機(jī)械外文文獻(xiàn)翻譯-高壓水射流技術(shù)對(duì)Al-Si合金的表面處理和摩擦學(xué)性能的影響 【中文4560字】【PDF+中文WORD】

機(jī)械外文文獻(xiàn)翻譯-高壓水射流技術(shù)對(duì)Al-Si合金的表面處理和摩擦學(xué)性能的影響?【中文4560字】【PDF+中文WORD】,中文4560字,PDF+中文WORD,機(jī)械,外文,文獻(xiàn),翻譯,高壓,水射流,技術(shù),Al,Si,合金,表面,處理,摩擦,性能,影響,中文,4560,PDF,WORD Journal of Surface Engineered Materials and Advanced Technology,2011,1,112-120 doi:10.4236/jsemat.2011.13017 Published Online October 2011(http:/www.SciRP.org/journal/jsemat)Copyright 2011 SciRes.JSEMAT High Pressure Water-Jet Technology for the Surface Treatment of Al-Si Alloys and Repercussion on Tribological Properties Md.Aminul Islam1*,Zoheir Farhat1,Jonathon Bonnell2 1Department of Process Engineering and Applied Science,Dalhousie University,Halifax,Canada;2Vector Aerospace,Engine Ser-vices-Atlantic,Summerside,Canada.Email:*Md.Aminul.IslamDal.Ca,Zoheir.FarhatDal.Ca Received June 27th,2011;revised August 3rd,2011;accepted August 16th,2011.ABSTRACT Recent developments in high pressure water-jet technology have brought the process to the forefront as a means of sur-face treatment.Water jet technology offers cleaning,cutting,processing as well as potential refinement of surface properties.By adapting the process parameters the surface characteristics can be changed while the profile remains the same.In the present study,water-jet technology was used for the surface treatment of Al-Si alloy to investigate its effect on tribological properties.Dry sliding wear behavior was investigated against AISI 52100 bearing steel ball using a reciprocating ball-on-flat configuration.Optical microscopy examination reveals that ploughing of grains,transgranu-lar and intergranular propagation of cracks;are the mechanisms by which material is removed during water jet treat-ment.While,on the other hand,SEM observation of the wear track reveals that plastic deformation and delamination are the dominant wear mechanism during the wear process.Water jet treatment was compared to hot isostatic pressing in terms of its effects on wear resistance and surface porosity of Al-Si alloy.It was found that,hot isostatic pressing reduces the total amount of porosity at the expanse of hardness while water jet treatment produces a compressed sur-face having higher hardness and compressive residual stress,which ultimately increases wear resistance.Keywords:Tribology,High Pressure Water Jet Treatment,Porosity,Surface Treatment,XRD,Reciprocating Wear 1.Introduction In recent years,the use of high-pressure water jet tech-nique became widely accepted practice in order to meet the requirements of production and maintenance.Wa-ter-jet technologies offer cleaning,machining,surface treatment and cutting of materials.Both roughening as well as polishing is possible with this procedure.It is therefore excellent for pre-treatment of engineering sur-faces.For the last few decades,water-jet technology cov-ered the following applications:removal of paint,grease,dirt from aircraft in the aviation industry;prevent pump cavitation;removal of cement lips,incrustations,lime,solidified dust from autoclave vessel;removal of worn protective coating,incrustations,solidified materials in the chemical industry;semiconductor frame cleaning in the electronic industry;removal of weld slag,water scale,mill scale and rust in steel mills;water jet cutting;vibration free demolition by abrasive water jets and so on 1-11.The conventional water jet technique has been used for cutting and polishing by mixing abrasive materials into the water which transport the abrasive materials to strike the substrate.On the other hand,high pressure water jet can produce a compressed surface layer without abrasive materials,while the core of the component remains un-changed.The new surface layer tribolayer is expected to have enhanced tribological properties due to possible work hardening,residual stress and reduction of surface porosity.Hence,water jet treatment may be an effective technique for improved surface properties.Aluminum-silicon based alloys have received consid-erable attention due to their high strength to weight ratio.The reduction in weight of components leads to signifi-cant impact on fuel economy in dynamic systems.In ad-dition,these alloys are reported to have a reasonably high wear resistance.The wear resistance of Al-Si alloys de-pends on a number of material related parameters,i.e.,size,shape,composition and distribution of micro-con-stituents,in addition to service conditions 12-16.Po-rosity is a common feature of sintered Al-Si alloys and High Pressure Water-Jet Technology for the Surface Treatment of Al-Si Alloys and 113Repercussion on Tribological Properties strongly influences their properties and applications.In general the presence of pores is accompanied by a drop in strength,ductility and wear resistance of materials 17.Not only the total volume percentage of porosity influ-ences the degradation of properties but also size,shape and interconnectivity of pores play an important role 18-20.Meanwhile,initial attempts on the mechanical characterization of these materials were focused on static tests.However,in order to employ these materials in new applications,their dynamic properties have to be care-fully assessed.In recent years,fatigue behavior has been studied in numerous investigations.The influence of pores on crack initiation and propagation under cyclic loading was examined as well 21,22.The effect of wa-ter jet peening toward improving fatigue resistance was examined by various experimental tests and proved as an effective techniques for preventing fatigue in structural components 23-25.The objective of this study is to assess the potential of employing high pressure water-jet technology for the surface treatment of Al-Si alloys to improve wear resis-tance for automobile and aerospace applications.2.Experimental Details In order to obtain a homogeneous distribution of porosity,samples were prepared using powder metallurgy method.Two powders(i.e.,Al-Si master alloy and Al-Mg master alloy),were mixed to produce the following alloy com-position;88.8 wt%Al,6.0 wt%Si,4.5 wt%Cu,0.5 wt%Zn,0.2 wt%Fe.A Lico wax C was used as a pressing lubricant.Specimens were pressed at 100,200 and 600 MPa and were sintered in a tube furnace in the presence of nitrogen gas at 5600 C for 20 minutes and then slow cool to 4800 C.The green and sintered densities of sam-ples were determined in accordance with MPIF Stan-dard 42 26 and are listed in Table 1.For micro-struc-tural experiments,specimens were cut,mounted and ground using 240,320,400 and 600 grit SiC abrasive papers and then polished using 1m,0.3m and 0.05m gama alumina suspension.Olympus BX51 research mi-croscope,equipped with bright-field objectives was used to analyze the microstructure at high resolution.Table 1.Basic properties of sintered Al-6 wt%Si alloy.Specimens Green Density(g/cc)Sintered Density(g/cc)Pressed at 100 MPa 2.11 2.33 Pressed at 200 MPa 2.29 2.46 Pressed at 600 MPa 2.58 2.6 Water-jet treatment was performed using a highly pressurized water-jet.The system is capable of employ-ing ultrasonic pulses of water-jet for more intense treat-ment.Several tests were performed using the ultrasonic system.During experiments,specimens were individu-ally mounted on a turntable inside the water jet enclosure.Specimens containing different amount of porosity(com-pacted at 100,200 and 600 MPa)were subjected to water jet at four different pressures,34,48,55 and 70 MPa,employing a nozzle with an orifice size of 1.6 mm.The standoff distances between the nozzle and the specimen were 25,64,76 mm and the translation speeds were 25,50,150 and 250 mm/s.Surface porosities of samples were calculated using image analysis software.A series of images were taken to cover the whole surface area of the sample.Porosities were identified based on their gray-level intensity differ-ences compared to the matrix.Gray-level threshold set-tings were selected to permit independent detection of porosity,using the flicker method of switching back and forth between porosity and the matrix.Second phase particles and dendrite,may be counted as porosity be-cause their gray level range is similar to that of porosity.The grey-level thresholds as well as boundary conditions,(i.e.,aspect ratio,min radius and area)were set to avoid second phase particles and dendrite.A counting protocol was chosen to correct for edge effects so that a porosity lying across a field boundary is counted only once 27.For each field the area fraction of the detected area of porosity was measured by dividing the detected area of porosity by the area of the measurement field.Surface porosity values are given in Table 2.Table 2.Surface porosity of sintered Al-Si alloy.Untreated After HIP After water jet Specimens%Surface Porosity Average Pore Size(m)%Surface Po-rosity Average Pore Size(m)%Surface Porosity%Sub surface Porosity Average Pore Size(m)Pressed at 100 MPa 6.71 0.01 82.4 2.13 0.01 34.6 9.14 0.01 5.33 0.01 86.5 Pressed at 200 MPa 4.22 0.01 70.2 1.34 0.01 23.5 4.37 0.01 3.65 0.01 73.2 Pressed at 600 MPa 2.35 0.01 46.9 0.75 0.01 20.4 3.80 0.01 1.97 0.01 48.8 Copyright 2011 SciRes.JSEMAT High Pressure Water-Jet Technology for the Surface Treatment of Al-Si Alloys and 114 Repercussion on Tribological Properties Micromet micro-hardness tester was used to measure the micro-hardness of the specimens.This was conducted to assess the effect of water jet treatment on the sub-sur-face properties of the Al-Si alloy.In this test method,Vickers hardness measurements were performed using a diamond indenter and 15 g load.The size of the indenta-tion was measured using a light microscope equipped with a filar type eyepiece.HV measurements were taken as a function of depth from the treated surface.In order to measure residual stresses induced as a re-sult of the high pressure water jet treatment,X-ray peak broadening diffraction techniques was employed.X-ray diffraction(XRD)experiment was carried out on a specimen treated ultrasonically at 48 MPa from 76 mm standoff distance and 150 mm/s translation speed,em-ploying a high-speed Bruker D8 Advance system using Cu-K radiation having a wave length()of 1.54,tube voltage of 40 KV,and tube current of 40 mA.The strongest four peaks(i.e.,(111),(200),(220)and(311)were selected for slow scan of 0.02/sec.The integral breadth of XRD peaks were analysed using EVA soft-ware package.Peaks were corrected for the effects of K2 radiation and the background was removed.A stan-dard stress-free Al from the same alloy was used to measure instrumental broadening.The data was treated according to Williamson-Hall method 28.The integral breadth(i.e.,total area under the peak/peak height)of the diffraction peak can be expressed as,obsinssizeBBBB (1)Here Bsize and B are the grain size and micro-strain contributions to the observed peak broadening Bobs re-spectively.Bins is the peak broadening at a stress-free state,referring to the instrument contribution.The crys-talline size t is related to Bsize by the Scherrer equation 29,0.9tBcossizeB (2)Here,B is Braggs angle.While the micro-strain can be calculated from,dB2tandB (3)Where,d is the interplaner spacing.Substituting equa-tion(2)and(3)into(1)gives,0.9dBBcos2sintdobsinsBB (4)For grain size larger than 10 m(as in the present case)X-ray peak broadening is due to micro-strain effect alone,hence,the first term in equation(4)can be eliminated.Dry reciprocating wear tests were performed using a Universal Micro-Tribometer.This test method utilizes a ball upper specimen that slides against a flat lower specimen in a linear,back and forth,sliding motion hav-ing a stroke length of 5.03 mm.All tests were conducted at room temperature and a relative humidity of 40-55%.The load is applied downward through the ball specimen against the flat specimen mounted on a reciprocating drive.The tester allows for monitoring the dynamic nor-mal load,friction force and depth of the wear track dur-ing the test.A 6.3 mm diameter AISI 52100 bearing steel ball having a hardness of HRA 83 was used as a counter-face material.The ball was mounted inside a ball holder which was attached directly to a suspension system,which,in turn,is attached to a load sensor that controls and records forces during the test.The weight of the specimen was measured before and after each wear test to determine individual weight loss at selected time in-tervals.10 N normal load and 15 Hz frequency were em-ployed for three different time intervals(10 min,45 min and 90 min).After wear tests,worn surfaces and cross section of the wear track were examined using optical and scanning electron microscopy to determine possible wear mechanisms.3.Results and Discussion The size,shape and amount of pores are largely depend-ent on processing parameters.As all the specimens were sintered under the same sintering conditions,the com-paction pressure plays the most significant role in deter-mining pore size,shape and amount.Increasing the compaction pressure decreases the amount of porosity and reduces pore size,while the pore shape changes from large irregular to small round shape.The percent surface porosity ranges from 6.71%to 2.35%,while the average pore size ranges from 82.4 m to 46.9 m respectively as the compaction pressure of the Al-Si alloy powder is raised(Table 2).The low standard deviations of surface porosities indicate a uniform distribution of porosity throughout the structures.To mitigate the detrimental effect of porosity,specimens were subjected to hot iso-static pressing(HIP)treatment.HIP gave rise to a de-crease of approximately 68%in surface porosity and 60%in pore size.On the other hand,when the sintered Al-Si alloy was subjected to high pressure water-jet(us-ing 70 MPa pressure,64 mm distance and 25 mm/s)it is found that there is an increase of 36%,4%and 61%in surface porosity at compact pressure of 100,200 and 600 MPa,respectively.Moreover,there is approximately 4.3%increase in the average pore size after water jet treatment.While there is around 16%reduction in poros-ity in the subsurface region(22 3m from the surface)Copyright 2011 SciRes.JSEMAT High Pressure Water-Jet Technology for the Surface Treatment of Al-Si Alloys and 115Repercussion on Tribological Properties after the water jet treatment.It has also found that treated surfaces exhibit high roughness.The discrepancy in the amount of porosities and size of pores between the sur-face and near surface(Table 2)is attributed to the diffi-culty of measuring surface porosity when surface rough-ness is high.Therefore,near surface porosity is a better indicator of the amount and size of porosity.In the water jet process,jet pressure,standoff distance and translation speed have a significant impact on final surface properties.During the water jet treatment,water jet strikes the surface at a relatively high velocity which causes a sig-nificant amount of material loss.The amount of material loss increases with water jet pressure.Figure 1 shows a representative image showing the surface damage after water jet treatment.In this particular situation,the specimen compacted at 100 MPa was subjected to 55 MPa jet pressure operated from 25 mm standoff distance at 50 mm/s translation speed reveal a depth of the mate-rial removal of about 900 m.The insert on the upper right corner of the image(Figure 1)shows the position where the depth measurement was taken.Optical microscopy examination reveals that plough-ing of grains,transgranular,intergranular propagation of cracks and as a consequence,break away and pulling out of grains are the mechanisms for material removal during water jet treatment.Figure 2(a)-(e)shows a cross-sec-tion of a specimen compacted at 600 MPa and treated ultrasonically at 34 MPa at a translation speed of 250 mm/s and standoff distance of 64 mm.In Figure 2(a),shows transgranular crack propagating through a grain,while Figure 2(b)reveals a portion of a grain breaking away.It was also observed that the water jet penetrates along the grain boundaries promoting intergranular cra-cks to develop(Figure 2(c).This leads to pulling out of grains as shown in Figure 2(d).Figure 2(e)shows plou-ghing of grains along the water jet path.SEM observation Figure 1.Surface damage due to 55 MPa water jet pressure,25 mm standoff distance and 50 mm/s translation speed.of the surface on the other hand,reveal that water jet pressure causes collapse of porosity underneath the sur-face.Figure 2(f)shows an SEM image of the specimen compacted at 100 MPa and treated ultrasonically at 48 MPa from 76 mm standoff distance and at 150 mm/s translation speed.It is observed from the figure what appears to be a sub-surface porosity collapse due to the pressure created by the water jet,hence,producing low porosity subsurface tribolayer.In order to investigate the effect of water-jet treatment on hardness,a series of micro-hardness measurements were conducted according to ASTM standard 30.Hardness was measured at three different depths,17 3,122 5 and 1000 5 m below the treated surface.Fig-ure 3 shows the variation of hardness at different dis-tance below the surface of the specimen compacted at 100 MPa and treated at 55 MPa water jet pressure from 25 mm standoff distance at 50 mm/s translation speed.Each hardness value in the figure represent an average of 10 measurements.It is evident from the figure that the hardness drops gradually away from the treated surface.There is about 15%increase in hardness in the vicinity of the surface as compared to hardness of the base metal about 1 mm from the surface.It is believed that the ob-served rise in hardness closer to the surface is due to work hardening effects as a result of the high pressure water jet treatment.Figure 4 shows the superimposed XRD diffraction peaks of the standard and water jet treated specimens as representative peaks.It should be noted that neither broadening nor sifting of peaks was observed for the un-treated sample.XRD of the all scanned peaks were only slightly broadened but significantly shifted to the right after water jet treatment.According to peak broadening calculations the microstrain of the treated specimen was found to be 10 10-5.It can be shown that the residual stress resulting from microstraiin analysis is 7 MPa(i.e.,resE,where,E=70 GPa 31).On the other hand,the macrostrain(uniform strain)calculated from the shifts in peak positions,as d d,is 6 10-4 and the residual stress was calculated to be 42 MPa.The meas-ured macrostrain and,hence,the associated residual stress are compressive as peak shifts are to the right(to-ward higher Bragg angle).As in the XRD diffraction method,only planes parallel to the surface contribute to the measured intensity,crystal planes parallel to the sur-face are under compression due to the water jet treatment.The measured compressive residual stress promotes the closing of surface and near surface pores,hence,raising the hardness in the tribolayer and retarding crack initia-tion and propagation during wear.In order to investigate wear performance of the sintered Copyright 2011 SciRes.JSEMAT High Pressure Water-Jet Technology for the Surface Treatment of Al-Si Alloys and Repercussion on Tribological Properties Copyright 2011 SciRes.JSEMAT 116 (a)(b)(c)(d)(e)(f)Figure 2.X-section of a specimen compacted at 600 MPa,(a)transgranular crack propagation;(b)break away of grains;(c)intergranular crack propagation,(d)pulling out of grains,(e)ploughing of grains,(f)collapse of porosity due to water jet pressure.High Pressure Water-Jet Technology for the Surface Treatment of Al-Si Alloys and 117Repercussion on Tribological Properties Figure 3.Variation of hardness at different depth below the surface.Figure 4.(220)X-ray diffraction peak of water jet treated and standard Al specimens.Al-Si alloy after water jet treatment and compare to un-treated and HIPed conditions,reciprocating wear tests were performed under 10 N normal load and 15 Hz fre-quency at three time intervals(10 min,45 min and 90 min).For the water-jet treated specimens,the operating condition are as follows:70 MPa from 64 mm standoff distance and 25 mm/s translation speed.Figure 5 depicts the data collected during wear tests.The figure shows a somewhat linear increase(with no transition)in