Asurface hardening process based on Ni coating and subsequent diffusion heat treatments was studiedfor Al7075 alloy. Nickel coatings with different thickness on 7075 Al alloy were obtained by electrolyticand electroless processes . Heat treatments in inert atmosphere at 500 °C and 530 °C for different timeswere performed in order to obtain surface hardening of the aluminum alloy by diffusion of Ni into thesubstrate. The effect of temperature and time on the depth of hardening was studied by SEM, EDS, Glowdischarge optical spectrometry, microhardness tests and tribological tests.Surface hardness higher than 1000 HV and depth of hardening higher than 100 mm were achieved bydiffusion of Ni layers and formation of Al3Ni2 and Al3Ni intermetallic phases .The tribological test on Al7075 alloy against a hard Cr coated steel cylinder in air showed an averagecoefficient of friction ? of about 0.5 with a wide variation range, while all the coated and treated samplesexhibited a coefficient friction of about 0.7. However, the wear scar depth of the hardened alloy is about30 times lower than that of the Al 7075 alloy

Brunelli, K.

Dabalà, M.

Martini, C.

Italian Group Fracture (IGF): E-Journals / Gruppo Italiano Frattura

T R A T T A M E N T I  T E R M I C I7-8/2006Memoriela metallurgia italiana 37Surface hardening of Al 7075 alloyby diffusion treatmentsof electrolytic Ni coatingsK. Brunelli, M. Dabalà, C. MartiniA surface hardening process based on Ni coating and subsequent diffusion heat treatments was studiedfor Al7075 alloy. Nickel coatings with different thickness on 7075 Al alloy were obtained by electrolyticand electroless processes . Heat treatments in inert atmosphere at 500 °C and 530 °C for different timeswere performed in order to obtain surface hardening of the aluminum alloy by diffusion of Ni into thesubstrate. The effect of temperature and time on the depth of hardening was studied by SEM, EDS, Glowdischarge optical spectrometry, microhardness tests and tribological tests.Surface hardness higher than 1000 HV and depth of hardening higher than 100 mm were achieved bydiffusion of Ni layers and formation of Al3Ni2 and Al3Ni intermetallic phases . The tribological test on Al7075 alloy against a hard Cr coated steel cylinder in air showed an averagecoefficient of friction µ of about 0.5 with a wide variation range, while all the coated and treated samplesexhibited a coefficient friction of about 0.7. However, the wear scar depth of the hardened alloy is about30 times lower than that of the Al 7075 alloy.Key words: Aluminum alloys, Hardening,  Ni coating, Heat treatments, Electron Microscopy, WearINTRODUCTION Aluminum and its alloys are attractive for many applicationin chemical, automobile and aerospace industries because oftheir excellent properties as height strength-to- weight ratio,high electrical and thermal conductivities and good formabi-lity. However their hardness, wear resistance and mechani-cal properties are poor in comparison to steel resistance andcontinuous efforts are made in the research into new possi-bilities for making use of the advantage of the aluminum  inapplication that were reserved up to now for harder and mo-re wear-resistant materials. The solution mainly adopted isto produce a thick hardened layer on the substrate by lasersurface alloying process [1-2], plasma vapor deposited PVD[3], chemical vapor deposited CVD [4] and diamond-likehydrocarbons coatings [5].Recently, a method based on Ni-B electroless plating fol-lowed by diffusion heat treatment was proposed for improvethe wear resistance of titanium alloys [6]. The diffusion ofnickel and boron present in the coating  allowed a remarka-ble hardening of the titanium alloy.  It has been chosen touse the same method for the aluminum alloys. In this work,the effect of the diffusion of nickel and boron coating, obtai-ned by electroless deposition, and Ni coating, obtaining byelectrolytic process, on the hardening of the Al 7075 wasexamined. EXPERIMENTALSpecimens of 7075 Al alloy with a surface area of 12 cm2were obtained from a untreated rod and mechanically poli-shed using standard metallographic procedures. The surfaceof the specimens was degreased with alcohol and air dried,then activated by chemical etching in a 6 % HF aqueous so-lution for 5 seconds. After chemical etching, the specimenswere rinsed  with a deionized water and immersed either inthe solution  for the electroless Ni-B deposition or in the so-lution for the electrolytic Ni deposition. The chemical com-positions of the solutions are reported in Table 1. The proce-dures for preparing the solutions were suggested in the lite-rature [7-10], even if the composition for electroless Ni-Bdeposition was optimized after several experiments, becausethe deposition was made without a preliminary Zn conver-sion coating on the samples, which is frequently used to im-prove electroless deposit adhesion.Diffusion heat treatment of the plated specimens were car-ried out in a tubular furnace at 500° C and 530°C for diffe-rent times in inert atmosphere. The coating morphology andalloy microstructure were characterized by optical micro-scope and by a Cambridge Stereoscan 440 SEM equippedwith Philips PV9800 EDS.  The specimen concentrationprofiles were obtained by a LECO GDS 750 A glow dischar-Katya Brunelli, Manuele DabalàDIMEG – Università di Padova - Via Marzolo, 9   35131 – PadovaCarla MartiniIstituto di Metallurgia- Università di Bologna - V.le Risorgimento, 4- BolognaComponent Electroless Electrolyticbath bathNickel chloride 24  g l-1 -Sodium acetate 36  g l-1 -DMAB 10  g l-1 -Sodium lauryl sulphate 0.1 g l-1 -Nickel sulphate hexahydrate - 225- 410 g l-1Nickel(II)-chlorid-hexahydrat - 30-60 g l-1Boric acid - 30-40 g l-1pH 7 2-3Temperature 65-70 °C 55 °CVoltage - 1 VCurrent Density - 0.1-0.2 A/cm2Table 1 – Composition of the deposition baths.Tabella 1 – Composizione dei bagni di deposizione.T R A T T A M E N T I  T E R M I C I7-8/2006Memoriela metallurgia italiana38ge optical spectrometer (GDOS) with an anode surface of 4mm2 at 700 V with a current of 20 mA.  The microhardnessprofile were obtained by a Leitz-Werlag microhardness te-ster with 100 g weight. The tribological behavior was inve-stigated by dry sliding tests with a slider-on-cylinder tribo-meter. The stationary sliders consisted of the plated samplesunder investigation, whereas the rotating countermaterialwas a hard chromium plated steel cylinder (thickness of theCr coating: 300 µm; micro-hardness: 900 HV0.3; roughnessRa: 0.1 µm).The tests were carried out at 5 N applied load with a slidingspeed of 0,6 m/s and sliding distance of 5000 m at roomtemperature. Both friction resistance and system wear (i.e.wear of the slider plus wear of cylinder) were continuouslymeasured, using a bending load cell and a LVDT respecti-vely, and recorded as a function of sliding distance. Valuesof the wear scar depths and widths on the slider and the cy-linder were also evaluated at the end of each test by stylusprofilometer. Wear tracks were examined by SEM and OM.RESULT AND DISCUSSIONNi electrolytic depositionOptical microscopy analysis of the electrolytic coatingshowed that the layer, constituted by metallic nickel, wasuniform, adherent and thick about 20-40 µm (fig.1). The diffusion  heat treatments, performed at 530° C for 4,12, 24 h, induced the migration of Ni into the substrate andthe diffusion of the alloying elements towards the surfacewith the formation of a diffusion layer about 40-60 µmdepth, as it is shown in the fig. 2. Increasing the time of heat treatment the thickness of diffu-sion zone increased and became less uniform (fig.3). EDSanalysis showed that after 12 h heat treatment, the zone ofdiffusion was constituted mainly by Al (82% at.) and Ni(18% at.). After 24 h heat treatment the composition was nothomogenous and it was possible to distinguish three diffe-rent zones (fig.3). The dark thin surface one of about 5 µmwas constituted mainly by Al (81 % at.) and Mg (17.39 %at.) and only by 0.77 % at. of Ni. In the intermediate layer,thick about 50 µm, it was detected a constant amount of20% at. of Ni with a variable composition of Al and Cu. Theconcentration of Al and Cu was 64% and 2% in the lightgrey phases, while 77% and 14% in the dark grey phases.The atomic ratio Al/Ni was near to 3, suggesting that Al3Niwas formed. At the interface with substrate there were somelittle areas rich in Fe (4% at). The hardness profiles of theheat treated at 530°C for 12-24 hours were reported in Fig.4. The sample 12 h treated showed a surface hardness of 770HV100 which rapidly decrease to about 300 HV100 in 50 µmwhich correspond to the depth of diffusion layer, suggestingthat the presence of Ni is the responsible of increase of hard-Fig. 1 –  Ni electrolytic coating on Alsubstrate.Fig. 1 – Rivestimento di Ni elettrolitico sulsubstrato di Al.Fig. 2 – Ni electrolytic coating after heattreatment at 530°C for 12 h.Fig. 2 – Rivestimento di Ni elettrolitico dopotrattamento termico a 530° C per 12hFig. 3 – Ni electrolytic coating after heattreatment at 530°C for 24 h.Fig. 3 – Rivestimento di Ni elettroliticodopo trattamento termico a 530° C per 24h.Fig. 4 – Hardness profiles of samples heat treated at 530°C fordifferent times.Fig. 4 – Profilo di durezza dei campioni trattati a 530° C perdiversi tempi.Fig. 5 – Nielectrolytic coatingafter heattreatment at 500°Cfor 24 h.Fig. 5 -Rivestimento di Nielettrolitico dopotrattamento termicoa 500° C per 24h.ness. For the samples 24 h heat treated a surface hardness of180 HV100 was recorded, which increases to about 800HV100 at 50 µm depth. This because, as showed form SEManalysis, the outer layer is rich in Al and Mg migrated fromthe bulk to the surface, while the intermediate zone is con-stituted mainly in Al3Ni, as revealed by EDS analysis, sug-gested that this intermetallic is the main responsible of thehardening of this layer. Moreover the high spread of thehardness values was ascribable to the different amount ofCu founded by EDS analysis and in particular the highestvalues have been measured at about 40 µm from the surfacewhere the concentration of copper is lower. Obviously thelonger diffusion time produces a thicker diffusion layer.Because both the front of diffusion and profile of hardnesswere not uniform it has been chosen to diminish the tempe-rature of heat treatment from 530°C to 500°C, to limit thephenomena of element migration from substrate to surface,maintaining the diffusion time of 24 h.Fig. 5 showed the sample treated at 500°C for 24 h, where ispresent an unique uniform and compact zone of diffusion ofT R A T T A M E N T I  T E R M I C I7-8/2006Memoriela metallurgia italiana 39Fig. 6 –  Nielectrolytic coatingof about 80 µmafter heat treatmentat 500°C for 24 h.Fig. 6 –Rivestimento di Nielettrolitico di 80µm dopotrattamento termicoa 500° C per 24h.Fig. 7 – Hardness profiles of samples  heat treated at 500°C for 24 h.Fig. 7 – Profilo di durezza dei campioni trattati a 500° C per 24h.Fig. 9 –  NiBcoating after heattreatment at 530°Cfor 24 h.Fig. 9 –Rivestimento diNiB dopotrattamento termicoa 530° C per 24h.Fig. 8 – GDOS depth profiles  of NiB coating on Al substrate.Fig. 8 – Profilo GDOS del rivestimento NiB sul substrato di Al.about 60 µm which from EDS analysis resulted to be consti-tuted mainly by Al3Ni. The hardness profile of the samplesare reported in fig.7. The values of hardness are almost con-stant at 800 HV for about 50 µm due to the presence ofAl3Ni.Samples with a coating of about 80 µm, twice of the otherssamples studied, showed, after 24 h heat treatment at 500°C, two different compact layers of diffusion: one outer zonewith a thickness of about 10 µm and one inner zone withthickness of about 80 µm (Fig. 6). From EDS analysis the outer zone resulted constitutedmainly of Al3Ni2, while the main constituent of the inner zo-ne was Al3Ni. The hardness profiles showed that in proxi-mity of the surface the hardness values of 1000 HV werereached, which correspond to the hardness of Al3Ni2 [11].The value of 800 HV of the inner zone is ascribable to thepresence of Al3Ni as main constituent. Ni-B electroless depositionAnalysis of the electroless coating revealed that the platingbath allowed the deposition of an amorphous, uniform anddense Ni-B layer on the 7075 Al alloy. The GDOS analysis showed  that the layer was about of 10µm with an average boron content of 1 wt %, (Fig.8). The heat treatment were carried out at 530°C for different ti-mes. Until 24 h the diffusion was limited. After 24 h heattreatment an irregular diffusion zone between the coatingand the substrate, thick only about 10 µm, was observed(fig.9). From EDS analysis, the diffusion zone was constitu-ted mainly by Al with a ratio Al/Ni near to 3. In the whitespots of fig.9, the presence of a relevant  amount of Cu (6%at.) has been recorded, which was migrated towards the sur-face during heat treatment. It was observed that the Ni diffu-sed along boundary grain. In fact, isles with a different con-centration of Ni (12 % and  6% at., at a depth of 30 µm and100 µm, respectively) were found until a depth of 100 µmfrom the surface, while only trace of Ni was detected aroundthe isles of diffusion, even if near to the surface layer of dif-fusion. An increase in hardness has been recorded in corre-spondence of the thin layer of diffusion and of the Ni-richisles, where values of about 550 HV100 and 250 HV100 havebeen measured, respectively. The boron diffusion seems tonot influence the hardness of the substrate, unlike in tita-nium alloys [6]. Tribological testsThe first set of tests was carried out at 5N load with a slidingspeed of 0,6 m/s for a sliding distance of 5000 m. Untreatedsamples have been compared with Ni coated and heat trea-ted (at 500° for 24 h) samples. The coefficient of friction ofuntreated samples as a function of sliding distance displayeda wide range of variation with an average value of about 0.5,while the coefficient friction of all the treated samplesshowed a smoother trend and an average value of about 0.7(fig.10). The values of the wear scar depths on the untreatedsamples were about of 300 µm, 30 times greater than thosemeasured on treated samples, because the surface hardnessof treated samples is higher than untreated ones (fig. 11).The lower hardness can also account for the lower averagevalue of the coefficient of friction of untreated samples: theploughing component of sliding friction is lower for soft un-treated samples than for hard treated samples. The wide va-riation range in the coefficient of friction of untreated sam-ples can be due to stick/slip motion and extensive groovingof the soft slider, whereas the smooth trend of the coefficientof friction of treated samples is due to gradual micropoli-shing of the hard slider.The presence of adhesive yielding and cracks was not ob-served in treated samples, because the stronger bondbetween coating and substrate after heat treatment, due tothe formation of intermetallics, ensures higher depositionadherence.CONCLUSIONSHeat treatments of Ni electroplated on 7075 Al substrate fordifferent times and temperatures allowed  the interdiffusionof Ni and element of the constituted principally of Ni and AlT R A T T A M E N T I  T E R M I C I7-8/2006Memoriela metallurgia italiana40Fig. 10 – Coefficient of friction of Al 7075 T6 and samples Nicoated and treated at 500°C for 24 h.Fig. 10 – Coefficiente di frizione della lega Al 7075 T6 e delcampione nichelato elettrolit. e  trattato a 500° C per 24 h.Fig. 11 – Scar depth of Al 7075 T6 and samples Ni coated andtreated at 500°C for 24 h.Fig. 11 – Profondità del solco della lega Al 7075 T6 e delcampione nichelato  elettrolit. e trattato a 500°C per 24 h.alloy with the formation of a hard diffusion layer. The betterresults have been obtained after 24h of heat treatment at500°C which led to a more uniform zone of diffusionSurface hardness higher than 1000 HV and depth of harde-ning of about 100 µm were achieved by diffusion of Ni layersand formation of Al3Ni2 and Al3Ni intermetallic phases.For Al alloys the presence of boron not increase the hard-ness of the substrate, and electroless coatings are not so effi-cient in hardening like electrolytic coatings, because theirthickness is lower.The tribological test on Al7075 alloy against a hard Cr coa-ted steel cylinder in air showed an average coefficient of fric-tion of about 0.5 with a wide variation range, while all thecoated and treated samples exhibited a coefficient friction ofabout 0.7. However, the wear scar depth of the hardened al-loy is about 30 times lower than that of the Al7075 alloy. REFERENCES [1]  S. Tomida, K. Nakata, Surf. Coat. Technol., 174-175,2003, p. 559[2]  S. Tomida, K. Nakata, S. Saji and T. Kubo, Surf. Coat.Technol. 142-144, 2001, p. 585[3] E. Lugscheider, G. Krämer, C. Barimani, H. Zimmer-mann, Surf. Coat. Technol, 74-75, 1995, p. 497[4]  K. T. Rie, A. Gebauer, J. Wöhle, Surf. Coat. Technol.,86-87, 1996, p. 498[5] G. W. Malaczynski, A. H. Hamdi, A. A. Elmoursi andX. Qiu, Surf. Coat. Technol., 93, 1997, p. 280[6]  M. Dabalà, K. Brunelli, R. Frattini, M. Magrini, Surf.Eng., 20, 2004, p. 103[7]  Y. M. Chow, W. M. Lau, Z.S. Karim, Surf. Inter. Anal.,31, 2001, p. 321 [8]  A. R. Giampaolo, J.G. Ordorez, J. M. Guglielmacci, J.Lira, Surf. Coat. Technol., 89, 1997, p. 127[9] F. Delaunois, J. P. Petitjan, P. Lienard, M. Jacob-Dulie-re, Surf. Coat. Technol., 124, 2000, p. 201[10] W. D. Fields, R. N. Duncan, J. R. Zickgraf, MetalsHandbook, 9th edn , ASM International, Materials Park(OH), 1982, p. 601 [11] M. A. Zamzam, A. S. El –Sbagh, M.M. Milad, Mate-rials and Design, 23, 2002, p. 161INDURIMENTO SUPERFICIALE DELLA LEGA AL 7075MEDIANTE TRATTAMENTI DI DIFFUSIONE DI RICOPRIMENTIDI NiParole chiave: trattamenti termici, alluminio e leghe, microscopia elettronica, tribologiaIn questo lavoro è stato studiato un processo di indurimentosuperficiale per la lega Al 7075 attraverso  deposizione  diuno strato  di Ni e successiva diffusione termica.I rivestimenti di Ni con spessori variabili tra 10 e 80 _m so-no stati ottenuti mediante un processo di deposizione  di Nielettrolitico o di Ni-B chimico (tab.1). I trattamenti termici,il cui scopo è quello di indurre la diffusione del Ni all’inter-no del substrato, sono stati effettuati in forno in presenza diatmosfera inerte alle temperature di 500° e di 530° C pertempi variabili. L’effetto della temperatura e del tempo ditrattamento termico sull’indurimento superficiale della legaè stato investigato mediante SEM, EDS, GDOS,  prove dimicrodurezza e tribologiche. I trattamenti termici condotti sulla lega di alluminio con lostrato di Ni elettrolitico  ha permesso la interdiffusione delNi e degli elementi del substrato portando alla formazionedi uno strato esterno indurito costituito principalmente daNi e Al. I risultati migliori sono stati ottenuti con il tratta-mento termico a 500° C per 24 h che ha permesso la forma-zione di uno stato di diffusione compatto ed uniforme costi-tuito da Al3Ni2 e Al3Ni (fig. 5-6). Con questo trattamentosono stati ottenuti valori di durezza di circa 1000HV100 peruno spessore di 100 _m (fig. 7). Temperature maggiori pro-ducono uno strato di diffusione meno uniforme e con valoridi durezza inferiori (fig. 3-4). Per le leghe di alluminio la presenza di boro non contribui-sce ad aumentare la durezza del substrato e inoltre la depo-sizione chimica risulta meno efficace nell’indurimento dellalega rispetto a quella elettrolitica  in quanto i rivestimentiottenuti presentano uno spessore inferiore (fig. 9). Le prove tribologiche condotte con un cilindro di acciaio ri-vestito al cromo mostrano un valore del coefficiente di frizio-ne di 0,5 per la lega Al7075 e di 0,7 per i campioni nichelatitrattati termicamente (fig. 10). Tuttavia la profondità dei sol-chi dei campioni sottoposti al trattamento di indurimento su-perficiale risulta inferiore di circa 30 volte rispetto alla legaAl 7075 per le prove condotte con un carico di 5 N (fig. 11). A B S T R A C T

Surface hardening of Al 7075 alloy by diffusion treatments of electrolytic Ni coatings

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Surface hardening of Al 7075 alloy by diffusion treatments of electrolytic Ni coatings

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