NEW TRENDS IN ZINC COATINGS AT SHEETS USING IN AUTOMOTIVE INDUSTRY. Michaela KOLNEROVÁ a,

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1 NEW TRENDS IN ZINC COATINGS AT SHEETS USING IN AUTOMOTIVE INDUSTRY Michaela KOLNEROVÁ a, Jiří SOBOTKA b, Pavel SOLFRONK c, Pavel DOUBEK d, Lukáš ZUZÁNEK e a,b,c,d,e Technical University of Liberec, Studentská 2, Liberec, Czech Republic michaela.kolnerova@tul.cz, jiri.sobotka@tul.cz, pavel.solfronk@tul.cz, pavel.doubek@tul.cz, lukas.zuzanek@tul.cz Abstract This paper deals with the evaluation of sheets utility properties with zinc coatings and their utilization at carbody stampings production. Analysis of utility properties is carry out both for commonly used zinc coatings for car-body stampings production (Z100) and for newly developed zinc coatings new type (ZM). Evaluation of coatings is done out in light of tribological drawing processes strip drawing test as part of tribological testing. Moreover coatings are tested also from adhesive bonding testing point of view because adhesive bonding is subsequent technology to joint parts and quality of such joints is one of the crucial parameters for car body production process. Keywords: Coatings, Zinc, Magnesium, Tribology, Car-body 1. INTRODUCTION These days is still steel the most using material for car-body production. One of its disadvantages is corrosion instability in many common environments. So corrosion resistance is very important economic and technological parameters in industrial production. Research of materials whose surface is treated against corrosion is still running and developing and there is still put attention on it. Metallurgical research in the branch of new coating types development thus plays a great role in the automotive industry. It is mainly important for car-body parts where is providing the corrosion resistance one of the basic requires for car properties. That is why also determination of layer protective function loss is another integral part at monitoring utility properties of surface coatings (materials). 2. MATERIALS FOR AUTOMOTIVE INSUDSTRY In automotive industry are mostly utilized sheets with coating based on the zinc layer. Zinc is suitable mainly due to its relatively low cost and very good protection against corrosion. For car-body stampings are very often used sheets both hot dip galvanized and electro-galvanized with protection layer on one or both sides. As a problem arising from using coated sheets there is decline of coated layers quality at stamping itself when there is adhesion between sheet coating and tool. Quite often there is galling or scraping of zinc layer at zinc coated sheets.[1] That is why sheet producers are still trying to find out new methods hot to improve coatings, types, properties and methods of their preparation. Besides conventional coating on the zinc basis are these days tested coating based on compound of zinc and magnesium. Great attention in also put to determination the properties loss of new coatings and their waste during own sheet drawing process. To obtain perfect information about real tribological system of coatings is quite difficult and that is why there are carry out laboratory tests.

2 3. SHEETS WITH PROTECTIVE ZINC/MAGNESIUM LAYER (ZM) Metal coatings based on the zinc and magnesium (ZM) represent another level in development of hot dip galvanized coatings with anti-corrosion properties. ZM coatings are produced by alloying of Zinc (Zn) by Magnesium (Mg) and Aluminium (Al). Thus there is created protective layer with many advantages in comparison to conventionally used zinc coatings.[2] ZM coatings are better than materials using these days from the corrosion protection point of view. Based on the excellent corrosion protection can be thickness of metal layer strongly reduced in comparison to common zinc layers (Tab1 a Fig.1.). Table 1 Coating Zinc Zinc+Magnesium Z 100 g/m 2 ZM 70 g/m 2 Z 275 g/m 2 ZM 120 g/m 2 Fig.1 Coatings thickness layer reduction at keeping corrosion resistance properties [2] In Fig. 2 is shown characteristic of Z100 coating, zinc coating with amount 100 g/m2 produced by hot dip galvanizing method (HDG). In Fig. 3 is shown characteristic of ZM coating (coating composition 1.5% Mg, 2.5% Al, 96% Zn). Fig. 2 Characteristics of Z100 coating [3] Fig. 3 Characteristics of ZM coating [3] ZM coating due to their protection against corrosion are superior to coatings using these days, thickness of metal layer in comparison to common zinc coatings can be markedly lower. In Fig. 4 and 5 is comparison of both layers from corrosion-proof tests (samples are after 90 cycles in salt mist). From Fig. 4 is evident that ZM layer reveals better corrosion resistance and surface exhibits lower corrosion level. Identification of sheet parts after normal automobile running would be less complicated.

3 Fig. 4 Coating ZM [3] Fig. 5 Coating Z100 [3] Laboratory tests proved better protection against white corrosion. White corrosion represents only visual affection of surface and there is not lowered corrosion resistance. Simultaneously there were proved better corrosion properties at ZM coating against red corrosion which causes damage of basic material. In the Fig. 6 and 7 are coatings after corrosion loading of 60 cycles in salt mist of corrosion chamber. Fig. 6 Corrosion of ZM coating [3] Fig 7 Corrosion of Z100 coating [3] To determinate technological utility properties of protective coatings was chosen tribological test (strip drawing test). 4. TRIBO-PROCESSES Description of tribological conditions in system device tool material is essential part of drawing process these days. Research in this field has easily found its utilization also for numerical simulations of drawing process which are commonly used during stamping proposal. At Technical University of Liberec and in with cooperation with ŚKODA AUTO a.s. there was developed testing device SOKOL 400 for evaluation tribological properties. Its design enables to carry out tests under conditions which are close to conditions during real car-body stamping.. In Fig 8 equipment SOKOL 400 for tribological testing and principle of test For experimental measurement was used variant 1A [4] simulation of drawing conditions over the drawing edge with drawing radius R = 3 mm (Fig. 8), as a tool material was used non-hardened cast iron GGG 70L, tool temperature was 60±2 C according to real stamping conditions. As a tested material there was chosen steel sheet Z100 (Hot Dip Galvanized HDG) and sheet with the new coating ZM.

4 Fig. 8 Equipment SOKOL 400 for tribological testing and principle of test variant 1A 4.2. Condition of experiment [3] Sliding velocity : v =1mms -1 Table 2 Lubricant: Prelube Oil (1,5g/m 2 ) Contact pressure : p = 4;6;8MPa Temperature tool: T = 60±2 C Contact area tool: S = 3120mm 2 Material tool: GGG70L (unhardened) Radius tool R = 3 mm Substrate: steel sheet (Z100, ZM) 90 Mechanical properties ZM and Z100 RP0,2 Rm A80mm r n t MPa MPa % - - mm ZM ,5 2,33 0,199 0,70 Z ,1 2,51 0,230 0,73 Mechanical properties of sheets in direction 90 (toward rolling direction) are summarized in Tab. 2 [3] 4.3.Results of tribotest As a result from measurement there is diagram of dependence force displacement which serves as basis for evaluation coatings tribological properties. In the Fig. 9 and 10 are shown courses from individual measurements. Fig. 9 Diagram of measurement for coating Z100, Fig. 10 Diagram of measurement for coating ZM, (pressures: p = 4; 6 MPa, v=1mms -1 ) [3] (pressures: p = 4; 6; 8 MPa, v=1mms -1 ) [3]

5 As a primary evaluation criterion of tested coatings suitability was selected the size of drawing force FA at drawing strip over the drawing edge which clearly provides the differences between coatings quality and their drawing suitability. Differences between drawing forces are graphically shown in Fig. 8. For ZM coating has Drawing Force FA lower by 36.25% for contact pressure 4MPa. For 6MPa contact pressure has FA lower by 42.4% a than the coating Z100. Fig. 11 Results drawing forces FA coatings [3] 5. CONCLUSION Properties analysis of sheets with coatings for car-body stampings was in tribo-processes carried out by experimental tribological tests which were focused on the determination of new hot dip galvanized zinc coating type (ZM) suitability with new composition beside basic zinc element and alloys of Magnesium and Aluminium (specific composition ZM: 1.5% Mg, 2.5% Al, 96% Zn). These sheets should replace conventionally used zinc-coated sheets for production car-body panels. As a requirement for their application in series production there is minimization of negative effects at drawing to lower friction and eliminate galling. With regard to reliability of tribological processes was measurement carried out at sliding speed 1 mm/sec when the galling effect has the strongest influence and qualitative difference between individual coatings becomes evident. From Fig. 6 and 7 where are presented graphs of individual forces courses is evident that coating ZM revels better tribological properties at drawing because at adjustment contact pressure 8 MPa there was not possible to draw strip with coating Z100. This sample revealed galling up to its fracture. However such strip drawing tests do not include coating ZM surface integrity changes after sheet forming. Knowledge obtained from these experiments is just partial component for evaluation tribological properties for system: tool material lubricant technological conditions because as a final criterion for coating suitability it is not only value of force necessary to draw material but it is important to also take into account used lubricant and tool material and another technological conditions (see chapter 4.2.). Values of measured forces are graphically shown in Fig. 11. At simple tribological test between flat grips do not origin substantial changes in ZM coating crystallic structure till necking creation. That is why final values of strip drawing tests for ZM coating reveal different results that its behavior at intense sheet forming.

6 The rising of holding force at ZM coating at real drawing is probably caused by way of lubricant intrusion into microcracks on the coating surface and by increasing of Rz roughness value (height of roughness). At sheet forming surface slightly cracks so there is strong increase of braking intense as subsequent effect. Such effect is similar to that one when there is increase of pressure at tribological test. That is why is so important to deal with new types of combined grips for tribological tests which would prove or rebut influence of surface coating change on its properties. From testing of sheet stamping with coating Z and ZM in industrial praxis are till today known only e.g. results shown in Fig. 12 where is example of stamping when sheet with ZM coating is markedly lower formed into tool at the same adjustment of press. Thus there is strain increase behind drawing radius which sometimes can results as fracture. To remove such defects causes is possible by lowering of holding force till the limit for waviness creation or eventually by more lubricant. Using of ZM coatings will in some cases require tool design modifications or to use materials with higher formability. Fig. 12 Example of ZM coating stamping [5] Fig. 13 Example of Z coating stamping Z [5] In the Fig. 13 is shown that surface Z coating revealed strong galling which corresponds to results from tribological tests at strip drawing tests. Whether or not ZM coating will be able to compete with common zinc coatings is topic and task for its future development and also for carrying out tests simulating processes at car-body production. ACKNOWLEDGEMENTS This paper was supported by project Student Grant Contest from the TUL /SGS 21005/ with the cooperation of ŠKODA AUTO a.s. Mladá Boleslav, Czech Republic LITERATURE [1] SOBOTOVÁ, L., DULEBOVÁ, L.: Technologická spracovatělnosť plechov. In: Strojárstvo č. 9 /2011, s , ISSN [2] JUPPA, J.: Vliv deformace na změnu tribologických vlastností plechu s povrchovou ochrannou vrstvou Zn-Mg, Bakalářská práce 2013, TU v Liberci 2013 [3] Zinc/Magnesium coated Steel in Car Body Manufacturing. Research report ŠKODA AUTO a.s. and Technical University of Liberec, 2013 [4] KOLNEROVÁ, M.: Vliv technologických podmínek na vznik zadírání pozinkovaných plechů při tažení, Disertační práce, TU v Liberci, 2005 [5] Company materials ŠKODA AUTO a.s., Mladá Boleslav