HETEROGENEOUS JOINTS BETWEEN STEEL AND ALUMINIUM MADE BY MODIFIED MIG PROCESS. Aleš FRANC

Transcription

1 HETEROGENEOUS JOINTS BETWEEN STEEL AND ALUMINIUM MADE BY MODIFIED MIG PROCESS Aleš FRANC The University of West Bohemia, Univerzitni 8, Pilsen, Czech Republic, Abstract Metal inert gas welding is an efficient technology used for many years across the whole industry. Advanced constructions and equipments especially in automotive industry combine wide variety of materials. Combination of commonly used construction materials - the aluminium and steel, bring high demands on welding technologies. It has been proven that conventional arc welding technologies are unable to join steel to aluminium, in industry applicable quality and reproducibility. This fact leads to the new development of welding technologies modifications. This article deals with heterogeneous joints between steel and aluminium alloy with focus on their major weldability problems. In experimental procedure the low carbon zinc coated steel sheet is joined with aluminium alloy sheet (6000 series) by MIG modification. Basic microstructure evaluation of these heterogeneous joints is also presented. Keywords: steel to aluminium joint, heterogeneous joint. 1. INTRODUCTION Dynamic research of materials and their potential industry use brings higher demands on processing technologies. There are many issues that need to be solved especially how to transform semiproducts made of these modern materials to final products with minimal degradation of their (e.g. mechanical) properties. It s obvious that one kind of material cannot satisfy increased demands on modern construction. Aluminium alloys, steel and their combination proved to be important for construction across various branches of industry. Strong interest comes also from transportation industry, mainly from the necessity to design lightweight vehicle body. A great weight saving is offered by the use of materials with a lower density than steel such as aluminium alloys. The second common approach to decrease the weight of vehicle body is based on modern steels (AHSS, UHSS) usage. Both these approaches have widely discussed advantages and disadvantages. Necessarily the problems dealing with mixed constructions where aluminium alloy and steel parts are project have to be solved. Joining steel to aluminium represents great challenge for nonmechanical (clinching, screwing, riveting) joining technologies, Fig 1. Fig. 1 Car body of Audi TT [1] 1

2 2. THERMAL JOINING OF STEEL TO ALUMINIUM The main problem for thermal technique of joining steel to aluminium is the diversity of physical properties of joined materials, such as differences in thermal conductivity, melting points, etc., see Table 1. From the metallurgical point of view the principal factor which is decreasing the thermal weldability of steel to aluminium is low solid solubility of Fe in Al and formation of brittle intermetallic phases [2], Fig. 2. Binary system Fe-Al is characterized by solid solution αfe and six intermetallic phases Fe 3 Al, FeAl, Fe 2 Al 3, FeAl 2, Fe 2 Al 5 and FeAl 3 whose hardness vary from 330 HV 1 (Fe 3 Al) to 1013 HV1 Fe 2 Al 5. [2]. Tab. 1 Differences in physical properties of steel and aluminium alloy [3]. Property EN AW-6082 Low carbon steel EN AW-6082/steel Density [g/cm 3 ] 2,70 7,86 0,3 Melting point [ C] ,4 Boiling point [ C] ,6 Electrical resistivity [x10-6 Ω.m] 0,038 0,218 0,2 Thermal conductivity [W/m.K] ,8 4,0 Linear thermal expansion [x10-6 / C] ,0 Specific heat capacity [J.g -1. C -1 ] 0,9 0,478 1,9 Thermal diffusivity [m 2 /s] 7,2 1,2 6,2 Fig. 2 Binary phase diagram of Fe-Al system [4] 2

3 Fusion welding used to join steel to aluminium leads to formation of intermetallic layer located on the boundary. Its thickness varies from several to tens microns [5]. Based on principle of this intermetallic layer formation there are possibilities to decrease its thickness and this way to improve mechanical characteristics of these joints. It could be done either by chemical barrier e.g. by filler material (alloying by Si or Zn) or by proper temperature-time parameter which will obstruct the diffusion between Fe and Al [6]. Both these approaches lead to the necessity of improving conventional joining technologies and researching new ones. Metal inert gas welding and metal active gas welding - MIG/MAG are well-know and well-used welding technologies for decades in automotive industry worldwide. They can produce low-cost welds with sufficient quality and reproducibility and have potential for further development. It has been proven that MIG has potential to be suitable welding technology for creating steel to aluminium joints. Unfortunately the unstable arc and metal transfer lead to poor joint quality and spatter. This fact drives researchers to improve conventional MIG process and optimize it for this difficult task. The CMT (so called Cold Metal Transfer) process invented by Fig. 3 Lap joint - steel and aluminium; (1) aluminium sheet, Fronius International GmbH is MIG/MAG based welding (2) steel sheet, (3) filler technology suited to joining steel to aluminium. It uses special material metal transfer from the filler material to the weld pool with lower heat input during short-circuit phase. This technology also incorporates a wire retraction movement into the welding process to help the droplet detachment. Precise digital control of the process leads to almost spatter-free joints. When joining steel to aluminium by CMT the typical weld joint configuration is a lap joint as shown in Fig. 3. Electrical arc melts only aluminium sheet while the steel remains in solid state. Thus, this joining process cannot be categorized as a fusion welding but as a combination of fusion welding and brazing. 3. EXPERIMENTAL PROCEDURE Aluminium alloy sheet EN AW 6082 with thickness of 1 mm was prepared to T4 state - solution heat treated and naturally aged. The chemical composition of this alloy is: Al %, Cu %, Mg 0.088%, Mn 0.47%, Si 1.033%, Fe 0.247%, Ti, Zn, Cr rest. Light microscopy (polarized light effect) provided the evidence of coarse grain structure with an average grain size of μm (±1 μm). During specific thermal treatment several processes take place such as the precipitation of intermetallics like discrete rounded Mg 2 Si (black) particles or angular intermetallics (Fe,Mn) 3 SiAl 12 (gray scriptlike), Fig. 4. This sheet was joined by CMT welding technology with the low carbon steel sheet DX51 Z140 (zinc coated by hot-dip Fig. 4 EN AW 6082 structure galvanizing) of the same thickness. The chemical composition is: C 0.04%, Mn 0.2%, Si 0.01%, P 0.006%, S 0.012%, Ti 0.001%. The ferritic-pearlitic structure with an average grain size of 7.9 μm (±1 μm) was observed using light microscopy, Fig 5. As a filler 3

4 material the common AlSi5 wire of diameter 1.2 mm has been chosen. Its chemical composition is: Si 5.1%, Mn 0.01%, Cu 0.01%, Ti 0.06%, Be %, Fe 0.1%, Zn 0.01%, Mg 0.02%. The sheets were cut into rectangular welding samples 400 mm long and 150 mm wide and after degreasing were longitudinally joined in the lap weld configuration with travel speed of 10 mm per second. Welding tests were provided by ABB IRB 1600 robotic system to ensure reproducibility of welding speed, torch angle and trajectory adjustments. All samples were joined under inert atmosphere of Argon with purity 4.6. Other main process parameters were: I=68 A and U=12V. Fig. 6 shows example of joint between steel and Fig. 5 DX aluminium with its both characteristic types of joining structure process. Aluminium sheet was welded with filler material 51, ferritic-pearlitic while steel was brazed by this filler. Optically attractive weld bead has been achieved without any spatter on both sheets, Fig 6. Fig. 6 Macrostructure of steel to aluminium joint / weld bead Macrostructure analyses also showed small amount of porosity located in the filler material only. Visually different regions (brighter) were observed on the outer and inner part of the weld bead. For further investigation of bright outer region (left side, Fig. 6) SEM analyses have been provided, Fig. 7. The microstructure analysis of this region showed dendritic structure. The electron Tab. 2 Chemical composition of analyzed weld bead region probe microanalysis Element wt% proved the enrichment Al 31,5 of the filler material by Zn 40,9 Zn from the steel Fe 1,6 coating which was not Si 5,5 presented (after joining) Mn 1,2 C,O rest as a compact layer on aluminium/steel interface, Table. 2. It is obvious that zinc Fig. 7 EDX analysis of weld bead region layer was partly dissolved into the filler material and partly evaporated resulting in pores, Fig. 6 and Fig. 7. As 4

5 documented SEM analyses the interface between steel and aluminium is separated by relatively thin intermatellical layer Fe x Al y, Fig. 8. Its average thickness was 2.11 µm. Different morphology of this intermatellic layer border was observed. It was irregular on the side close to the aluminium but obviously smoother on the opposite side close to the steel, Fig 8. Fig. 8 Intermetallic layer between steel and aluminium Other welding tests were focused on the influence of zinc coating (produced by hot-dip galvanizing) to the joining process. It was investigated that zinc coating plays an important role for sufficient wetting of the steel surface by the aluminium. Experiments proved that it is not possible to achieve adequate wetting of steel without zinc coating even when different torch angles and positioning were used. 4. CONCLUSIONS The investigations proved that CMT process is suitable for manufacture steel to aluminium joints even when common filler material like AlSi5 is used. Above described experiments demonstrate that zinc layer vaporizes and partly dissolves to the filler material especially in the outer and inner regions of the weld bead. The intermetallic layer Fe x Al y has been located on aluminium/steel interface and its thickness varied from 2 µm up to 3 µm. Further investigation is planned to identify and quantify this steel/aluminium interface as well as the investigation of mechanical characteristics of these joints. REFERENCES [1] Lightweight Design - It s an Art: The Audi ASF Design Principle. AutomobilesReview [online] [2] KOBAYASHI, Shigeaki, YAKOU, Takao. Control of intermetallic compound layers at interface between steel and aluminum by diffusion-treatment. Materials Science and Engineering. 2002, vol. 338, no. 1-2, s [3] MatWeb : Material Property Data [online] [4] ASM Handbook Volume 3 : Alloy Phase Diagrams. Hugh Baker. [USA] : ASM International, s. [5] M. Kreimeyer and G. Sepold, Laser steel joined aluminium-hybrid structures, Proceedings of ICALEO'02, Jacksonville, USA (2002). [6] Ryabov V.R. Welding of Alloys to Steels/Ed. by B.E.Paton; Electric Weld. Inst.,Ukr.Acad. of Science. Amsterdam:Harwood Academic publishers, p. 5