|Period:||01.01.2013 - 30.04.2019|
|Partner:||Prof. Dr.-Ing. G. Wagner, M. Thomä; TU Chemnitz, Lehrstuhl Verbundwerkstoffe
Dr.-Ing. B. Wolter, Dr.-Ing. B. Straß; Fh IZFP, Saarbrücken
|Project Manager:||Dr. Sigrid Benfer|
Aim of the Project
Only with hybrid construction is it possible to combine the advantages of dissimilar materials. Force-fitted or form-closed joints can be realized in an easy manner, but the strength of welded joints can rarely be achieved. However, fusion welding of dissimilar materials is often impossible as a result of the different melting ranges. Moreover, in many cases undesirable mixed phases occur in the weld area, whereby the attainable strength of the joints will be reduced.
The aim of the research project is to use friction stir welding (FSW) to join material groups that are well established in industrial applications, but that cannot be joined by fusion welding. These are the light weight materials aluminum and magnesium as well as aluminum and steel. FSW enables a joining by plastic deformation. To eliminate strength reducing effects by occurring brittle phases and to improve the stirring in the joining area power ultrasound is additionally transferred into the welded area (USE-FSW). An estimation of the quality of the produced welds will be carried out by monotonic and cyclic as well as non-destructive investigations. To determine the complex problems of corrosion of the hybrid material joints, detailed examination of the corrosion behavior, especially of the welding area, are made.
The USE-FSW process has been successfully applied to Al/Mg-hybrid joints (see publications) and was now transferred to Al/steel-joints. Investigations on joints made out of an Al wrought alloy (EN AW-6061) and cold rolled DC04 steel have been in the focus of the second project period.
The hybrid joints were characterized by Scanning Kelvin Probe (SKP) measurements first. At the beginning of the measurements the Volta potential difference between the Al alloy and the steel is about 1.0 V. Both materials exhibit a shift of the Volta potential to more positive values within the measurement time due to surface oxidation processes. The Volta potential shift for the steel is about 0.1 V whereas the aluminum alloy shows a shift of 0.2 to 0.3 V. This leads to a slight reduction in the difference between the materials during the measurement. The Volta potential maps show an influence of the ultrasound on the welding area of Al/steel hybrid joints but it is much less pronounced than in the case of the Al/Mg-hybrid joints. This can be explained by the different joining parameters for the different joints. For the Al/Mg-hybrid joints the pin-offset was 1 mm to the Mg which results in a mechanical mixing of both materials during the process. In case of the Al/steel-hybrid joints, because of the high difference in the melting points of the materials, the pin only rotates within the Al with a distance of 0.3 mm to the faying surface of the steel. Therefore the influence of the ultrasound introduced via the steel sheet is less visible. Only the shoulder of the rotating tool has mechanical contact to the steel and may transport material from one side to the other. The SKP measurements resulted in about 1 V more negative Volta potential values for the EN AW-6061 Al alloy compared to the DC04 steel. This is a similar difference to that measured between AZ80 Mg alloy and AC-48000 Al alloy where the Al alloy has shown the more positive values. From that result one would expect that the EN AW-6061 Al alloy will show stronger corrosion in corrosive media than the steel particularly if both phases are in simultaneous contact to an electrolyte.
The measurements in air are not directly transferable to that of in a liquid electrolyte, as the open circuit potential (OCP) measurements in 0.5 molar NaCl solution show. The OCP of the EN AW-6061 alloy exhibits a shift from about -0.7 V to -0.47 V within the first 5 to 10 min and then stabilises around that value. The OCP of the steel declines continuously from about -0.23 V to -0.45 V within the first 10 min and then further to -0.52 V. After 15 to 20 min the OCP value of the DC04 steel is more negative than that of the EN AW-6061 Al alloy. This behaviour is different to that of the Al/Mg-hybrid joints where an OCP difference of 0.9 V was measured between the base materials in the sodium chloride electrolyte similar to the Volta potential difference in air (SKP measurement). The nugget area and the Al/steel transitional area show nearly constant OCP values immediately after the immersion in the electrolyte. The values are similar to that of the EN AW-6061 aluminum alloy. The OCP values of the base materials and the nugget phase within the hybrid joints are independent of the joining conditions. After the immersion time of 60 minutes the OCP difference between the materials is only about 50 mV or less. Therefore, the risk of an enhanced corrosion caused by a galvanic element is much less pronounced than in the case of the Al/Mg-hybrid joints.
The EN AW-6061 aluminum alloy exhibits a free corrosion potential (Eoc) around -0.46 V and a pitting potential around -0.42 V which is only slightly more positive than the free corrosion potential. The corrosion current density measured for the aluminum alloy at the Eoc is around 1-2 μA/cm2 indicating a low corrosion rate. At the pitting potential the current density increases immediately up to values of several hundred μA/cm2. The free corrosion potential of the DC04 steel is around -0.54 to -0.53 V. The polarization curve of the steel shows a continuous increase in the anodic region. The corrosion current density at the Eoc is slightly higher than that of the EN AW-6061 aluminum alloy (2-3 μA/cm2). The free corrosion potential of the nugget area shows during the potentiodynamic polarization a small shift to the anodic direction compared to the OCP measurement. The pitting potential of the nugget area is identical to that of the EN AW-6061 aluminum alloy. The corrosion current density of the nugget area of the FSW-hybrid joint at Eoc is somewhat higher than that of the USE-FSW-hybrid joint and higher than that of the EN AW-6061 aluminum alloy. This may be attributed to the higher amount of intermetallic phases within the measured area. These phases show a higher corrosion tendency. The Eoc value of the Al/steel transitional area of the FSW-joint is about 50 mV more negative than that of the USE-FSW-hybrid joint, which may be a result of a different Al to steel ratio or different phases within the measured area. However, both polarization curves exhibit in the anodic range first a continuous increase of the current density - comparable to the DC04 base material - and then a pitting potential and further progress comparable to that of the EN AW-6061 aluminum alloy. The corrosion current density at the Eoc is lower than that of the DC04 base material. This is a distinct difference to the Al/Mg-hybrid joints where an increase of the corrosion current density of several orders of magnitude was observed at the Mg/nugget transitional area.
The USE-FSW process will be used to join other alloy combinations (e.g. Al/high strength steel). The mechanical and corrosion properties of the new hybrid-systems will be characterized as well.
Dr. Sigrid Benfer
Tel.: 069 / 75 64-382
B. Strass, G. Wagner, C. Conrad, B. Wolter, S. Benfer and W. Fürbeth
Advanced Materials Research 966-967 (2014) 521-535
S. Benfer, B. Straß, G. Wagner, W. Fürbeth
Surface and Interface Analysis 48 (2016) 843-852
Marco Thomä, Guntram Wagner, Benjamin Straß, Christian Conrad, Bernd Wolter, Sigrid Benfer, Wolfram Fürbeth
Chapter Friction Stir Welding and Processing IX, Part of the series The Minerals, Metals & Materials Series pp 179-194