Advanced Materials Research Vols. 966-967

Abstract: The life time of roller bearings can be increased by inducing compressive residual stresses in the subsurface region of the raceway. These stresses can be induced by overloading in the first numbers of revolution. It would be much more useful to create the surface integrity within the manufacturing process. In this paper a method is presented to improve the process chain from grinding and honing to hard turning and deep rolling. As a result the surface finish is comparable to ground bearings. Due to the deep rolling process the maximum compressive stresses can be induced to higher values and depth. For the evaluation of the surface roughness in hard turning process and the maximum compressive stresses in deep rolling empirical models based on D-optimal experimental design are used.

Abstract: In this work, the use of a nanocrystallization surface treatment is considered on a cobalt based alloy. Since the Co28Cr6Mo is widely used as an articular bearing surface for artificial joints like hip and knee prostheses, the improvement of its tribological properties is a matter of concern to extend the lifespan of implants. By means of SMA (Surface Mechanical Attrition) treatment, a nanostructured layer is formed at the surface of a CoCrMo alloy establishing an optimized hardness gradient down to a depth of a few hundred of microns. Different treatment times (5, 15 and 25 minutes) are assessed comparatively and several surface polishing methods are studied: with clothes, brushes and a liquid filled with abrasive particles. The influence of processing parameters is discussed regarding hardness and topography. Moreover, the impact of surface modification is examined in terms of wear strengthening through scratch testing. The use of increasing loads mode gives some evidence of the benefit of SMAT. A significant decrease of penetration depth is noticed, from 30% to 60% on average. A straight correlation is also noticed depending on the surface finish method. This study illustrates both the ability of micro scratch testing to assess comparatively treated surface layer and to highlight the benefit of SMAT for wear strengthening in an abrasive wear mode.

Abstract: Cold welding, e.g. by cold forging, is a smart manufacturing technology, enabling novel multi material designs. A material combination, which is particularly attractive for manufacturing, though challenging to handle in a cold welding process, is steel and aluminum. We investigate the bond formation between cold forged C 15 (mainly primary heat treated) and AW 6082. Analysis starts with numerical simulations using the finite element analysis (FEA) to identify optimum conditions for bond formation. The bond strength was determined by tensile tests from samples eroded from the cold-welded specimen. Best performing samples showed a maximum tensile strength of ~200 MPa with ductile failure in the AW 6082. Transmission electron microscopy (TEM) inspection of the bonded area between aluminum and steel show a reaction layer consisting of iron and aluminum of few nm thickness throughout the sample. The formation of such a reaction layer is hypothesized to be crucial for bond formation.

Abstract: Cold pressure welding is a versatile process that can be used to weld most metals. It is, however, difficult to achieve a weld at room temperature through the application of pressure. As in every welding process, several different parameters have to be considered: the metals to be welded and their properties in terms of surface condition, cleanliness, strength of the metal, and so on. The surface condition and surface preparation methods are the most important parameters, since the cold weld is based solely on the contact between the two metal surfaces that are involved. Joining with electrochemical support (ECUF) is a new approach that overcomes most of the current process barriers and limitations through optimized electrochemical surface treatment and an improved process technology based on a pilger roll. This paper provides an overview of the ECUF process and current research results on the pressure welding of pure copper (CW004A). The formation of the weld has been investigated and the welding interface characterized in respect of its microstructure.

Abstract: A backward extrusion forged bonding using low carbon steel and pure aluminum is conducted. The bonding strength between the materials is evaluated by a micro tensile test that is cut out at the bonding boundary. The maximum bonding strength is larger than that of the aluminum. In addition, the metallurgical mechanism of the joining of the backward extrusion forged bonding is investigated by means of a scanning transmission electron microscope (STEM). An intermetallic compound (IMC) layer is produced at the boundary with a thickness of about 3 nm. The process is applied for bonding between aluminum-nickel and between aluminum-copper. The bonding strength between the materials was evaluated by using a micro tensile test and the maximum bonding strength is shown. Fractured surfaces of the tensile specimens are observed by scanning electron microscope (SEM) and relationship between bonding strength and position on the boundary is discussed.

Abstract: A part with optimized material characteristics can be realized by cladding of two or more materials. In the aerospace industry high strength aluminum alloys like AA2024 are commonly used. Due to their susceptibility to atmospheric corrosion a protective surface layer has to be provided, e.g. pure aluminum. Because of high differences in material strength problems occur during bonding. This study discusses if and how active cooling can be used to create a temperature field which compensates the material strength difference and thus improves roll bonding of two materials of different strength. Cooling simulations were carried out to investigate the influence of the boundary conditions and cooling time before hot rolling for different layer thicknesses. For the example of a thick core (50 mm) and a thinner cover layer (10 mm) the optimal cooling time was determined to be in a range of 3 - 14 s. Furthermore, roll bonding experiments were performed at various height reductions and cooling times to investigate the influence of the material strength differences on the rolling and bonding behavior. Due to the implementation of a cooling operation a varying elongation of the surface layer and the core material has been successfully reduced from 30 to 22 mm.

Abstract: Roll bonding is a joining-by-forming process, in which two or more metals are permanently joined through pressure and plastic deformation, which causes the creation of a metallic bond. The bond formation is a complex process based on various process conditions in the joining zone, such as strain, normal pressure, temperature, strain rate, shear strain and surface condition. Since an individual variation and analysis of the influencing parameters is usually not possible during the rolling process, a specific experimental setup for the investigation of the joining mechanisms is necessary. In this paper, a testing procedure has been developed to determine the bond strength in joining-by-forming processes. The material combination chosen was AA2024/AA1050 as used in aircraft applications. AA2024 sheets are cladded with pure aluminum to improve the corrosion resistance. The performed experimental parameter study confirms the expected influencing factors and is used to determine parameters of a bonding model, which can be integrated in a finite element simulation.

Abstract: The implementation of multi-material concepts and the manufacturing of modern lightweight structures, for example in automotive engineering, require appropriate joining technologies. The ability to join dissimilar materials without additional mechanical elements, chemical binders, or adverse influences of heat on the joining partners is key in reaching the desired weight reduction in engineering structures. The Magnetic Pulse Welding (MPW) process meets these demands, making it a viable alternative to conventional thermal welding and mechanical joining processes. The present paper focuses on the analytical determination of the impact velocity as one of the key parameters of MPW processes. On the basis of experimentally recorded data concerning the course of the discharge current and geometrical parameters of the welding setup, the respective velocity is determined. A comparison with measurement data gained by Photon Doppler Velocimetry is performed.

Abstract: The two common processes of impact welding, explosion welding and electromagnetic pulse welding, may offer great technological advantages, but at the same time exhibit poor observability due to the use of explosives and a highly transient behavior, respectively. A novel test rig is developed and enhanced to collide and weld specimens purely mechanically. Besides its simple build-up and the easy and safe operation, the test rig allows setting crucial process parameters almost independently. The test rig’s construction and improvement is described. A trigger method for high speed imaging is developed and tested. The numerical simulation of the impact shows that the conditions directly at the welding zone are predictable and can be adjusted accurately. Finally, the preparation of specimens to evaluate the influence of surface roughness and grain structure is discussed.

Abstract: Friction Stir Welding (FSW) is a suitable technology for joining dissimilar materials. As the process temperature during FSW typically does not exceed the solidus temperature, like in fusion welding, high quality joints can be produced with a minimum of intermetallic phases. A comprehensive description of the effective joining mechanisms of friction stir welded dissimilar material joints is still subject of research. In this study the results of an analysis of the effect of the pin length, which is supposed to have a significant influence on the characteristics of the joining mechanisms, are presented. Especially the influence on the bonding conditions and the mechanical properties of the joints has been investigated. For this purpose combinations of aluminum and titanium have been welded with varying pin length at different rotational speeds. The experiments show that at a sufficient distance between the interface zone and the pin tip the bonding is realized by a substance-to-substance bond and microscopic form-fit. As this distance decreases, a visible macroscopic form-fit is generated. However, this macroscopic form-fit causes no significant elevation of the joint strength. First scanning transmission electron microscopy (STEM) images reveal an interfacial layer, which indicates a diffusion of the two materials.