Development of the heating and forming strategy in compound forging of hybrid steel‐aluminum parts

In times of increasing energy costs automotive light weight construction is gaining more importance. The production of hybrid compounds by forging is a promising method for manufacturing functional parts by applying resource‐saving process steps. The mechanical properties of these parts can be specifically adapted to the requirements. In compound forging of steel‐aluminum parts the two materials need to be heated to different forming temperatures. In this paper, the challenges and their methods for the development of a heating and forming strategy based on different material characteristics are presented.     

Verschleißfeste Beschichtungen von Aluminium‐Stahl‐Hybridstrukturen mittels thermischer Spritztechnik

Spraying of wear‐resistant coatings of aluminium‐steel‐hybrid‐structures By means of high velocity oxy fuel flame spraying of aluminium‐hybrid‐structures consisting of a Al Zn 5,5 Mg Cu 1,5 (ENAW7075) and a NiCrBSi‐coating as well as a Cr₃C₂ 25NiCr‐ coating are manufactured. The hybrid composite structures are analysed and compared with each other regarding to hardness, surface roughness, wear‐resistance and coating density.     

Diffusionslöten von Aluminium mittels PVD applizierter Lotzusatzwerkstoffe

Diffusion brazing of aluminium by PVD applied filler metals Diffusion brazing of aluminium and aluminium alloys precoated with filler metal components enables fluxless wetting and obtains braze joints of high strength at moderate brazing temperatures. Previously deposited components of filler metals on the base materials as thin film, using Arc‐PVD‐process lead during a subsequently diffusion brazing process to the formation of a local liquid phase (transient liquid phase). The liquid phase is formed from the deposited thin film material and the base material and is solidified isotherm due to diffusion procedures. In doing so braze joints of higher melting point than brazing temperature can be realised. In this work, vacuum brazing of the two systems, Al‐Cu and Al‐Cu‐Si have been investigated. Cu and Al‐Cu‐Si were deposited on the base material using Arc‐PVD‐process. The base materials were pure aluminum and EN‐AW6060. Metallographic and scanning electron microscope analyses proved that the braze seam area after the completed diffusion brazing process shows similar structure and composition as the base material.     

Die Integration der Lebensdauerberechnung in den Fahrzeugentwicklungsprozess am Beispiel MIG‐geschweißter Bauteile aus Aluminium im Fahrwerksbereich

Integration of the lifetime calculation in the automobile development process based on MIG‐welded aluminium chassis components The lifetime prediction results for metallic components depend significantly on the required calculation inputs. These inputs include the finite‐element‐meshing of the component geometry, the material properties and the load time histories for the component. In the present study the influence of the finite‐element‐modelling on the lifetime prediction is investigated, based on a line‐welded aluminium chassis component. Of special interest is the modelling of the weldline. For the lifetime calculation the software tool FEMFAT from “Engineering Center Steyr” is used. The evaluation of the lifetime prediction results for weldlines is based on a local stress approach. The qualification of the calculation for the product development process is described, based on typical BMW weldline S‐N‐curves.     

Einfluss der Beschichtungen beim stoffschlüssigen Lichtbogenfügen von Stahl mit Aluminium

Influence of the coating by bonded arc joining of steel to aluminium The firmly bonded joint between steel and aluminium has, so far, not been achieved successfully in joining technology. The problems of joining these metals thermally are caused by the low up to non‐existing solid solubility of the metals and by the development of brittle intermetallic phases. The quality of the joints depends on many influential factors which are, sometimes, also interacting. One of those influential factors is the coating of the steel sheets. Using the example of coatings which are common and others which are unusual in automotive engineering it has been tried to demonstrate the influence on the wetting length and wetting angle of those factors with, otherwise, constant boundary parameters. The results are part of a test series which is currently carried out at the Joining and Welding Institute, Aachen.     

Alkoholatkorrosion von Aluminium (EN AC‐48100), Einfluss von Temperatur sowie Ethanol‐ und Wassergehalt

The corrosion behaviour of the aluminium alloy EN AC‐48100 (Al‐17wt%Si‐4wt%Cu‐Mg) was investigated in ethanol fuels with different contents of water (0.05 vol.‐%, 0.2 vol.‐%, 0.35 vol.‐%), at different temperatures (20°C, 60°C, 80°C), and with different contents of ethanol (10 vol.‐%, 25 vol.‐%, 50 vol.‐%, 85 vol.‐%). A interdependency of the corrosion with the temperature and the water content in the fuels was determined. At a temperature of 80°C in an ethanol fuel with less than 0.05 vol.‐% water, strong corrosion starts soon after immersion. An influence of the ethanol content to the degree of corrosion was determined for fuels with ethanol contents below 25 vol.‐% notably. Based on the observations and measurements, a theory for the multi stage character of the corrosion mechanism of aluminium alloys in dry ethanol fuels was developed.     

Einfluss der Werkstoffzähigkeit auf das Festigkeitsverhalten unter mehrachsiger Beanspruchung am Beispiel von Schweißverbindungen aus Stahl und Aluminium

Influence of Ductility on the Multiaxial Fatigue Behaviour by the Example of Welded Joints of Steel and Aluminium The multiaxial fatigue behaviour of materials with different ductility under constant and changing principal stress directions is also applicable to welded joints of different materials. For this, welded flange tube connections of the fine grained steel StE 460 and the artificially aged aluminium alloy AlSi1MgMn T6 were investigated under constant amplitude combined bending and torsion. Out‐of‐phase loading, i. e. changing principal stress directions, of the steel joints led to a decrease of fatigue life, which is observed at ductile material states. However, for the aluminium joints out‐of‐phase loading resulted same behaviour as in‐phase loading, which indicates a semi‐ductile material behaviour. The results for the welded steel joints were evaluated on basis of local stresses by the integral hypothesis of the Effective Equivalent Stress EES (WVS). This hypothesis for ductile material states takes into account the life decreasing influence of out‐of‐phase loading by considering the interaction of the shear stresses on different planes. The fatigue behaviour of the aluminium welds is described by the critical plane based combination of shear and normal stresses (KoNoS), which is valid for semi‐ductile material states.     

Modellbasierte Optimierung und experimentelle Charakterisierung der Thermo‐ und Kryo‐Mechanik werkstoffhybrider Bauteile

Model based optimisation and experimental characterisation of thermo and cryo‐mechanics of hybrid material components The design of hybrid material components with thermo‐mechanical requirements is supported by optimisation techniques. From a set of examples some general conclusions will be derived. Severe requirements come from cryogenic environment not only because of individual material behaviour but also due to possible mismatches between different materials to be combined. This also puts specific requirements for test set ups for determination of related material and component properties.     

Comparative corrosion studies of zirconia particle reinforced high‐alloyed 14‐6‐6 CrMnNi‐steel by potentiodynamic polarisation and one year outdoor exposure

The corrosion behaviour of a spark plasma sintered CrMnNi high‐alloyed steel, without and with 10 vol.% zirconia particle reinforcement was investigated by potentiodynamic polarisation in 5 wt.% sodium chloride solution and by an one year outdoor exposure test. After the polarisation test both materials revealed a similar damage behaviour as in the outdoor exposure test. The unreinforced material was attacked by pitting corrosion, whereas the particle reinforced material showed a more homogeneous corrosion attack involving an intensified particle detachment. Furthermore, investigations of the unreinforced steel by white light interferometry revealed pit depths of approximately 65 µm after potentiodynamic polarisation and 36 µm after one year outdoor exposure. In contrast, for the composite a damage evolution due to particle detachment was found in both experiments.     

Kerbgrundkonzepte für die schwingfeste Auslegung von Aluminiumschweißverbindungen am Beispiel der naturharten Legierung AlMg4,5Mn (AW‐5083) und der warmausgehärteten Legierung AlMgSi1 T6 (AW‐6082 T6)

Fatigue design of aluminium welded joints by the local stress concept exemplarily shown on the naturally aged wrought aluminium alloy AW‐5083 and the artificially aged wrought aluminium alloy AW‐ 6082 T6 Local fatigue design concepts based on material‐ and microstructural‐related parameters, e.g. the microsupport‐concept, cannot be regarded as easily applicable. The investigations, which compared the micro‐support‐concept with the local stress concept with a fictitious notch radius r_f, were carried out with different types of MIG‐welded joints of the aluminium alloys AW‐5083 and AW‐6082 T6 under fully reversed and pulsating axial loading. The evaluation of the results showed that the local stress concept using the fictitious notch radius of r_f = 1.0 mm can be applied to aluminium welded joints from plates with thicknesses t ≥ 5 to 25 mm independently from the alloy and weld geometries (fully or partially penetrated butt welds, transversal stiffener). Master design curves are proposed for different stress ratios, i.e. R = ‐1, 0 and 0.5, which allow the consideration of residual stresses as well as load induced mean stresses. The results permit also the suggestion of Δσ = 70 MPa as FAT‐value for the IIW‐Fatigue Design Recommendations     

Numerische und experimentelle Untersuchungen zur Herstellung von stranggepressten Aluminium/Magnesium‐Werkstoffverbunden und zur Festigkeit des Interface

Numerical and experimental investigations of the production processes of coextruded aluminium/magnesium‐compounds and the strength of the interface In the research project SFB 692 the aim of the subproject B3 is to analyze aluminium/magnesium‐compounds, especially the strength and fracture mechanical properties. Furthermore a numerical model of the interface should be created. In the following paper the main results of the investigations of the subproject are presented. Different influencing variables were analyzed, which determine the quality of the interface. It has been shown, that the compound quality and also the strength of the interface is based on several parameters of hydrostatic and indirect extrusion processes. If the quality of the interface is good, without cracks, the strength is very high. Furthermore the behaviour of the interface under loading in an extended temperature range is presented. A numerical model was found to visualize the behaviour of the compound during the loading tests.     

Schwingfeste Auslegung von Schweiß‐ verbindungen aus der Magnesiumknetlegierung AZ31(ISO‐MgAl3Zn1) mit dem Konzept der fiktiven Ersatzradien von rf = 1,0 mm und 0,05 mm

Fatigue design of welded joints from the wrought magnesium alloy AZ31 (ISO‐MgAl3Zn1) by the local stress concept with the fictitious notch radius of r_f = 1.0 mm and 0.05 mm The investigations were carried out with three different types of MIG‐ and TIG‐welded magnesium joints of the alloy AZ31. The evaluation of the results showed that the local stress concept using the fictitious notch radius of r_f = 1.0 mm can be applied to magnesium welded joints from plates with thicknesses t ≥ 5 mm independently of the weld geometries (fully or partially penetrated butt welds, transversal stiffeners). Design curves are proposed for different stress ratios, i.e. R = ‐1 as well as 0 and 0.5, which allow the consideration of residual stresses as well as load induced mean stresses. The results permit also the suggestion of Δσ = 28 MPa as FAT‐value for the IIW‐Fatigue Design Recommendations. Further, the FAT‐value Δσ = 73 MPa for the fictitious radius of r_f = 0.05 mm to be applied to welded thin magnesium joints is derived, too. These FAT‐values are compared with already known data for steel and aluminium joints. A linear relationship between the FAT‐values and the Young’s modulus is determined.     

Aluminium alloys exposed to borderline solutions – a methodology to evaluate the susceptibility to corrosion fatigue with respect to corrosive de‐icers

In addition to high mechanical loads certain components for the automotive industry are exposed to corrosive environments, especially during winter, when corrosion promoting de‐icers are essential to sustain road traffic. The underlying research work contributes to the evaluation of the corrosion fatigue performance of aluminium alloys relevant for automotive application generally, while the present text focuses on wrought alloys. Aluminium alloys are of special interest because of the opportunity to reduce expenses for an additional corrosion protection and to support lightweight construction of vehicles. Components exposed to simultaneous mechanical and corrosive service loads as well as corrosion tests of different scale level (immersion and cabinet testing, long term outdoor exposure of components, usage on test carriers) were analysed to evaluate typical forms of corrosion. Based upon the prevailing damaging mechanism of material conditions with known differences in susceptibility to specific forms of corrosion, like intergranular corrosion or pitting, methods were derived to evolve service relevant types of corrosion during laboratory tests.     

Ermittlung von Berechnungskennwerten an Karosseriewerkstoffen – Bericht über ein Gemeinschaftsprojekt der Stahl‐ und Automobilindustrie

Determination of input data for numerical design of sheet steels – Report on a common research project of the steel and automotive industry Within the scope of a common research project of the steel and automotive industry, 20 sheet steels have been investigated to obtain input data for FE‐analysis. In detail, elastic, plastic and fatique characteristical values were determined by several testing institutes for a period of 3 years. The investigated sheet steels differ with regard to the microstructure and the steel concept. Beside several ferritic steels, multiphase steels like dual phase‐, complex phase‐ and TRIP steels as well as 2 austenitic stainless steels were characterised. The starting materials and selected steels with a defined predeformation and heat treatment were investigated. Within this project, the partners developed a testing and documentation precept in which the ways and means were fixed to reach the defined steel condition and to enable a standardised testing and data output for the material database, realised by the automotive industry. Before the actual elastic, plastic and fatique testing, a reception test for all steels was carried out to characterise the materials with regard to the microstructure, surface condition, chemical composition and mechanical properties, obtained in the quasistatic tensile test. The results of the different testing institutes (elastic, plastic and fatique) will be presented in separate publications in detail. As a result of this project it became obvious that the investigated steels can be divided into steel groups which show a similar strain hardening behaviour. Thus, a prediction of mechanical values and flow curves for cognate steels within one steel group seems to be possible. This subject will be the focus of further investigations within the scope of a new project started on January 1^st, 2003.     

Effect of quenching temperature on microstructure and performance of Al‐bearing cast boron steel

The effects of quenching temperature on microstructure and performance of Al‐bearing cast boron steel (ACBS) containing 0.25–0.45%C, 1.5–1.8%B and 1.4–1.6%Al were investigated by means of the optical microscopy (OM), the scanning electron microscopy (SEM), X‐ray diffraction (XRD), Rockwell hardness and Vickers micro‐hardness tester. The results show that the solidification structures of cast steel consist of high hardness boride, ferrite, pearlite and a small quantity of martensite when 1.5–1.8%B and 1.4–1.6%Al are added into the carbon steel. The metallic matrix of ACBS changes into single martensite from the mixed structure of ferrite, pearlite and martensite along with the increase of quenching temperature. The increase of quenching temperature also leads to the transformation of boride from continuous shape to isolated shape. Moreover, the micro‐hardness of matrix and macroscopical hardness increase with the increase of quenching temperature. When the quenching temperature excels 1000°C, the hardness has a slight decrease. ACBS has good comprehensive properties after heat treatment at 1000°C.     

Mikrostrukturelle Pressschweißnahtcharakterisierung im strangpressten Zustand für Al‐Mg‐Si‐Legierungen

Weld seams form when profiles are extruded using porthole or bridge dies. These are inevitable when producing industrial relevant hollow profiles but imply the weak spot if the profiles are used for applications with high mechanical requirements. The characterization of formed weld seams together with their mechanical properties is problematic or sometimes even impossible due to the complex profile geometries. Expansion, bending or tensile tests of profiles or parts of the same were hitherto often used for their analysis lacking the possibility of basic assumptions about their properties. The investigations described herein circumvent the problem by using a flat profile exhibiting the weld seam in the middle. Flat profiles offer the possibility of standardised specimens for tensile testing. The extrusion experiments focused on the aluminium alloys EN‐AW 6060 and EN‐AW 6082. During the experiments billet temperature and extrusion speed have been varied. The microstructure was analysed subsequent to explain the obtained results and to offer a possibility of characterisation.     

Corrosion behavior of 5Cr0.5Mo steel in sulfur‐bearing solution at high temperature

Corrosion behavior of 5Cr0.5Mo steel in sulfur‐bearing solutions as a function of temperature, test time, and sulfur content was investigated by weight loss measurements in this study. The results indicate that the corrosion rate of the steel increases with increasing test temperature. Besides, the corrosion rate increases during the initial test time and then decreases with longer test duration. Moreover, the corrosion rate rises with a higher amount of sulfur in the solution, and consequently a lower amount of sulfur leads to a decreased corrosion rate. In addition, the effect of Cr and Mo elements has been discussed.     

Hybrides Walzen am Beispiel von Titan‐Aluminium‐Verbunden

Hybrid rolling as exemplified by titanium‐aluminium laminates Triple layered titanium‐aluminium laminates composed of titanium alloys TiAl6V4 (ASTM grade 5) and Ti 99.8 ASTM grade 1) together with the aluminium alloys AlMgSi 0.5 (EN‐AW 6060) and AlCuMg 1 (EN‐AW 2017) are manufactured by hot rolling and the deformation behaviour is investigated subject to alternating deformation parameters. The focus is the investigation of the differences between stepwise and continuous increases in true strain. True strains of 20 … 60 % are tested at temperatures from 400 … 500 °C. The contact zone of the manufactured laminates is then metallographically examined and the interlayer bond strength is mechanically tested. Torsion tests are presented for qualitatively determining the bond strength of the laminate. Bond forming already initiates at true strains of 35 % and temperatures of 350 °C within the rolling gap.     

Foreword – Materialwissenschaft und Werkstofftechnik 10/2011

Load controlled fatigue tests were performed up to 10⁷ cycles on flat notched specimens (K_t = 2.5) under constant amplitude and variable amplitude loadings with and without periodical overloads. Two materials are studied: a ferritic‐bainitic steel and a cast aluminium alloy. These materials have a very different cyclic behaviour: the steel exhibits cyclic strain softening whereas the Al alloy shows cyclic strain hardening. The fatigue tests show that, for the steel, periodical overload applications reduce significantly the fatigue life for fully reversed load ratio (R_σ = –1), while they have no influence under pulsating loading (R_σ = 0). For the Al alloy overloads have an effect (fatigue life decreasing) only for variable amplitude loadings. The detrimental effect of overloads on the steel is due to ratcheting at the notch root which evolution is overload's dependent.