq.(1)[13].

where CR is the corrosion rate,ais the Molar mass(for magnesium 24.3g/mol),icorris the corrosion current density in μA/cm2,nis the valance andDis the density(1.74 gm/cm3).The obtained CR was converted into mm/year by considering 1 mils/year equal to 0.0254mm/year.

Fig.1.a)Photograph showing the workpiece during turning operation and b)photographs of machined specimens.

Fig.2.Optical microscope images of the samples:a)0h,b)6h,c)12h and d)24h.

3.Results and discussion

The optical micrographs of the specimens as shown in Fig.2 clearly demonstrate the effect of heat treatment on altering the microstructure.Usually in AZ series Mg alloys,solid solution of magnesium and aluminum is formed,when the aluminum content is less than 1%.Mg17Al12(β-phase)appears at the grain boundaries if the aluminum content is more than 1%at room temperature.The solubility of aluminum can be increased with respect to the temperature,and at 437°C,the maximum solubility of 12.6%can be achieved[3].The solid solution of magnesium and aluminum givesα-grains.In AZ91 Mg alloy,the presence ofβ-phase as a network like structure can be clearly seen at the boundaries ofα-grains as shown in Fig.2(a).Adjacent to theβ-phase regions,mixture ofαandβregions(eutectic regions)can be seen as indicated by black arrows in Fig.2(a).After 6h of heat treatment,the microstructure was greatly affected as shown in Fig.2(b).The eutectic regions which contain a mixture ofα+βregions were completely disappeared.The reason behind the disappearance of these eutectic regions after 6h of heat treatment is the increased solubility limit at 410°C.A fraction ofα-grains immediately next to theβ-phase is super saturated with more aluminum as indicated by white arrows in Fig.2(b).The microstructure after 12 and 24h of heat treatment also shows the increased solubility of aluminium.The amount and the distribution ofβ-phase was significantly altered due to the heat treatment.Interestingly,after 6 and 12h of heat treatment a distinct boundary can be seen which was formed due to the diffusion of aluminium fromβ-phase intoα-grains[4]as indicated by black arrows in Fig.2(b)and(c).In 24h heat treated specimen,the aluminium rich regions were observed as widen and a diffused barrier was appeared as indicated by thin black arrows in Fig.2(d).Manyα-grain boundaries were observed as free fromβ-phase.From the microstructural observations,it can be clearly understood that the heat treatment has a greater effect on reducing the amount and distribution ofβ-phase and results in generating super saturatedα-grains.The average grain size was measured as 166.6±5.5 Hv,169±9.2 Hv,170±8.1 Hv and 173.7±6.8 Hv for 0h,6h,12h and 24h samples respectively.The average grain size was observed as marginally increased after heat treatment but the difference is insignificant.

Fig.3.Microhardness measurements of the samples:a)hardness distribution and b)average hardness.

Fig.4.Schematic representation of cutting forces produced during turning operation.

The hardness measurements of all the specimens at different heat treatment conditions are shown in Fig.3.The distribution of hardness,demonstrates a large variation in the measurements of 0h sample.This is due to the presence of three different regions(α,β,andα+βzones).Among the measurements,least hardness was observed when the indent was placed onα-grains(86.3 Hv)and highest hardness was observed when the indent was placed onβ-phase(133.5 Hv).Therefore,the large variation in the measured hardness values can be attributed to the combination of these regions.The variation within the heat treated samples was found to be reduced after 6h of heat treatment.Further,the variation was reduced after 12h and 24h of heat treatment.This can be understood clearly by referring to the microstructure of the samples.

The decreased hard and brittle phase(Mg17Al12)and the eutectic regions played a major role on reducing the variation in the hardness of heat treated samples.As AZ91 Mg alloy contains different phases(hard and soft)at different regions,the hardness variations are obvious as demonstrated in our earlier study[11].When the measurement was done onβ-phase,higher hardness was observed.Whereas the indent was placed on eutectic region or theα-phase,lower hardness was measured.Therefore,variations within the hardness were more for unprocessed AZ91.In the present study,β-phase and eutectic regions were decreased and theαgrains became super saturated after heat treatment.Hence,the variations were observed as decreased after heat treatment as shown in Fig.3(b).Amount of secondary phase,formation of super saturated grains and change in average grain size are the important factors which influence the hardness after the heat treatment.Reduced amount of secondary phase and grain growth decrease the hardness and formation of super saturated grains increases the hardness.The overall combined effect of these three factors resulted hardness to marginally increase after the heat treatment.This result indicates that the contribution from the formation of super saturated grains was higher to increase the hardness compared with the other two factors to decrease the hardness.

Fig.5.Comparison of typical cutting force(Fz)profiles obtained at 0.4 DOC and 0.16mm/r feed rate during turning at 325rpm:a)0h,b)6h,c)12h and d)24h.

Fig.6.Cutting force measurements obtained at 0.1mm DOC and 0.04mm/r feed rate:a)Feed force(Fx),b)thrust force(Fy)and c)cutting force(Fz).

Hardness of the material influences the cutting characteristics.In addition hardness distribution within the material also influences the cutting forces and type of chip formation during machining.Therefore,the changes in the hardness variations within the samples certainly influence the machinability particularly,cutting forces.Fig.4 shows schematic representation of typical cutting forces produced during the turning operation.

Fig.5 shows typical cutting force(Fz)profiles of the samples obtained at 0.16 feed rate and 0.4 DOC machined at 325rpm speed.Fig.6 shows feed force(Fx),thrust force(Fy)and cutting force(Fz)obtained while turning at 0.1 DOC,0.04mm/r feed rate with 325 and 715rpm.Interestingly,all the forces were increased for 6h samples and further decreased for 12 and 24h samples.After 6h of heat treatment,theα-grains became super saturated with aluminium and possibly led to lattice distortion[14,15].Machining is considered as severe plastic deformation process,in which chips are produced due to shear with an aid of a sharp edge known as cutting edge which is made to move against the workpiece as shown in Fig.4[16].The required force to shear and to produce chips from the workpiece was increased for 6h samples compared to 0h samples.The reason behind the increased cutting forces can be attributed to solid solution strengthening ofα-grains.Similar trend was noticed at both the cutting speeds(325 and 715rpm).

Fig.7.Photographs of chips obtained at different cutting parameters.

The cutting forces were observed as reduced for 12h and 24h samples in spite of increased hardness compared with 0h and 6h samples.12h and 24h samples became super saturated after heat treatment that led to introduce lattice distortion.The level of lattice distortion is increased with an increase in the amount of the solvent which further promotes brittleness.In the present study,more amount of aluminum was dissolved into magnesium in the basichcpcrystal structure of 12h and 24h samples.Therefore,due to the difference in the atomic radius between aluminum and magnesium,the lattice distortion was increased with the increased solubility of aluminum that promoted higher hardness[14]and introduced brittleness in 12h and 24h samples.Additionally,the amount of hard intermetallicβ-phase was also decreased for 12h and 24h samples compared with the other samples.Hence,the required cutting force to shear the material was decreased while machining 12h and 24h samples.Similar trend was observed for 12h and 24h samples compared with 0h and 6h samples with the other cutting parameters.

Fig.7 shows the photographs of the chips collected after turning with different cutting parameters.Continuous chips were produced during turning of 6h samples compared with the other samples when the feed rate was 0.04mm/r.In the case of other samples,combination of continuous and discontinuous chips was noticed at different parameters.Interestingly,for 0.4mm DOC and 0.04mm/r,the chips were observed as continuous for all the cases.It can be understood that the speed played an important role on heat generation during turning operation and also formation of continuous chips[10,17]which is the reason why many times continuous chips were produced when speed was increased from325 to 715rpm for all the samples.Whereas at lower speed(325),majority times,discontinuous chips were produced particularly for 12 and 24h samples.From the results,it can be demonstrated that the heat treatment led to develop super saturatedα-grains in AZ91 Mg alloy and the machinability was observed as increased considering the cutting forces.Whereas from the information on type of chips formed during cutting,it can be understood that the machinability is higher for 6h heat treated specimens compared with other specimens.

Table1 Electrochemical parameters and corrosion rate of the samples.

Fig.8.Potentiodynamic polarization curves of the specimens.

Fig.8 shows the potentiodynamic polarization curves of all the samples.Electrochemical test parameters were obtained from the Tafel slope and corrosion rate was calculated and listed in Table 1.All heat treated samples showed higher corrosion current density(icorr).The sample heat treated for 6h shows a noble behavior in corrosion potential(Ecorr)buticorrwas higher compared to 0h sample.Similarly,highericorrvalues were measured for 12 and 24h samples.From the corrosion rate calculations,the poor corrosion resistance of heat treated samples can be clearly seen.In the presence of secondary phase such as Mg17Al12at the grain boundaries of AZ series Mg alloys,localized galvanic couple is established between theα-grains and the secondary phase(β-phase)and galvanic corrosion is inevitable[18].Pitting corrosion is another common mechanism,which is responsible for the degradation of magnesium alloys particularly in the presence of chloride ions[18].Combination of these two mechanisms aggravates the corrosion rate of magnesium alloys.In the present study,the amount ofβ-phase was decreased with respect to the heat treatment soaking time.However,from the corrosion studies it is clear that the corrosion resistance was decreased for heat treated samples which have lower amount ofβ-phase.The distorted lattice of super saturatedα-grains played predominant role in increasing the corrosion.It can be understood that the contribution from the super saturated grains to elevate the corrosion rate was higher compared with the reduced galvanic corrosion with lower amount ofβ-phase.Therefore,the synergetic effect was significant to degrade the corrosion resistance after the heat treatment.Hence from the present study,it is clearly demonstrated that the machinability of AZ91 Mg alloy can be improved with microstructural modification particularly by producing super saturated grains and reducing the secondary phase by heat treatment.However,if the structures or components which are intended to work in the environment where corrosion susceptibility is higher,heat treatment of AZ91 Mg alloy is not advised as it deteriorates the corrosion resistance.

4.Conclusions

AZ91 Mg alloy was heat treated for 6,12 and 24h at 410°C with an aim to study the effect of the modified microstructure on machinability and corrosion resistance.From the results,the following conclusions can be drawn:

i)Heat treatment of AZ91 Mg alloy at least up to 24h greatly affects the amount and the distribution of the secondary phase at the grain boundaries.

ii)Super saturatedα-grains can be produced by decreasing eutectic regions and theβ-phase.

iii)Hardness variations were decreased after heat treatment but the difference in the average hardness became insignificant after the heat treatment compared with untreated AZ91 Mg alloy.

iv)Machinability of AZ91 Mg alloy can be improved by developing super saturated grains and reducing the amount of secondary phase.

v)Corrosion resistance was greatly affected after the heat treatment due to the super saturated grains which promoted higher corrosion rate

Hence from the present study it is suggested that the machinability of AZ91 Mg alloy can be improved by heat treatment and due care should be given while heat treatment,if the structure is meant to work in corroding environment.

Acknowledgments

The authors would like to thank Mr.Dileep,Lab assistant,Department of MME,RGUKT Nuzvid for his help in conducting electrochemical tests.Thanks are due to Mr.Siva Krishna,Mr.Khan and Mr.Joshi,Lab assistants,Department of Mechanical Engineering,RGUKT Nuzvid for their help in carrying machining experiments.

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