M. Jahazi

The non-equilibrium segregation of boron on original and moving austenite grain boundaries

Abstract The influences of boron addition, testing temperature and holding time were studied on the kinetics of austenite recrystallization and on the extent of boron segregation in a series of HSLA steels containing B, Nb, and Nb+B. By means of particle tracking autoradiography (PTA), it is shown that recrystallization, quenching, and deformation all induce the non-equilibrium segregation of boron to austenite grain boundaries. The observations indicate that boron segregation decreases with quenching temperature and that deformation induces segregation on the original boundaries, which persists until recrystallization starts. The non-equilibrium segregation of boron on moving boundaries is described, which decreases to the equilibrium state of segregation when recrystallization is complete, at which point it is no longer detectable by the PTA technique. Finally, a comparison is made between the segregation behaviors of the B and Nb–B steels under different experimental conditions. The non-equilibrium segregation of boron during quenching is interpreted in terms of the solute-clustering model proposed by Aust and coworkers. Boron segregation on moving boundaries is attributed to the time required by a boundary to incorporate a dislocation into its structure.

The influence of flow-forming parameters and microstructure on the quality of a D6ac steel

Abstract The influences of flow-forming parameters and the state of the microstructure on the quality and mechanical properties of a D6ac steel were studied. The effects of feed rate, the shape of the contact line, the roller angle and the percentage reduction on the elimination of spinning defects such as a wave-like surface, microcracks and bore were studied. Also, the influence of the preheat temperature, the holding time and the cooling rate on the microstructure and mechanical properties of the material was investigated. The experiments were carried out on a 3-roller spinning machine and the optimum conditions for the elimination of the above-mentioned defects were determined. Both optical and electron microscopy were used for the study of the microstructure, and the appropriate heat-treatment leading to the best strength–toughness combination was found. The results obtained are interpreted within the framework of metal working theories and examined critically with existing models.

Continuous Wave ND:YAG Laser Welding of Sand-Cast ZE41A-T5 Magnesium Alloys

ABSTRACT A continuous wave 4 kW Nd:YAG laser system was used to weld 2-mm butt joints of sand-cast ZE41A-T5 magnesium alloys at a power of 2.5 kW, welding speed of 6.0 m/min, and defocusing distance from − 2 to + 3 mm for the material in the machined surface conditions. It was found that the adjustment of defocusing distance greatly influences the establishment of conduction or keyhole mode welding. Conduction welding is obtained at a power density of 4.0 × 10 5 W/cm 2 . Keyhole welding is reached at a threshold irradiance of 1.5 × 10 6 W/cm 2 . The fusion zone consists of refined equiaxed grains formed through cellular growth in the Zr-containing magnesium alloys. The partially melted zone is rather narrow, only a few grains wide. No grain growth or coarsening but softening is observed in the heat affected zone (HAZ). The weld defects observed include three main types: imperfect shape, cavities, and weld cracks. The mechanisms of their formations are discussed. In addition, the original cast quality was found to have a significant influence on the formation of defects such as underfill, surface depression, porosity, and burn-through during laser welding.

Linear Friction Welding of IN-718 Process Optimization and Microstructure Evolution

Linear Friction Welding (LFW) of IN-718 Superalloy was investigated under several processing conditions. The influence of process parameters such as frequency (60Hz to 100Hz), amplitude (2mm to 3mm) and frictional pressure (50MPa to 110MPa) on the microstructure and mechanical properties of welded specimens was determined. Optical and scanning electron microscopy, and micro-hardness testing were used to characterize the welded areas as well as the Thermo-Mechanically Affected Zones (TMAZ). In-situ thermocouple measurements were performed to follow temperature evolution in the specimens during the different phases of the LFW process. The analysis of the results indicated that for some specific conditions (f=80Hz, a=2mm and P=70MPa) a maximum temperature of 1200°C was attained during the last stage of the welding process, the burn-off phase. This temperature, very close to the alloy melting range, would be sufficient to cause partial liquation in this zone. Microscopic examinations revealed the presence of oxide particles aligned around the weld interface. Their concentration and distribution, varying with process parameters, affect the weld integrity. The TMAZ characterised by a global loss of strength (from 334HV to 250HV) is associated with temperatures exceeding 800°C and causing γ’ and γ’’ reversion. A narrow band of the TMAZ, exposed to high strains and temperatures, showed evidences of dynamic recovery and recrystallization (up to 67% of reduction in the matrix grain size). Visual and microscopic examination of the flash layer, revealed two distinct zones. Microstructure evolution and microhardness variations were associated to process parameters and the optimum conditions for obtaining defect free weldments were determined.

Optimization of Processing Parameters During Laser Cladding of ZE41A-T5 Magnesium Alloy Castings Using Taguchi Method

A continuous wave 4 kW Nd:YAG laser welding system was employed to clad single beads on machined 6.2-mm thick ZE41A-T5 aerospace magnesium alloy sand castings using nominal 1.6-mm filler rods of the parent metal. Based on the quality criterion of minimum dilution ratio, the Taguchi experimental method was used to optimize different process parameters and to identify the dominating factors. It was found that, for the process window investigated in this work, the optimal levels were 5.0 m/min for wire feed rate, 2.25 kW for laser power, and 2.5 m/min for cladding speed, with a minimum dilution ratio of 0.155. With 95% confidence, the wire feed rate is the dominating factor with the most significant influence on the dilution ratio. The dilution ratio usually decreases with increased wire feed rate while it increases with increased laser power. By contrast, the cladding speed appears to have insignificant influence on the dilution ratio over the cladding speed range used in this study.

The influence of hot rolling parameters on the microstructure and mechanical properties of an ultra-high strength steel

Abstract The influence of reheat temperature, percentage deformation, interpass time and finish rolling temperatures on the microstructure and mechanical properties of AISI 4130 steel were studied. Reheat temperatures of 900, 950, 980, 1050, 1100 and 1200°C were used and reductions per pass of 15, 30, 40, 50 and 60% were employed. Finally, finish rolling temperatures from 500 to 850°C were used. The samples were air cooled after the final pass and the variations of their hardness, tensile and impact properties as a function of thermomechanical treatment parameters were measured and their microstructure studied using both optical and scanning electron microscopy. By a judicious control of thermomechanical treatment it was possible to determine the rolling conditions leading to the optimum combination of strength and toughness. The possible mechanisms responsible for the observed behavior are presented and the results are discussed within the existing models.

MICROSTRUCTURAL EVALUATION OF FRICTION STIR PROCESSED AZ31B-H24 MAGNESIUM ALLOY

Abstract The microstructural characteristics in an AZ31B-H24 magnesium alloy after friction stir processing (FSP) were examined. The effects of FSP parameters including forge force and traverse speed on the microstructure were evaluated. It was observed that the grain size increased from about 4 μm in the base metal to about 8 μm at the centre of the stir zone after FSP. The aspect ratio of the grains decreased towards the centre of the stir zone. The changes in the grain size and shape resulted in a drop in micro-indentation hardness from 75 HV in the base metal to about 55 HV at the centre of the stir zone. Increasing the forge force or decreasing the traverse speed increased the grain size due to a greater heat input. It was also observed that the annealing effects (recrystallization and subsequent softening) of FSP were less pronounced with increasing distance horizontally or vertically from the pin tool due to the presence of temperature gradient. Furthermore, the Hall-Petch type relationships between the microhardness and the grain size were found to be valid after FSP.

High temperature deformation of nickel base superalloy Udimet 520

Abstract The high temperature deformation behaviour of nickel base superalloy Udimet 520 was characterised using hot compression isothermal tests. Hot compression tests were conducted between 900 and 1150°C with strain rates of 0.001, 0.01, 0.1 and 1 s-1. Testing at ≤ 950°C led to sample fracture for all the applied strain rates. The flow behaviour at 1000, 1050 and 1075°C indicated the occurrence of dynamic recovery. For specimens tested at 1100, 1125 and 1150°C, recrystallisation is the softening mechanism. The strain rate sensitivity factor m was estimated for various thermomechanical histories. The activation energy for the hot deformation was determined to be 780 kJ mol1. The Zener–Hollomon parameter was also determined and its variation with grain size was studied with deformation conditions. The microstructures of all samples were examined by both optical and scanning electron microscopy. The presence and variations in the morphology and size distribution of deformed and recrystallised grains were determined and related to the deformation conditions.

Simulation of friction stir welding using industrial robots

– The purpose of this paper is to establish a model‐based framework allowing the simulation, analysis and optimization of friction stir welding (FSW) processes of metallic structures using industrial robots, with a particular emphasis on the assembly of aircraft components made of aerospace aluminum alloys., – After a first part of the work dedicated to the kinetostatic and dynamical identification of the robotic mechanical system, a complete analytical model of the robotized process is developed, incorporating a dynamic model of the industrial robot, a multi‐axes macroscopic visco‐elastic model of the FSW process and a force/position control unit of the system. These different modules are subsequently implemented in a high‐fidelity multi‐rate dynamical simulation., – The developed simulation infrastructure allowed the research team to analyze and understand the dynamic interaction between the industrial robot, the control architecture and the manufacturing process involving heavy load cases in different process configurations. Several critical process‐induced perturbations such as tool oscillations and lateral/rotational deviations are observed, analyzed, and quantified during the simulated operations., – The presented simulation platform will constitute one of the key technology enablers in the major research initiative carried out by NRC Aerospace in their endeavor to develop a robust robotic FSW platform, allowing both the development of optimal workcell layouts/process parameters and the validation of advanced real‐time control laws for robust handling of critical process‐induced perturbations. These deliverables will be incorporated in the resulting robotic FSW technology packaged for deployment in production environments., – The paper establishes the first model‐based framework allowing the high‐fidelity simulation, analysis and optimization of FSW processes using serial industrial robots.

A Review on Inertia and Linear Friction Welding of Ni-Based Superalloys

Inertia and linear friction welding are being increasingly used for near-net-shape manufacturing of high-value materials in aerospace and power generation gas turbines because of providing a better quality joint and offering many advantages over conventional fusion welding and mechanical joining techniques. In this paper, the published works up-to-date on inertia and linear friction welding of Ni-based superalloys are reviewed with the objective to make clarifications on discrepancies and uncertainties reported in literature regarding issues related to these two friction welding processes as well as microstructure, texture, and mechanical properties of the Ni-based superalloy weldments. Initially, the chemical composition and microstructure of Ni-based superalloys that contribute to the quality of the joint are reviewed briefly. Then, problems related to fusion welding of these alloys are addressed with due consideration of inertia and linear friction welding as alternative techniques. The fundamentals of inertia and linear friction welding processes are analyzed next with emphasis on the bonding mechanisms and evolution of temperature and strain rate across the weld interface. Microstructural features, texture development, residual stresses, and mechanical properties of similar and dissimilar polycrystalline and single crystal Ni-based superalloy weldments are discussed next. Then, application of inertia and linear friction welding for joining Ni-based superalloys and related advantages over fusion welding, mechanical joining, and machining are explained briefly. Finally, present scientific and technological challenges facing inertia and linear friction welding of Ni-based superalloys including those related to modeling of these processes are addressed.