Key Engineering Materials Vols. 622-623

Abstract: Use of light materials such as aluminum, magnesium and carbon fiber reinforced plastic (CFRP) has been increased to achieve the light-weight car body in automotive industry. For successful multi-material design of automotive body, the joining method for dissimilar materials is required to assemble the automotive components produced by various materials. Especially, hole clinching process is effective to fasten dissimilar materials without any additional joining element. In this study, effect of tool shape on the hole clinching is investigated by FE-analysis and experiments. The parameters related to clinching tool shape are punch diameter, punch corner radius and die depth. The geometrical interlocking is evaluated by the neck-thickness and undercut. Joint strength using single lap shear test is evaluated also to verify the effectiveness of hole clinching as automotive joints.

Abstract: Linear friction welding is a solid-state joining process developed for non-axisymmetric components in which the joining of the specimens is obtained through reciprocating motion and pressure. In the process, the friction forces work due to the high frequency oscillation and the pressure between the specimens is converted in thermal energy. In order to design an effective machine, relevant issues derive from the high frequency and the large inertial forces involved in the process. In this study, the authors describe the redesign of a preexisting prototypal machine for LFW processes. A machine redesign is needed when welding high resistant materials, i.e. steels or titanium alloys, with high frequencies, up to 72 Hz. The sensors equipping the machine allows in process measurements of key process variables as temperatures of the specimens, tangential forces, accelerations and speeds. At the same time through the acquired data, the main weaknesses of the machine can be highlighted allowing for effective redesign.

Abstract: Ultra-thin Cu/Al clad strip with 0.12 mm thickness was successfully fabricated by accumulative roll bonding and the interfacial structure of Cu/Al clad strip has been characterized by means of optical microscope (OM), energy dispersive spectroscopy (EDS), scanning electron microscopy (SEM) and also X-ray diffraction (XRD). The intermetallic phases and their formation sequence at the interface was experimentally verified. And the growth kinetics of each phase was also modeled considering the diffusion controlled reaction mechanism. The effect of interfacial compounds on tensile fracture of ultra-thin Cu/Al clad strip was also studied. The obtained results indicate that, intermetallic compounds formed in the interface region of ultra-thin Cu/Al clad strip within the experimental condition are confirmed to be Al₂Cu, Al4Cu9 and AlCu in sequence. The calculated activation energies for the growth of Al₄Cu₉, AlCu and Al₂Cu are 101.45、114.30 and 95.15 kJ/mol, respectively. The major cracks propagate through the AlCu intermetallic layer and the Al₂Cu / AlCu interface.

Abstract: Hybrid metallic structure, which is composed of aluminum alloy as a representative of light metal and steel as that of high strength metal, has been paid attention remarkably in these days, especially in the transportation vehicle industry from the energy saving viewpoint. To perform the hybrid structure, friction stir welding FSW has been recognized to be an effective way for the welding between dissimilar metals, like between steel and Al alloy. Based on our previous findings on the butt welding between dissimilar metals with straight weld line, welding between dissimilar metals with circular weld line has been tried to open a new application field of FSW. The objective of this research is to get a fundamental finding on the welding between dissimilar metals with circular weld line and to clarify the effect of material character of Al alloy and tool design on the FSW between dissimilar metals. Typical results obtained in this study are summarized as follows: 1) Macroscopic defects often observed at the bottom part of the welding in case of ADC12 material. It indicates that insufficient stirring was given in the stirring zone due to the higher thermo-mechanical property of ADC12. 2) Tensile strength of the specimen welded with scroll shoulder tool was 266 MPa, while that with flat shoulder tool was 161 MPa. Tensile strength of the weld joint was improved by a factor of 1.65 by introducing the scroll onto the tool shoulder with normal thread probe. 3) High performance defect free ADC12/S45C weld structure with circular weld line could be fabricated by the tool with combination of scroll shoulder and thread probe.

Abstract: Superplastic forming has been known for the ideal process for manufacturing complex parts. Also, diffusion bonding can give a higher design flexibility, which allows a better performance with a lower overall manufacturing cost. Fine grained INCONEL 718 alloy sheet has been known to show superplastic behavior with the combination of high strength and corrosion resistance at the elevated temperatures. In the present study, high temperature deformation characteristic of INCONEL 718 sheet with 15m was investigated firstly. Then, blow forming process with cylindrical cavity was tried. Also, best diffusion brazing and bonding condition was tried to be defined in terms of temperature, pressure and time. Bonding strength was characterized by using lap shear type test and interface observation. Characteristics of deformation and diffusion bonding at high temperature were influenced greatly with grain size while Nb precipitate also played an important role.

Abstract: In this work, the pressure-assisted induction sintering (PAIS) method, is being presented. The specimens (a cylindrical shape, diameter of 50 mm, height of 10 mm) was heated by a induction coil with heating speed of about 220 K/s to temperature 800°C, 850oC, 900oC, 950oC and 1000oC. During this process, the temperature of specimen was measured by five thermocouples (Φ 0.5 mm) which were located into graphite punch with sampling frequency of 2 Hz. Firstly, the die with specimen was located in the center of the coil. Next, the die was located out of the coil center (1 and 2 cm in four directions) and heated again. The analyzed samples show a difference of heat flow in the specimens. The studies also showed the difference between the temperature at the surface of the matrix (measured by a pyrometer) and the temperature of the sample (measured by a thermocouples). Two types of thermocouples placement were tested, parallel and perpendicular to the door of the vacuum chamber. In addition, an analysis of the temperature distribution along the diameter of the sample was performed.

Abstract: High-frequency electric resistance welded pipes are used for high-grade line pipes. To address the significant need for weld seam reliability, it is important to clarify the associated welding phenomena. In this study, a numerical analysis model is developed to clarify the behaviours of the molten steel and oxide in HFW pipes. The temperature distribution of the HFW is calculated using electromagnetic and heat conductive finite element analysis methods. The molten metal and oxide flows are analysed by modelling heat conductive and plastic flows. The movement behaviour of the oxides in the molten steel is successfully analysed with this technique. The material properties as a function of the temperature of the steel pipe are calculated using general-purpose simulation software. With pressurisation by the welding rolls, the molten steel moves to the upper part, and the oxide, which exists in the internal parts, rapidly decreases such that almost all of the oxide transitions to the excess metal part. The internal oxide content rate after pressurisation at 0.08 m/s is less than 0.01. To decrease the oxide content, the pressurisation rate must increase such that the molten steel and an oxide pressurised at high temperatures transition to the excess metal part.

Abstract: Friction Stir Welding (FSW) is a solid-state welding process used to weld difficult to be welded or unweldable materials as aluminum alloys. In the last years other materials have been successfully tested as magnesium, titanium and nickel based alloys. Mixed joints can be obtained by FSW but issues arise in the correct choice of the process parameters. In the paper a numerical model is presented for the prediction of the main field variables distribution and the occurring material flow in FSW of dissimilar AA6061 and AZ31 butt joints. Important insights are obtained on the effect of the mutual position of the two materials with respect to the advancing and retreating side of the joint has been highlighted.

Abstract: Robotic friction stir welding experiments were performed on AA5754 aluminium alloy sheets, 2.5 mm in thickness, in two different temper states (H111 and O-annealed). A six axes robot with a hybrid structure, characterised by an arm with parallel kinematics and a roll-pitch-roll wrist with serial kinematics, was used. The effect of the process parameters on the macro-and micro-mechanical properties and microstructure of joints was widely analysed. It was shown that, under the same process condition, the mechanical properties of the joints are strongly influenced by the initial temper state of the alloy. In particular, as AA5754-H111 is welded, the ultimate tensile strength is not significantly affected by the process parameters whilst the ultimate elongation significantly depends on the welding speed. In AA5754-O, both ultimate values of tensile strength and elongation are affected by the welding speed whilst a negligible effect of the rotational speed can be observed. Irrespective of the welding parameters, the H111 temper state leads to mechanical properties higher than those given by the O-annealed state. An investigation has been also carried out in order to evaluate the micro-hardness profiles and microstructure of the FSWed joints in order to understand the mechanisms operating during robotic friction stir welding.

Abstract: This work reports an investigation on an innovative assembly system for the components of a coolant pump designed for automotive industry. After having identified the main parameters involved in the process, the authors develop a numerical model aimed to investigate the main characteristics of the process which has been subsequently validated experimentally. To do this a simple mock-up of the pump was built that was employed for roll hemming tests and then to the main proof tests to assess the functionality of the assembled product. Experimental tests were conducted on some specimens in order to check the quality of the assembly between parts. The specimens passed the leak test as well as the torque proof as required by the design specifics which prove the validity and feasibility of this type of process.