THE INFLUENCE OF THERMAL LOAD ON PROPERTIES OF HETEROGENEOUS JOINTS BETWEEN STEEL AND ALUMINIUM ALLOY A. FRANC 1, J. TESAŘ 2

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1 CO 2 /Km [g] THE INFLUENCE OF THERMAL LOAD ON PROPERTIES OF HETEROGENEOUS JOINTS BETWEEN STEEL AND ALUMINIUM ALLOY A. FRANC 1, J. TESAŘ 2 1 Faculty of Mechanical Engineering, University of West Bohemia, Univerzitni 8, Pilsen, Czech Republic, afranc@kmm.zcu.cz 2 New Technologies - Research centre, University of West Bohemia, Univerzitni 8, Pilsen, Czech Republic tesar@ntc.zcu.cz Abstract Current concepts of lightweight car body combine different types of materials. The use of aluminium alloy and steel combination in one construction is quite common in automotive industry. The main still remaining issue is how to join these two different materials. When fusion joining is used, brittle intermetallics can occur in the joint. The amount of these intermetallics depends on thermal cycle of joining process. Presented experimental work deals with the influence of thermal load on Al-Mg-Si/steel joints properties. The analysis of temperature distribution has been done by infrared camera. Keywords: steel to aluminium joint, temperature distribution, infrared camera measurement 1. INTRODUCTION Current trends in automobile design reflect increasingly strict requirements for the composition of exhaust emissions. Throughout the world, CO 2 receives most attention of all exhaust gases. Regulations for car manufacturers vary across countries and organizations. As Fig. 1 shows, however, the reduction of CO 2 emissions continues to be a common trend. One of the efforts that can help to meet this demand is reducing the car weight. Hence, automotive design becomes ever more complex and relies on a number of different types of materials in manufacturing the car body. Combining steel and aluminium alloys has raised a number of materials and manufacturing difficulties CAFE Europe U.S. Government initiative Chinese Government statement Year Fig. 1 Planned CO 2 emission reduction The main still remaining issue is how to join these two different materials. When fusion joining is used, brittle intermetallics can occur in the joint [1]. The amount of these intermetallics depends on thermal cycle of joining process. The thermal cycle of the joining process strongly affects diffusion phenomena at the joint's interfaces and the degradation of properties of the aluminium alloy and steel. Consequently, it has a significant impact on end-use properties of the entire joint. Key aspects in controlling this process are monitoring and analysis of the temperature field during the creation of the joint. Data obtained by this means can be used for numerical modelling of properties of the joint and for its quality prediction. Measurement of temperature fields by infrared camera is a promising technique for development projects and industrial applications. It is well suited for

2 exploring temperature fields of surfaces with known emissivity [2]. Where the surface emissivity value is either not known or is too low, a surface paint with high emissivity value can be used [3]. This method was used for monitoring the temperature fields in aluminium alloy and steel sheets being joined. The joints were created by GMAW process. 2. EXPERIMENTAL PROCEDURE First parent material was aluminium alloy EN AW It is an Al-Mg-Si hardenable alloy. End-use properties of this alloy can be altered primarily by heat treatment and thermomechanical treatment. Mg x Si y precipitates have significant impact on its properties and are employed to achieve the hardening effect. Good corrosion resistance combined with relatively good weldability and machinability make the alloy very versatile in terms of application potential. The second parent material was dual phase (DP) ferritic-martensitic steel. It is an advanced fine-grained high-strength steel. Its characteristic is the presence of two basic microstructure components: martensite and ferrite. Martensite islands have positive effect on strength and hardness of the material. Ferrite matrix provides relatively high ductility, formability and toughness. Filler material selected for the experiment was AlSi5. Chemical composition and basic mechanical properties of the experimental materials are given in Tab. 1. Table 1 Chemical composition and basic mechanical properties of parent materials and filler material DP steel Chemical composition [w t%] Fe C Si Mn P S N Cr Ni Cu Mo Al Nb V B bal Rp 02 [MPa] 732 Aluminium alloy EN AW-6082; T6 Chemical composition [w t%] Al Cu Mg Mn Si Fe Ti Zn Cr bal Basic mechanical properties State Rm [MPa] R p0.2 [MPa] A 10 [%] T Basic mechanical properties / zinc coating Rm [MPa] A 80 [%] Hot-dip galvanizing 121 g/m Zn ~ 8.5 μm thickness per side Filler material AlSi5 Chemical composition [w t%] Si Mn Cu Ti Al Be Fe Zn Mg bal Basic mechanical properties Rm [MPa] Rp0.2 [MPa] A 5 [%] Dimensions of parent material specimens were mm. The sheets being joined were coated with a paint of known emissivity of The filler material was in the form of a bare wire of 1.2 mm diameter. As shown in Fig. 2, the sheets were clamped in lap joint configuration. The joint was made by gas metal arc welding process (GMAW). Infrared camera FLIR SC0 was placed to capture the joining process from above. The camera with the resolution of pixels can capture objects with temperatures of up to 0 C. The temperature range of the camera for the experiment was selected in reference to the known emissivity of sheet surfaces, Fig. 2 Lap joint configuration rather than to those of the electric arc and weld pool which were not known. The parts of sheets to be joined, where the filler material was transferred, were not painted. The main reason was to allow the electric arc to be established and stable. Three technological variants of specimens were prepared. The process speed of 10 mm/s was constant in all cases. Sets of

3 T [ C] T [ C] parameters for the first, second and third specimen were 41 A, 12 V, wfs 3 m/min; 52 A, 12 V, wfs 3.5 m/min and 64 A, 12 V, wfs 4 m/min, respectively. The parameters were chosen to provide comparison between conditions with increasing amounts of heat input to joined sheets. 2.1 Analysis of Temperature Field An image of the sheets to be joined taken with the infrared camera is shown in Fig. 3. In the middle of the infrared image, an area of the joint was clearly to be seen, whose emissivity was not known. The torch is shown in the left part of the image where temperature is the highest as well. It is the region where metal from the filler material is transferred to the weld pool. The infrared image also reveals higher transfer of heat to the aluminium alloy: Fig. 3, sheet no. (2). This highly heterogeneous temperature field is caused by the difference in thermal diffusivity of the materials to be joined. Consequently, one can also expect higher internal stresses in the area of the joint. The infrared image reveals that the heat source provided only minimal preheating of the materials to be joined. Using the ThermaCAM Researcher thermograhic software, Fig. 3 Temperature field of sheets being joined (1) steel sheet with less diffused heat (2) aluminium alloy sheet with further through conducted heat points spaced 5 mm apart were distributed in the infrared image in the area close to the joint and additional points spaced 10 mm apart were laid out in a greater distance from the centre of the joint for tracking the time dependence of temperature. The points were laid out in both directions perpendicular to the trajectory of the torch, as shown in Fig. 3. Temperature vs. time curves were obtained for each of the 33 points, see Fig. 4, Fig Time [s] Fig. 4 Temperature - time curves for monitored locations on the side of the aluminium alloy sheet, specimen Time [s] Fig. 5 Temperature - time curve for monitored locations on the side of the steel sheet, specimen 1 Fig. 4 and Fig. 5 show the time dependence of temperature in the aluminium alloy sheet and in the steel sheet at various distances from the centre of the joint. Points, which were closest to the centre of the joint and, at the same time, in a region with known emissivity, were 10 mm from the centre of the joint. The highest temperature found at these points was C on the surface of the aluminium alloy sheet (Fig. 4) and C on the surface of the steel sheet (Fig. 5). With increasing distance from the centre of the joint,

4 T [ C] T [ C] the maximum temperatures of the points decrease and shift to later times. The time interval from t = 0 s to approx. t = 12 s is the time required for the torch to arrive at the monitored points in the centre of the sheets. A slight temperature oscillation can be seen at the monitored points prior to the actual increase in temperature. In view of the arrangement of the experiment, this occurrence may be attributed to the torch s movement across areas closest to the centre of the joint, i.e. points 10 and 15, Fig. 4, Fig Specimen 1 Specimen 2 Specimen 3 Specimen Fig. 6 Maximum temperatures at monitored points on the aluminium alloy sheet at various distances from the joint, specimens 1, 2 and 3 0 Specimen 1 Specimen 2 Specimen 3 Specimen Fig. 7 Maximum temperatures at monitored points on the steel sheet at various distances from the joint, specimens 1, 2 and 3 Fig. 6 and Fig. 7 show maximum temperatures measured at the monitored points on aluminium alloy and on steel at various distances from the joint s centre for variants 1 through 3. Measured data shows that temperatures increase with increasing welding current at all points. When constant current is used, the maximum temperature decreases with increasing distance from the joint. 2.2 Examination of Joints Metallographic cross sections the joints were prepared for observing their macrostructures and for HV hardness measurement. Test specimens were made for (room temperature) tensile test. As Fig. 8 shows, the weld bead geometry changes: its height decreases and its length increases with increasing heat load. These changes are strengthened by the fact that a greater amount of filler materials melts. The maximum length of the joined area was found in specimen 2: 48 μm. Decohesion of the sheets occurred in the right-hand part of specimen 3. The images show a clear increase in porosity in the sequence of specimens from no. 1 to no. 3. This porosity is assumed to be caused by the zinc coating evaporating from the steel sheet surface and being trapped in the filler material. Fig. 8 Macrostructures of the joints; specimens 1, 2, 3.

5 F max [N] Hardness HV 0.05 HV 0.05 hardness was measured on specimens 1 through 3. In all cases, indentations were on a centre-line of the section of aluminium alloy sheet. The zero point of the coordinate system was on the left-hand side of Distance [mm] Specimen 1 Specimen 2 Specimen 3 Fig. 9 Hardness profiles of the specimens the weld bead. Measured data showed that AlSi5 filler material had lower hardness than the unaffected aluminium alloy in T6 condition. Hardness of the HAZ is lower than that of the unaffected parent material, as shown in Fig. 9. The HAZ widths were as follows: 10.7 mm, 11.7 mm and 16 mm in specimens 1, 2 and 3, respectively. The hardness profiles for all tested specimens are typical of the structure degradation taking place in fusion joints in 6xxx-type aluminium alloys in T6 condition [4]. Local minimum of the hardness levels in HAZ in the aluminium alloy in specimens 2 and 3 were almost equal: 65 HV 0.05 and 68 HV 0.05, respectively. These values are almost equivalent to those of the filler material (approx. 65 HV 0.05). Local minimum of hardness in specimen 1, which exhibited the mildest heat load, was 78 HV Specimens of the aluminium alloy-steel lap joint were 20 tested in tension. Thickness of the tensile test 2800 specimens was 10 ± 0.15 mm. Five tests were conducted for each joint variant. Values of maximum 2700 force at fracture are shown in Fig. 10. The scatter of the data of maximum force at fracture for variant (F max = 2575 N) was relatively high: ± 70 N. This was 20 caused by the fact that in some specimens the fracture occurred in the region rich in intermetallic phases, i.e at the interface between the aluminium alloy and steel Maximum force was measured in case of Specimen 1 Specimen 2 Specimen 3 Specimen specimen 2: F max = 2837 N. Fractures in these specimens occurred predominantly in the weld metal Fig. 10 Tensile tests (F max ) of the aluminium alloy and in HAZ. Fractures in specimens 3 (F max = 2517 N) occurred predominantly at the interface between aluminium alloy and steel. 3. CONCLUSION AND FUTURE WORK Temperature fields in joints between aluminium alloy and steel can be monitored using infrared camera. Temperature distribution of the sheets being joined reflects material and technological parameters. In addition to data on temperature in the vicinity of the electric arc, it would be useful to obtain additional information by another temperature measurement technique, e.g. a pyrometer. The measurement provided information on time dependence of temperature at various locations of the sheets being joined. The temperature field values will be used as input for numerical simulation of joining of these materials. HV 0.05 hardness was measured along a line in the HAZ of the EN AW-6082; T6 aluminium alloy. Degradation of the aluminium alloy based on hardness profiles was measured. Tensile test data proved that the amount of heat input has significant impact on strength of these dissimilar joints.

6 ACKNOWLEDGEMENTS The result was developed within the project reg. no.sgs and within the CENTEM project, reg. no.cz.1.05/2.1.00/ that is cofounded from the ERDF within the OP RDI programme of the Ministry of Education, Youth and Sports. REFERENCES [1] BRUCKNER. J. Der Cold Metal Transfer (CMT) - Prozess von Stahl - Alu Verbindungen und seine Möglichkeiten. In Eurojoin 5 : Internationaler Kongress der EWF. Wien : [s.n.], 5. s. V17-1-V17-8. [2] MALDAGUE X.P.V.: Theory and Practice of Infrared Technology for Nondestructive Testing, Wiley-Interscience, ISBN: , 1. [3] LUONG, M.P. Fatigue limit evaluation of metals using an infrared thermographic technique, Mechanics of Materials 28 (1998) [4] ALIA, B., et al. ASM Handbook : Welding, Brazing, and Soldering. Theodore B. Zorc. 4th rev. edition. United States of America : ASM International, s. ISBN