SOUTĚŽNÍ PŘEHLÍDKA STUDENTSKÝCH A DOKTORSKÝCH PRACÍ FST 2007

LOW ENERGY WELDING OF HETEROGENEOUS JOINTS BETWEEN STEEL AND ALUMINIUM

Aleš Franc

ABSTRACT The welding of metals with dissimilar physical and chemical properties has become a significant sphere of interest thanks to specific demands on the components and construction of various kinds of equipment. Arc welding has proved to be an economically affordable and efficient method of joining heterogeneous materials. The highest usage represents the combination of steel and aluminium. Steel is very suitable for stable and strong components and aluminium for filling. These joints have a large application in many industrial branches. Thanks to their small weight, good plasticity and corrosion resistance they are used e.g. in automotive industry. This study deals with the influence of changes in the basic welding parameters on the quality of heterogeneous joints of steel and aluminium produced by low energy welding method. The microstructure analyses of these joints and their mechanical properties are also presented. Low carbon (zinc coated) steel sheets ČSN 41 1331 and aluminium alloy sheets ČSN 42 4400 have been chosen as parent material. AlSi5 is used as filler material. KEYWORDS Aluminium – steel welding, heterogeneous joints, low energy welding, arc welding. 1. INTRODUCTION As a result of specific requirements on components and construction of a variety of equipment, the importance of welding metals depending on their physical and chemical properties is increasing. In many cases it is not possible to say whether one material itself is able to satisfy these requirements. This and economic reasons drive the development of new welding technologies of heterogeneous materials, particularly thin sheet metals. Using welding techniques with a relatively high level of heat transfer, e.g. MIG and TIG, the materials undergo complex tension fields and intermetallic intermediate phases of high hardness and strength, but low ductility. For example, when welding aluminium alloy with steel (namely galvanized sheets) technology is sought to create a welded connection where only the aluminium alloy melts and the steel remains in the solid phase. Our research partner has developed a method of GMAW where less heat is transferred to the material, despite achieving a secure welded connection. The complete welding process operates in the power source without mechanical impact on the feed of the welding wire. 2. PROBLEM CHARACTERISTIC 2.1 Classical welding arc Short (classical) welding arc is created when MIG/MAG welding at lower performance, i.e. at low welding voltage and low current. This is when the conventional metal transfer is formed which is characterized by a repeated phases of welding arc and shorting - fig.2-1.

Fig. 2-1 Metal transfer (schematic), voltage Us and current Is during shorting of welding arc

During repeated firing of the welding arc, part of the welding pool may evaporate explosively if the increased welding current is not sufficiently damped in the current area. This results in either the creation of spatter or a very low dynamic welding process leading to its instability. When welding thin sheets, e.g. 1 mm (approximate limits), with repeated firing of the welding arc, the welding bath ‘drops’ and holes are formed in the sheet. When welding surface treated sheets, e.g. galvanized, a danger exists that the plated surface near the weld and on the lower side will evaporate (arising from its being burned). This makes arc welding unsuitable for heat sensitive and surface treated sheet metals. Since the 1980s welding has been carried out with the aim of lowering splatter but not heat transfer [1] [2]. Another step on the way was the modified short arc ChopArc [3] and [4], with which MAG welding achieved notable progress, especially in the field of thin sheets of 0.2 - 0.8 mm. An adaptive regulation system was also developed which optimizes the quality of the process in real time [5]. The latest development solves welding with discontinuous mechanical feed of welding wire. This requires Push-Pull drive with an adjustable motor with high dynamics located in the welding nozzle. This process is suitable for automated welding but not for manual work. 2.2 Arc modification - joining with low heat input The reason for developing welding process using arc modification was to achieve a low energy process without mechanical impact on the feed of the welding wire (vwire speed = const.). The electrical power of the welding process with repeated firing of the electric arc is a deciding factor for the successful welding of thin metal sheets. This is shown actively throughout the whole process - fig. 2-2.

Fig. 2-2 Metal transfer, voltage Us and current Is

2.3 Metallurgical weldability of welded joints between steel and aluminium When welding galvanized steel sheet and aluminium sheet it is essential to take into account their specific properties. It is important to consider their differing physical and chemical properties, e.g. thermal expansion coefficient, corrosivity, and also atomic properties, e.g. crystal lattice and lattice constant. Heat transfer during welding causes changes in the crystal lattice and leads to the creation of an intermetallic phase (IMF) – fig. 2-3 [6].

Fig. 2-3 Phase diagram Al – Fe [6]

A notable problem is posed by the thickness (generally volume) of the intermetallic phase, which is dependent on the amount of heat transferred during welding. Therefore until now all tested welding technology for heterogeneous materials has concentrated on how to limit this phase or completely remove it. The intermetallic phase is a diffusion controlled process. To create a welded joint between steel and aluminium, the steel sheet must have guaranteed wettability. This requirement is met by the surface being galvanized. The creation of the intermetallic phase is shown in fig. 2-4.

Fig. 2-4 Origin of intermetallic phase (IMF) a)Formation of locally saturated solid solution in surrounding imperfection

b)Formation of nucleus of new phase c)Horizontal growth of IMF nucleus along the join between steel and aluminium

d)Formation of second IMF and further growth of first IMF e);f) Growth of second phase IMF

According to Rjabow’s model [7] at normal surrounding temperatures Fe-Al on the side of the steel phase η - Fe2Al5 and on the side of the aluminium phase Θ - FeAl3 can be found on the constitutional diagram. Comparison with the other intermetallic shows that these two phases exhibit the worst useable properties. Phaseη ,which has an extremely high value of microhardness (up to 1100HV), has a markedly negative effect. It must also be remembered that the differing physical and chemical properties of steel and aluminium bring corrosion problems to welded joints. In the resulting high electrochemical potential difference between the iron and aluminium, which reaches 1.22V, it is important to take into account increased corrosive susceptibility (e.g. intercrystalline corrosion of iron alloy). The potential difference between the zinc coating on the steel sheet and the aluminium alone is 0.899V.

3. EXPERIMENT The best method for selecting parameters of test samples and their sampling was designed. The structural and mechanical properties of 1 mm welded steel plates with aluminium plates and the basic materials were also ascertained. 3.1 Parent materials The parent materials used for welding were as in tab. 3-1 and tab. 3-2.

Tab. 3-1 Basic mechanical values and chemical composition of galvanized steel sheets

C Mn Si P S Ti0,04 0,2 0,01 0,006 0,012 0,001

Rm [MPa] A80 [%]357 36

Steel ČSN 41 1331Chemical composition [wt.%]

Mechanical values

Cold galvanizing 245 g/m2

Tab. 3-2 Basic mechanical values and chemical composition of aluminium alloy sheets

Al Cu Mg Mn Si Fe Ti Zn Cr96,65 0,0213 1,0497 0,6184 1,008 0,2255 0,0293 0,0104 0,0646

Rm [MPa] Rp0,2 [MPa] A [%]117,7 51,5 29,03

Aluminium alloy ČSN 42 4400 (AlMgSi)Chemical composition [wt.%]

Mechanical values

3.2 Filler material 1.2 mm AlSi5 filler wire was used for welding as in tab. 3-3

Tab. 3-3 Basic mechanical values of welded iron and chemical composition of AlSi5 filler wire

Si Mn Cu Ti Be Fe Zn Mg5,1 0,01 0,01 0,06 0,0002 0,1 0,01 0,02

Rm [MPa] Rp0,2 [MPa] A5 [%]165 55 18

Filler material AlSi5Chemical composition [wt.%]

Mechanical values

.

3.3 Technological testing of welded joints 3 technological variants for welding joints were designed. They were welded sheets as in tab. 3-1 and tab. 3-2 (galvanized steel-aluminium alloy), from which samples no.1 to 9 were prepared. Lap-jointed welded joints were tested. 3.4 Evaluation of welded joints 3.4.1 Visual check After welding the test samples were visually checked using a magnifying glass of 7x magnification (ČSN EN 970). No defects were detected. 3.4.2 Capillary colour indication test Carried out in accordance with ČSN EN 571-1 with 100% coverage. No cracks were detected. Details of this test are shown for sample no. 5 in fig. 3-1.

Fig. 3-1 Detail of test on weld no. 5

3.4.3 Tensile test Tensile testing was carried out on the parent materials, (steel and aluminium sheet) and the welded joint. Testing was performed at 200C according to ČSN EN 1002-1. The dimensions of the test bars conform to ČSN EN 895. In all welded joints fracture occurred in the parts made of aluminium alloy, outside the welded area in the HAZ area – fig. 3-2. From the point of view evaluation of ductility, 96% of the breaks are located in the measured length (L0 ). The results of the tensile tests show that the change in parameters in the selected area has no substantial effect on the strength characteristics of the welded joints.

Fig. 3-2 Test bars after destruction

3.4.4 Macroanalysis One weld sample was taken from each technological variant. In fig. 3-3 can be seen the porosity in the welded metal, which is caused by the evaporation of the zinc layer. In this image is also clearly seen the dual character of the weld: on the steel side the brazing process, and on the aluminium side the welding process.

Fig. 3-3 Macrophoto of weld sample 1, magnified 15x, stereo microscope NIKON SMZ 800

3.4.5 Microanalysis In fig. 3-4 of weld sample 1 is clearly visible detail of the boundary (melted) of the Al sheet and the welded metal. Micropores can also be observed in the welded metal without any defects in the melted boundary. In fig. 3-5 of weld sample 2 the brazing boundary (steel - welded metal) without defects is visible.

Fig. 3-4 Microstructure - weld sample 1

Fig. 3-5 Microstructure of the base material – welded metal

On the basis of microscopic investigation of all welded joints (i.e. weld samples 1 to 9) no differences in microstructure related to changing parameters of welding technology. 4. CONCLUSION In the framework of this study low energy welding method was tested for heterogeneous welded joints: 1 mm thick galvanized sheet ČSN 41 1331 and 1 mm thick aluminium sheet ČSN 42 4400 (AlMgSi). This method was developed by our research partner and is currently undergoing further development and applications in industry are being investigated, primarily in the automobile industry. It was found that in a given area of use this automated welding technology ensures an appropriate quality of welded joints from both a mechanical and structural point of view. To be able to use this automated welding technology for manual welding in shielding gas for butt and 3D welds, further testing must be carried out. Additional analyses of the intermetallic phase and corrosive properties as well as the stability of the zinc layer all deserve further investigation.

REFERENCES

[1] Kabushiki Kaisha Kobe Seiko Sho: Output control of SC welding power source, PatNr.: US 4546234, Kobe Steel, Japan, 1984.

[2] The Lincoln Eletric company: STT – Surface Tension Transfer, Pat.Nr.: EP 0324960 B1, USA, 1989, a EP 1232825 A3, USA, 2002.

[3] Goecke, S.-F.; Dorn L.: Untersuchungen zum Einfluss der Prozessregelung und Schutzgaszusammensetzung auf Spritzerbildung und Nahtgeometrie beim MAG-Kurzlichbogenschweissen von Stahl-Dünnblech unter 0,5 mm Dicke, Final Report DFG (Deutsche Forschungsgemeinschaft) Do 202/26-3 (2000)

[4] Goecke, S.-F.; Dorn L.; Hübner M.: MAG-ChopArc-Schweissen für Feinstblech > 0,2 mm. Konferenz-Einzelbericht im Tagungsband Grosse Schweisstechnische Tagung – GST 2000, Nürnberg 27.-29. Sep. 200, DVS-Berichte Band 209, (2000), S. 163-168

[5] Goecke, S.-F.; Metzke, E.; Spille-Kohoff, A.; Langula, A.: ChopArc – MSG-Lichtbogenschweissen für den Ultraleichtbau, bmb+f-gefördertes Verbundprojekt, Abschlussbericht, Fraunhofer IRB Verlag, 2005, ISBN 3-8167-6766-4

[6] Massalski, T. B.: Binary Alloy Phase Diagrams. American Society for Metals, Metals Park, Ohio, 1986, 2224 p.

[7] Rjabov, W. R.: Schmelzschweissen von Aluminium mit Stahl, Kiev, 1969

Ing. Aleš Franc, The University of West Bohemia in Pilsen. Univerzitni 22, 306 14 Pilsen, tel.: 377 638 330, e-mail: afranc@kmm.zcu.cz