Key Engineering Materials Vol. 966

Abstract: Currently, heavy-haul and passenger rails are joined by a welding process, which can be either flash-butt welding or thermite. The joining process has increased the overall rails strength, but the welding parameters optimization is tricky and must be performed and studied to improve the weld quality. Heavy-haul rails are high carbon steels, containing alloying elements and as such, the weld presents a series of difficulties. On one side, martensite should be avoided during the cooling step, while on the other, the HAZ should be minimized as it is known to be prone to localized wear and rolling contact fatigue. Finite element simulations were performed to map the weld cooling rates and corresponding heat-affected zone (HAZ) width. CCT curves of rail steels were determined using dilatometry for different austenitizing temperatures. Comparing the simulations with the CCT data, processing windows able to prevent martensite formation were determined, usually corresponding to a critical cooling rate of 2 °C/s. The correlation with the simulations showed that the shorter the HAZ length, the greater the chance of martensite formation due to the higher cooling rate. The methodology developed and presented in this paper can be used for simulations considering phase transformations or determining the microstructure formed from different thermal welding cycles, depending on the distance from the heat source during the welding process.

Abstract: QP steels and other third generation AHSS possess outstanding combinations of strength and ductility, making them very attractive for the automotive sector. However, an Achilles heel of these materials is their rather limited weldability. Despite the obvious importance of this problem, very few works have been published characterising 3^rd generation AHSS welded joints. For the current contribution, 3 novel QP stainless steels were investigated. Resistance spot welded joints were prepared following QP treatment of sheets. Results following paint baking revealed a high nugget hardness and a microstructure of martensite, retained austenite and delta ferrite. Cross tension strength was highest for the alloy with an optimum dispersion of retained austenite which improved weld metal toughness despite the high hardness. A further improvement in cross tension strength was realised on tempering at high temperatures, leading to an 60% improvement in strength, thanks to improved toughness of the martensite constituent.

Abstract: The composites based on reactive metals (Zr, Ta, Nb, Ti) sheets explosively welded to stainless steel plates were investigated using X-ray synchrotron radiation, TEM and SEM to characterize phase transformations in near-the-interface layers. SEM and TEM investigations of the solidified melt regions unveiled amorphous and nanocrystalline non-equilibrium phases of variable chemical compositions, incorporating elements from the joined components. Phase analysis in layers near the interface carried out using high-resolution synchrotron radiation show predominantly reflections coming from the main elements of parent sheets/plates. Nevertheless, a closer look at the diffraction patterns shows the presence of reflections coming from the phases based on the two-component equilibrium phase diagrams. The measurements performed at the interface, but including only the steel plate, revealed significant amounts of α-Fe, γ-Fe and ε-Fe phases. Their appearance was attributed to the high pressure and fast cooling rates, which promoted a martensitic transformation in steel.

Abstract: As a result of R&D efforts about the advanced materials, an ultra-high strength steel (UHHS) and a carbon fiber reinforced thermos-plastic (CFRTP) have been developed. In this study, various advanced multi-material dissimilar lap joints using UHHS, CFRTP and an aluminum alloy of A5083P-O were fabricated by the resistance spot welding (RSW), refill friction stir spot welding (RFSSW), friction stir spot welding (FSSW) and laser irradiation assistance plastic welding (LIAPW). Where, the total heat input for each joining method was varied in order to examine the influence of welding condition on joint performances, which were the tensile shear strength, shear fatigue property and corrosion shear fatigue property. The tensile shear strength of UHHS and A5083P-O joined by RFSSW is almost two times of that produced by RSW, and this difference seems to be mainly caused by the difference of joint area. The difference of joint area also affects the tensile shear strength of UHHS & CFRTP or A5083P-O &CFRTP lap joints made by FSSW and LIAPW, and the joint strength of LIAPW is larger than that of FSSW. As for the shear fatigue tests, it is found that the fatigue strength seems to decrease almost monotonically with increasing the applied load regardless of the types of dissimilar materials. Moreover, through the corrosion shear fatigue tests for A5083P-O and CFRTP joined by FSSW or LIAPW, it is found that the corrosion fatigue property seems to be controlled by the crack propagation behavior.

Abstract: In the ultrasonic bonding process, oxides existing on the metal surfaces are removed, and bonding is achieved by bringing clean surfaces to be in contact with each other. However, the bonding process with microstructure variation is not well understood due to experimental difficulties. In this study, using a newly developed sample holder, which enables ultrasonic bonding in a TEM, we directly observed the bonding process at the nanoscale. The bonding process of Au foils with a clean surface was investigated and compared to that of Al foils with a stable oxide film, a bonding inhibitor, on the surface. During the Al ultrasonic bonding process, the nanoparticles generated dispersed over the entire bonding interface and finally formed a fine grain region at the interface. In contrast, in Au bonding, the nanoparticles generated tended to accumulate at the local area of the Au surface and form bridge-like connections between Au foils. It was considered that these differences in bonding behavior were caused by the surface conditions of the materials to be bonded.

Abstract: Gleeble 3800-thermomechanical simulator was used to simulate the heat affected zone of quenched and tempered 0.16 wt.% C steels with variation of molybdenum-and niobium contents. The purpose of the study was to evaluate the effect of alloying content on the properties of the coarse-grained zone of HAZ region (CGHAZ) and partially re-austenitised inter-critical zone (ICHAZ) with two different t_8/5 times (5 s and 15 s). Results showed that Mo and Nb decreased the amount of softening in the HAZ-region, especially with longer t_8/5 -time (15 s). 0Mo steel had mixed microstructure of bainite and martensite in the CGHAZ region with t_8/5 time of 15 s, which led to higher degree of softening compared to other steels. Shorter t_8/5 time of 5 s produced martensitic microstructure in CGHAZ region in all cases leading to higher hardness values. Impact energy values at-40 °C were at least 34 J/cm² in all cases. Highest hardness values in the ICHAZ-region were achieved in the case of 0.5Mo steel. Also, at-40 °C impact energy values of ICHAZ were at least 34 J/cm² in all cases, however Mo-free steel achieved clearly higher impact energies in ICHAZ region, which is result from softer microstructure with relatively low hardness compared to other steels. Overall, it can be concluded that longer t_8/5 time can be used, which corresponds to higher heat input in welding, if Mo and/or Nb alloying is used.

Abstract: A technique for fabricating structural joints by closed-die-type friction-stir forming (FSF) is introduced in this study. The process is as follows. First, a steel sheet with a prepared hole is placed on an aluminum alloy plate. A die with a through-hole cavity is then placed above it to press down the steel sheet tightly. Next, a rotating stepped cylindrical tool is inserted into the through-hole cavity. To enclose the die cavity, the upper side of the tool is then positioned such that it is nearly in full contact with the inner surface of the through-hole. Finally, the top part of the tool is allowed to penetrate into the aluminum alloy plate through the prepared hole of the steel sheet to cause the material to extrude backward. Consequently, the material fills the whole of the space between the tool and die to generate a hollow-rivet-like aluminum alloy structure fastening the steel sheet to the aluminum alloy plate. This technique enables easier alignment between the die and the prepared hole of the steel as compared with the conventional joining technique which uses FSF. In addition, the new technique uses a one-sided approach (i.e., from the side of the harder material with a higher melting temperature) to join dissimilar materials, a process which is difficult for conventional methods of friction-stir welding and forming.

Abstract: This paper is devoted to the numerical computation of a steady-state thermo-fluid modeling related to the Friction Stir Welding Process in a two-dimensional cylindrical geometry. It analyzes the efficiency of the implementation on parallel architectures of two finite-difference schemes on a structured grid. The first one applies the Newton-Raphson method to compute a numerical solution of this non-linear elliptic type equation, and uses an iterative sparse solver. The second one is based on a time-marching approach converging to the steady state solution thanks to a time-explicit computation. Their respective performance is presented and discussed. Some numerical simulation results are presented to validate the proposed approach.

Abstract: When investigating the heterogeneous properties of welded joints, mechanical testing of certain Heat Affected Zone (HAZ) regions, using standard approach test specimens, is very difficult or sometimes even impossible. Inability to precisely position and extract mechanical test specimens, even ones of the subsize dimensions, from the narrow HAZ regions is a limiting factor in the mechanical testing implementation. Detailed investigation of the HAZ is made possible by the use of thermo-mechanical simulations on the Gleeble welding simulator. In scope of this paper several characteristic HAZ microstructures of S690QL grade High Strength Steel (HSS) are being simulated. Multi-pass welding simulations are done on special 10x10 mm square section bar specimens in order to reproduce thermal gradients and characteristic microstructures at any location in a weld. Such simulated HAZ microstructures are of a sufficiently large volume, with homogeneous and repeatable properties, that standard specimen methods for mechanical testing can be readily implemented. Metallographic optical examinations, as well as hardness measurements were done initially. Mechanical properties are focused on determining stress-strain curves for each characteristic weld region. The paper investigates whether the mechanical properties of Gleeble simulated hard-soft combined HAZ regions are better in comparison to exclusively hard or soft HAZ regions. The obtained results can subsequently be used for the material model development.