Adhesiveness of Cold Rolled Steels for Car Body Parts

Transcription

1 Mterils Reserch, Vol. 10, No. 3, , Adhesiveness of Cold Rolled Steels for Cr Body Prts Kleiner Mrques Mrr, Evndro de Azevedo Alvreng, Vicente Tdeu Lopes Buono b * Usimins Reserch nd Development Center, Rod. BR 381, Ipting - MG, Brzil b Federl University of Mins Geris, Deprtment of Metllurgicl nd Mterils Engineering, Ru Espirito Snto, 35, Belo Horizonte - MG, Brzil Received: Jnury 4, 2007; Revised: June 23, 2007 The im of this work ws to evlute the dhesiveness of uncoted nd zinc-electroglvnized steel sheets used in the utomotive industry. Three types of dhesives, one crylic nd two epoxy resins, were employed to join low crbon cold rolled steels, one uncoted nd nother electroglvnized, both previously degresed or chemiclly pickled. Mechnicl strength of the joints ws evluted by the T-peel nd tensile strength tests. Steel grde, surfce condition nd heting below the cure tempertures did not influence the joints mechnicl strength. However, their sher strength decresed drsticlly s the test temperture incresed. The exposure of the joints to n tmosphere with 90% reltive humidity t 40 C cused reduction of their sher strength. Epoxy dhesives showed higher mechnicl strength, but exhibited higher degrdtion by humidity. Keywords: low crbon steels, dhesives, degrdtion, epoxy resins, crylic resins 1. Introduction Adhesives re used to join prts of similr or dissimilr mterils, being constituted by mixtures of substnces with severl functions. The min components of dhesives re mcromoleculr or polymeric substnces. They cn be pplied to the dherent s surfces in liquid, psty or solid stte, but in the finl stge of the joining process the dhesives should hve low viscosity in order to wet entirely the surfces to be joined nd to produce good dhesiveness fter curing. In generl, dhesion mechnisms cn be grouped s follows 1-3 : i) mechnicl interlocking, in which the dhesive penetrtes into micro-irregulrities present on the dherents surfces, contributing to strong dhesion. A good wettbility is n importnt requirement for this dhesion mechnism; ii) electronic dhesion, bsed on chemicl bonding ssocited with the trnsference of electrons between the dhesive nd the dherent to blnce the Fermi energy levels. Electrosttic forces re responsible for this type of dhesion; nd iii) physicl dsorption, in which the dhesive nd the dherent re joined by forces estblished on their interfces, ssocited with pproprite intermoleculr contct. The use of dhesives to join steel nd luminum prts is growing continuously in the cr nd irplne industries. More recently, dhesive bonding hs been pplied to repir pipes in fluid trnsport systems 4, nd the possibility of its use in steel constructions hs been demonstrted: dhesive connections showed higher strength nd stiffness thn bolted connections 5. In the beginning of the lst decde, dhesives were used in more thn fifty different cr prts, to join metllic mterils, glsses, lether, plstic nd others 6. When structurl dhesives re pplied to join steel prts of the cr body, the benefits re relted to reducing the weight, incresing the rigidity, reducing vibrtion nd noise nd lso closing gps nd holes. Also, prticulr ttention is directed towrds the finishing chrcteristics, since dhesive bonding of steel prts of cr bodies reduces considerbly the incidence of undesirble surfce irregulrities ssocited with welds, screws, nuts or rivets 6,7. Besides tht, it is well known tht dhesive bonding is the preferred jointing method when ftigue of structures involving bonded prts is considered 8. *e-mil: vbuono@demet.ufmg.br The surfce condition of dherents generlly plys n importnt role in estblishing strong nd durble bond. Adequte surfce clening tretments of the dherent prts, before the ppliction of dhesives, re thus of fundmentl importnce. Adhesively bonded steel prts re commonly degresed with chemicl solvents, followed by pickling with hydrochloric or sulfuric cids. More recently, gritblsting hs been shown to increse dhesion of steel joints 9. Despite the dvntges promoting the use of dhesive bonding of metllic prts, importnt issues remin to be solved, becuse the mechnicl properties of bonded joints re generlly influenced by number of fctors, such s the type of dhesive employed, the nture nd surfce roughness of the metllic dherents, the wet ngle nd the test temperture 1,2,7-11. Adhesive degrdtion due to exposure in tmospheres with high reltive humidity nd reltively high tempertures deserves specil ttention for the use of dhesive bonding in the utomotive industry Under certin tmospheric conditions, chemicl rections cn led to loss of dhesion nd decrese in the mechnicl strength of the joints. Adhesive embrittlement nd oxide formtion t dherent/dhesive interfces hve been pointed out s the min cuses of this loss of dhesion 10. In this work, the dhesiveness of uncoted nd zinc-electroglvnized steel sheets used in the utomotive industry ws evluted, using three types of dhesives, one crylic nd two epoxy resins, of the type employed in cr bodies. Loss of dhesive strength ws evluted by mens of mechnicl tests performed t different tempertures nd in joints previously het treted under usul nd high humidity tmospheres. The im ws to identify the influence of the vrious prmeters investigted on the mechnicl properties of the most common dhesive steel joints used in the utomobile industry. 2. Experimentl The dhesive clled A1 in this work is non-structurl type nd hs n crylic resin s the bse substnce. It is normlly used to close gps nd holes in cr bodies. Two other dhesives were lso employed.

2 268 Mrr Mterils Reserch They re structurl dhesives, designted s E1 nd E2, nd hve epoxy resins s the bse component. Tble 1 presents the informtions bout them given by the suppliers for the utomobile industry. The two dherents employed in this investigtion re low-crbon cold-rolled steels used in cr bodies. One of them, clled here the bsic steel - BS, is uncoted, nd the other, referred to s the electroglvnized steel - EGS, ws electroglvnized with zinc (70/70 g.m -2 ) in n industril production line. Their chemicl composition is bsiclly the sme nd both mterils were evluted s 0.70 mm thick sheet specimens. Due to the necessity of good contct between dhesive nd dherent to promote joints with suitble mechnicl strength, the wet ngle ws mesured fter nneling the joints t 180 C for 20 minutes. This condition corresponds to the curing recommendtions stted in Tble 1. Since the surfce roughness is nother importnt fctor to produce good mechnicl strength, this property ws evluted by the use of Mitutoyo roughness surfce tester. In the cse of BS steel specimens, the joints were prepred using two surfce conditions: i) only degresing with tetrchloroethylene; nd ii) degresing followed by pickling with hydrochloric cid (15%). The EGS steel ws only evluted under the degresed condition. The dhesives were pplied to the dherent surfces with sptul, covering completely the re to be joined with fine lyer of pproximtely 0.5 mm in thickness. The mechnicl strength of the joints ws evluted by two types of mechnicl tests: the T-peel test (ASTM D 1876), in which tensile stress is pplied to the dhesive/dherent interfce, nd the tensile strength test (ASTM D 1002), involving the induction of sher stresses on the interfces. The mechnicl tests were performed in n Instron test mchine, t cross hed speed of 5 mm/min. The rtio between the rupture lod nd the joined re ws tken s the mechnicl strength of the joints tensile tested. The rupture lod ws the mximum vlue reched during the test. In the T-peel test, the results re expressed s the rtio of the verge lod by the width of the dhesion re. All results of mechnicl strength reported here re the verges of three tests performed under identicl conditions. Averge stndrd devitions were smller thn 20% in ll tests. Frcture surfces were observed by scnning electron microscopy SEM, in Cmbridge Stereoscn 360 microscope. To verify the influence of temperture on the mechnicl strength of the joints, tensile strength tests were relized t the following tempertures: 20, 0, 25, 50, 90, 130 nd 180 C. Another evlution, using the sme type of test, ws performed t room temperture, with joints previously het treted t 50, 90, 130 nd 180 C for 30 minutes. In this cse, the joints were tested to verify whether the dhesives re susceptible to heting embrittlement. The sensitivity to humidity degrdtion ws evluted in joints of the BS steel (degresed nd pickled) nd of the EGS steel (degresed), previously exposed in humid cbinet, t 40 C nd 90% of reltive humidity. The temperture nd reltive humidity degree were chosen to simulte severe tmospheric conditions. During one yer, within intervls of two months, specimens were tken out of the humid cbinet nd submitted to the tensile strength test. 3. Results nd Discussion 3.1. Adherents surfce roughness The surfce roughness of the dherents, mesured in the rolling direction, is shown in Figure 1. The electroglvnized steel exhibits the highest roughness, ssocited with the presence of zinc crystls in the coting, giving rise to more irregulr surfce. The BS steel in the degresed nd pickled condition shows rougher surfce, s compred to the sme mteril tht ws only degresed. This mens tht the pickling cid solution cused strong chemicl ttck of the steel surfce Wet ngles Tble 2 shows tht the dhesives, fter curing t 180 C, exhibit wet ngles vrying from 30 (dhesive E1, degresed BS steel) to Surfce roughness ( m) BS/DS BS/DS + DP EGS/DS Steel/Surfce tretment Figure 1. Longitudinl verge roughness of the dherents. Tble 2. Wet ngles of dhesives on the dherents fter nneling t 180 C. Adhesive Adherent Clening Wet ngle ( ) E1 BS DS 30 E2 BS DS 45 A1 BS DS 70 E1 BS DS + DP 35 E2 BS DS + DP 45 A1 BS DS + DP 90 E1 EGS DS 55 E2 EGS DS 60 A1 EGS DS 85 Clening: DS = degresed with tetrchloroethylene, DP = pickled with HCl 15%. Tble 1. Informtion bout the dhesives given by the suppliers (flow time nd density vlues t 25 C). Adhesive Flow time (s) Density (g.cm -3 ) Mx. storge time t temperture Curing conditions E months/10 C or 6 months/25 C 170 C/20 min E months/10 C or 3 months/25 C 140 C/40 min or 180 C/13 min A months/10 C or 2 months/25 C 180 C/20 min Flow times determined in viscosity tests.

3 Vol. 10, No. 3, 2007 Adhesiveness of Cold Rolled Steels for Cr Body Prts (dhesive A1, degresed nd pickled BS steel). Considering tht t room temperture ll the dhesives presented psty consistency, nd tht the dhesive A1 hd smller density nd viscosity thn the others (Tble 1), this mens tht the dhesive A1 becme more viscous fter heting t 180 C, the curing temperture. In this condition, the wet ngles decresed from A1 to E1, while dhesive E2 occupies n intermedite position. Resin E1 lwys presented the best wettbility. Although, in generl, the EGS steel showed higher vlues of wet ngle thn the BS steel fter curing, the het tretment turned dhesive E1 less viscous, while the behvior of dhesive A1 ws the inverse, independently of the dherent. Tble 2 lso shows tht the surfce condition of the dherents influenced the wet ngle. After degresing nd pickling, the BS steel showed higher wet ngle thn the sme mteril in the degresed condition. These differences cn be explined by the surfce tension estblished between the dhesive nd the dherent, which increses s the surfce roughness increses Mechnicl tests The verge vlues of the results obtined in the T-peel nd tensile strength tests re shown in Tble 3. Although different dhesives produced different results, it cn be observed tht the metllic substrtes did not significntly ffect the mechnicl strength of the joints. In the tensile strength tests, the crylic dhesive joints A1 showed inferior strength s compred to the epoxy dhesive E1 nd E2. However, in the T-peel tests, dhesive A1 nd E2 prcticlly show the sme strength. As usul in mechnicl testing of dhesive bonded joints, two filure modes were observed in the frctured specimens observed by SEM: the dhesive mode, tking plce t the interfce between dhesive nd dherent (Figure 2), nd the cohesive mode, which occurs in the dhesive bulk (Figure 2b) 8. Tble 3 shows tht the cohesive filure occurred more frequently in the peeling test. In the tensile strength test, joints mde with dhesive E1 showed higher susceptibility to interfce filure thn the other dhesives. The predominnce of filures in the dhesive mode in the tensile strength tests indictes tht the dhesion force, under sher lods, corresponds to the intrinsic mechnicl strength of the interfce. The lowest mechnicl strength shown by dhesive A1 is relted to the fct tht it is not structurl dhesive, nd thus hs reduced intrinsic strength Vrition of the sher strength with temperture The vlues of verge sher strength obtined t different test tempertures re shown in Figure 3. The joints with the dhesive E1 mintined their mechnicl strength up to 90 C. Above this temperture, there ws n effective decrese in the sher strength. On the other hnd, dhesive E2 exhibited this trnsition temperture round 50 C. It ws lso observed tht the filures t higher tempertures occurred in the bulk of the dhesives. This behvior cn be ssoci- L = SE1 EHT = 20.0 KV WD = 21 mm 200 m () L = SE1 EHT = 20.0 KV WD = 21 mm 200 m (b) Photo = 3549 Photo = 3543 Figure 2. Secondry electron imges of the frcture surfce of tensile strength test specimens showing ) dhesive filure in EGS steel bonded with dhesive E1 nd b) cohesive filure in EGS steel bonded with dhesive A1. Tble 3. Results of the peeling nd tensile strength tests. Adhesive Adherent Clening Peeling tests Tensile strength tests Strength (N/mm) Filure type b (%) Strength (MP) Filure type b (%) E1 BS DS C C-30A E2 BS DS C C-40A A1 BS DS C C-10A E1 BS DS + DP C C E2 BS DS + DP C C-20A A1 BS DS + DP C C-10A E1 EGS DS C A E2 EGS DS C C-20A A1 EGS DS C - 15A C-05A Clening: DS = degresed with tetrchloroethylene, DP = pickled with HCl 15%; nd b Filure type: A = dhesive, C = cohesive.

4 270 Mrr Mterils Reserch Sher strength (MP) BS/E1 BS/E2 BS/A1 EGS/E1 EGS/E2 EGS/A1 Sher strength (MP) BS/E1 BS/E2 BS/A1 EGS/E1 EGS/E2 EGS/A Temperture ( C) Figure 3. Effect of test temperture on the sher strength Temperture ( C) Figure 4. Effect of het treting temperture on the sher strength. ted with the decresing of the intrinsic mechnicl strength of the dhesives s the temperture rises. The results of verge mechnicl strength presented in Figure 4 show tht het treting the joints between 50 nd 180 C did not chnge their mechnicl strength mesured fter cooling to room temperture. It is lso shown in Figure 4 tht the mechnicl strength of the epoxy dhesives used with the BS dherent slightly incresed between 50 nd 100 C. This effect ws not observed in the joints of the EGS dherents. Although it could be ttributed to further cure of the dhesives, ccording to the literture 1, this increse in the mechnicl strength of the epoxy dhesives is ssocited with humidity diffusion through the dhesive/dherent interfces, due to the presence of porous iron oxides, which were not totlly removed during clening, or my hve formed fter the ppliction of the dhesives to the BS surfce. This process cn led to dhesive drying, thus incresing its strength Humidity degrdtion of mechnicl strength The exposure of the joints to 40 C nd 90% reltive humidity cused reduction of their sher strength, s shown in Figure 5. This wekening of the joints cn be ssocited with dhesive embrittlement nd/or the formtion of oxides on the dherents surfce 9,12. Both phenomen re cused by the penetrtion of humidity into the joints, ssocited with the severe tmospheric conditions simulted in the tests. According to the literture 1,7, humidity cn diffuse into the joints by cpillrity, through microcrcks formed in the dhesives or through microgps present in the dhesive/dherent interfces. The joints of both steels with dhesive E1 showed compct structure, without porosity nd with little oxidtion on the dherents surfce. On the other hnd, in ll joints, the A1 dhesive lwys presented porous structure. However, in spite of tht, both EGS nd BS joints with dhesive A1 showed high resistnce to humidity degrdtion, which cn be considered s n intrinsic property of the dhesive itself. The E2 dhesive lso showed some porosity nd suffered surfce oxidtion, especilly when joining the EGS steel. In fct, the EGS joints with dhesive E2 exhibited ccentuted humidity degrdtion nd the lrgest nd fstest loss of mechnicl strength, in reltion to the other dhesives. This result cn be ssocited with the type of oxides formed t the interfce. Nkzw 10 studied the embrittlement of epoxy dhesives in zinc coted steel joints nd found tht the fst decrese in mechnicl strength ws relted to formtion of zinc oxides on the dhesive/dherent interfces. He lso observed tht the use of phosphtized or chromtizted zinc coted steels incresed the Sher strength (MP) Figure 5. Effect of time of exposure t 40 C nd 90% reltive humidity on the sher strength. protection ginst zinc corrosion nd ttenuted dhesion degrdtion. The behvior of the epoxy dhesive joints of the uncoted BS steel found in the present work, chrcterized by smller humidity degrdtion thn tht of the EGS dherent (Figure 5), thus indictes tht the bonding forces between the epoxy dhesives nd iron oxides possibly formed on the BS dherents surfce re higher thn those estblished with the zinc oxides formed on the surfce of the EGS steel. 4. Conclusions BS/E1 BS/E2 BS/A1 EGS/E1 EGS/E2 EGS/A Time (months) The mechnicl strength of crylic nd epoxy joints ws not influenced by the type of metllic dherent, (uncoted or zinc coted steel) nor by the clening procedures pplied to the dherents surfce (degresing or decresing followed by chemicl pickling). For ech dherent type, the mechnicl strength of the epoxy dhesive joints showed superior vlues s compred to crylic dhesive joints. The sher strength of the joints ws continuously reduced s the test temperture incresed up to 180 C. The joints with the E1 epoxy dhesive conserved high vlues of mechnicl strength t tempertures up to 90 C. The other two dhesives exhibited ccentuted loss of strength t lower tempertures. Het treting the joints t different tempertures below the cure temperture produced the recovery of their mechnicl strength. After one yer exposure under severe humidity conditions (40 C nd 90% reltive humidity), the epoxy dhesives exhibited

5 Vol. 10, No. 3, 2007 Adhesiveness of Cold Rolled Steels for Cr Body Prts 271 strong decrese in sher strength, ssocited with humidity diffusion, which produced degrdtion of the dhesives nd wekening of the bonding between dhesive nd dherent. The zinc coted steel joints with the E2 epoxy dhesive showed the highest reduction in mechnicl strength, ssocited with the weker bond estblished between zinc oxides probbly formed t the dherents surfce nd the E2 dhesive. References 1. Kinloch AJ. Durbility of Structurl Adhesives. New York: Elsevier Applied Science; Engineered Mterils Hndbook. Adhesives nd Selnts. vol. 3. Mterils Prk: ASM Interntionl, 9639 Kinsmn Rod Mterils Prk, OH USA; Cotter JL, Hockney MGD. Metl joining with dhesives. Interntionl Metllurgicl Reviews. 1974; September 19: Jensen MK, Love BJ, Grnt JW, Cotton J, Keiser JR, Wilson DF. Comprison study of dicyndimide-cured epoxy bonded steel joints nd polymidomine-cured epoxy bonded steel joints. Interntionl Journl of Adhesion nd Adhesives. 2000; 20(6): Psternk H, Schwrzlos A, Schimmck N. The ppliction of dhesives to connect steel members. Journl of Constructionl Steel Reserch. 2004; 60(3-5): Hyshi K. Adhesive bonding for utomobiles. Techno Jpn. 1991; 24(10): McNmr DK, Ahern JS. Adhesive bonding of steels for structurl pplictions. Interntionl Mterils Reviews. 1987; 32(6): Schijve J. Ftigue of Structures nd Mterils. Dordrecht: Kluwer Acdemic Publishers; Hrris AF, Beevers A. Effect of grit-blsting on surfce properties for dhesion. Interntionl Journl of Adhesion nd Adhesives. 1999; 19(6): Nkzw M. Mechnism of dhesion of epoxy resin to steel surfce. Nippon Steel Technicl Reports. 1994; 63:16-22, October. 11. Mlucelli G, Priol A, Ferrero F, Qugli A, Frigione M, Crfgn C. Polyurethne resin-bsed dhesives: curing rection nd properties of cured systems. Interntionl Journl of Adhesion nd Adhesives. 2005; 25(1): Srgent JP. Durbility studies for erospce pplictions using peel nd wedge tests. Interntionl Journl of Adhesion nd Adhesives. 2005; 25(3):

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