7. Conclusions 7.1 Microstructure Features of Single Lap Joints with Filler Material Al99.8

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1 7. Conclusions Hybrid steel/al-alloy single lap joints as well as butt joints in the voestalpine welding geometry manufactured by the CMT-joining method developed by Fronius GmbH were studied, specifically: 1. The microstructure features of single lap joints with filler material Al99.8 [94]. 2. The microstructure and mechanical properties of hybrid butt joints with filler materials Al99.5, AlMn1, AlSi5 and AlSi3Mn1 without varying the plate parent materials [97]. 3. The microstructure and mechanical properties of hybrid butt joints with filler material AlSi3Mn1, comparing the use of AW5182-H111 and AW6181-T4 Alalloy plates [98]. 4. The residual stress state of hybrid butt joints with filler material AlSi3Mn1 [99]. Microstructure analyses of hybrid steel/al-alloy joints show that welding between aluminum alloy and filler and brazing between filler and steel took place. The zinc coating of the steel sheet improved the wetting of the steel by the molten filler material, so in the centre of the overlap joint zinc shifted towards the interface edges. At the edges of the joint Al-Zn eutectoid and areas with hypoeutectoid Al-Zn compositions were obtained and Zn partially diffused on the grain boundaries of the filler at their surroundings. 7.1 Microstructure Features of Single Lap Joints with Filler Material Al99.8 At the interface between the steel and the Al99.8 filler two intermetallic phases, Fe 2 Al 5 and FeAl 3 form the intermetallic phase seam (IMP). The thickness of the IMP is about 2.3µm, which is significantly thinner than similar intermetallic phase seams obtained by other joining processes dealing with the same system. The Fe 2 Al 5 is characterized by trapezoidal, nearly equiaxial single grains surrounded by Al-rich remains of the steel grains. The growth direction of the Fe 2 Al 5 crystals was not observed to correlate with any crystal axis. The FeAl 3 grains are significantly smaller than the Fe 2 Al 5 grains and most often elliptically shaped. The longest axis of the ellipsoids is nearly perpendicular to the original interface between the steel and the Al99.8 filler. Most of the FeAl 3 grains contain 64

2 a large amount of microtwins, presumably due to high stress caused by the volume expansion associated with the formation of the FeAl 3 phase. In the region near the intermetallic phase seam the Al99.8 filler contains iron enriched precipitates, which are presumably due to the diffusion of iron into the filler and precipitation from the oversaturated melt Microstructure and Mechanical Properties of Hybrid Butt Joints with Filler Materials Al99.5, AlMn1, AlSi5 and AlSi3Mn1 The process nature and choice of a non-precipitation-hardenable alloy as aluminium plate limits the HAZ to a range not detectable by standard characterization methods. The weld bead dimensions attained by the optimized joining parameters allow for weld bead strengths as high as the parent materials, regardless of the filler materials studied in this work. The intermetallic phase seam growth at DX54D plate/filler interface is hindered to less than 5µm, allowing for reaching plastic strains of at least 8% in tensile tests. As in single lap joints, the intermetallic phases θ-al 13 Fe 4 and η-al 5 Fe 2 form the IMP in CMT hybrid butt welds with Al99.5 and AlMn1 fillers, but in the latter case the η-al 5 Fe 2 grains appear to have grown slightly larger than in the former one. The phase α c -AlFeSi additionally appears in the IMP by adding up to 5%wt Si. Silicon containing filler materials hinder significantly the development of the IMP already in the early stages of its growth, but in excess Si is detrimental to the mechanical properties of the weld bead. Even though the presence of up to 1%wt Mn does not influence the nature of the IMP, it improves the overall behavior of the joint. That means, the choice of filler materials AlMn1 and AlSi3Mn1 allows for optimized mechanical behavior, provided that process fluctuations remain minimal Microstructure and Mechanical Properties of Hybrid Butt Joints with Filler Material AlSi3Mn1, and AW5182-H111 and AW6181-T4 Al-alloy Plates In joints manufactured with two different aluminium plate types, AW5182-H111 and AW6181-T4, the same intermetallic phases, α-al 19 Fe 4 MnSi 2, θ-al 3 Fe and η-al 5 Fe 2, were identified in the IMP. Mechanical testing revealed that joints with an undermatched steel parent material reached the maximum expected values, i.e., the mechanical properties of 65

3 the steel parent material, and presented ductile fracture by using both types of aluminium plates. Crash tests on hybrid test pieces comparable to those of crash elements in automotive applications revealed high toughness without failure in the joint seam Residual stress state of hybrid butt joints with filler material AlSi3Mn1 Residual stress analyses reveal a typical welding residual stress state of a weld without phase transformation. Both in longitudinal and in transversal direction compressive residual stresses are present in the steel plate parent material, while tensile residual stresses were determined steel and the aluminum alloy base materials at the vicinities of the weld seam. The area containing tensile residual stresses due to its higher heat conductivity is larger in the aluminum alloy than in the steel, extending ~20mm and ~10mm, respectively. The residual stress distributions at the top and at the bottom side of the weld are very similar due to controlled welding parameters and the symmetric welding geometry. 7.2 Remarks The CMT welding process has shown to produce aluminium-to-steel hybrid joints in both the single lap and butt configuration, the latter case of which has been proven a reliable quality from the mechanical point of view. Thus, the CMT process represents a viable alternative when considering the implementation of fusion welding techniques for the production of hybrid pieces in the automotive industry. A proper scaling study would be required to consider economical and technical singularities of implementation in the industry. The use of high resolution electron backscattered diffraction (HR-EBSD) and transmission electron microscopy (TEM), complemented by analytical means has proven to be necessary for accurately identifying the intermetallic phases present in the IMP for the reasons exposed formerly in 2.2. Nevertheless it must be mentioned that the preparation of reliable samples for investigation of the IMP via both EBSD and TEM requires much attention and can only be attained by overcoming the strong material property differences present between the hard steel, the much harder IMP and the soft aluminium alloy filler. In the case of EBSD, a flat transition surface between these three 66

4 components is crucial in obtaining suitable EBSP, whereby no induced deformation by sample preparation is also necessary. In the case of TEM, a proper preparation technique must be utilized to address specifically the IMP area and, as in the case for EBSD sample preparation, the difficulty of material differences must be overcome when aiming at obtaining a flat electron transparent foil without sample preparation artifacts. Furthermore, the use of EBSD may also be implemented as a powerful technique to characterize orientation related aspects of the different components in the joint, i.e., the aluminium filler, steel and the IMP. The respective differences between filler materials Al99.8 and Al99.5, between deep drawing steel plates DX56D and DX54D, between steel Zn-coatings Z140 and Z200 and between the aluminium alloy plates A6061-T4 and AW6181-T4 did not influence significantly the IMP characteristics at the interface of the studied hybrid joints. The joint configuration, i.e., overlap or butt joints, thus presented the same type of IMP and is qualitatively comparable. Given that the residual stress state investigated here involves macrostresses (of the type I) a similar behaviour would be qualitatively expected from dissimilar hybrid aluminium-tosteel joints with the other parent materials used in the work. 7.3 Outlook In the present work it has been shown that the hindering effect of Si on the growth of the phase η-al 5 Fe 2 is already very strong at early growth stages. It would be suggested to implement HR-TEM or HR-STEM in order to clarify if Si strategically incorporates in the low occupation Al sites in the structure of η-al 5 Fe 2 stated by [43] to be the diffusion path of Al atoms in its structure, and thus determine if the growth-hindering mechanism of this phase by Si occurs actually by such substitution. During the conduction of TEM and EBSD measurements on the IMP of the different joints, attention was called by orientation dependences appearing between different phases at the interface of hybrid joints. Further work in that direction would be of use in order to understand the nucleation and growth mechanisms of the phases in the IMP. Complementing experiments in order to obtain a wider understanding on how Si and Mn affect the development of the IMP in hybrid aluminium-to-steel joints would useful. It 67

5 would be suggested that welding parameters like the applied current, working voltage or the Si and Mn content in the filler material be varied. Also the implementation of experimental setups which allow regulating variables like time and temperature at different scales would be helpful to understand the growth kinetics of the IMP. For example, sessile drop experiments to determine wettability of different fillers and correlate fast solidification rates with measured temperatures or immersion tests, where later stages of IMP growth could be investigated. The implementation of nanohardness tests for localized measurement of properties of the different phases at the IMP could be beneficial for determining the real hardness step between the different components at the interface. Measurement of abaxial mechanical properties of the welds investigated, like in bending tests, would help complement the results obtained here. In the same manner, mechanical properties of overlap welds would be of use to compare their mechanical behaviour against butt welds. 68