A. Vairis

High frequency linear friction welding of a titanium alloy

Abstract A new hysteresis-free linear friction welding machine is described, capable of welding at variable frequencies and amplitudes of oscillation with adjustable friction and forging pressures. Experiments were performed with Ti 6Al 4V up to a frequency of 119 Hz and for two amplitudes of oscillation (3 and 0.92 mm). The minimum power input required to achieve welding conditions is shown to depend on both frequency and amplitude of oscillation. Weld impact strength is shown to depend on the friction pressure applied for the large amplitude case. Also, it is shown that forging at the end of the process can help to improve weak welds in situations where the energy input is just below the minimum required to achieve optimum welding conditions.

Modelling the linear friction welding of titanium blocks

Abstract Analytical and numerical models of linear friction welding of Ti6Al4V (numbers indicate wt.%) are established to predict temperature rises during the initial phase of the process, which is of importance in forming sound welds. Both model predictions are compared with experimental data. The numerical model is used to explain the physical conditions that control the ability to weld under certain process conditions. The effects of the exothermic reaction of titanium with oxygen during the extrusion phase are also discussed.

On the extrusion stage of linear friction welding of Ti 6Al 4V

Abstract The extrusion phase of linear friction welding of Ti 6Al 4V, which has been shown to have an important effect on weld integrity, is investigated. The position of the area at the rubbing interface where the maximum frictional heat input develops is shown to depend on the amplitude of oscillation. This is corroborated with macroscopic examination of the flash and axial shortening data from experiments. An analytical model is developed to predict the strain rates that the material at the plasticised zone of the interface is exposed to, using phenomenological experimental data.

Linear and rotary friction welding review

Friction welding (FW) is a high quality, nominally solid-state joining process, which produces welds of high structural integrity. Rotary friction welding (RFW) is the most commonly used form of FW, while linear friction welding (LFW) is a relatively new method being used mainly for the production of integrally bladed disc (blisk) assemblies in the aircraft engine industry. Numerous similar and dissimilar joints of structural metallic materials have been welded with RFW and LFW. In this review, the current state of understanding and development of RFW and LFW is presented. Particular emphasis is placed on the process parameters, joint microstructure, residual stresses, mechanical properties and their relationships. Finally, opportunities for further research and development of the RFW and LFW processes are identified.

Design and Commissioning of a Friction Welding Machine

An experimental lightweight linear friction welding machine was designed and commissioned. The machine was designed to explore higher frequencies of oscillation (up to 1 kHz) than those commercially available, for a number of different amplitudes of oscillation. During the commissioning stage problems related to rig stiffening were recognized and the operating envelope of the machine was identified. Important friction welding process parameters like stress and temperature are monitored to evaluate process development and weld integrity.

In situ SAXS investigation of dibromomethane adsorption in ordered mesoporous silica

A SAXS/WAXS apparatus with the aid of a specially designed sample cell capable for performing both SAXS and WAXS experiments was used for adsorption studies in nanoporous materials. The applicability of the instrument for structural investigations and its ability for adsorption experiments because of the advanced sample environment were demonstrated by carrying out in situ SAXS measurements during gas physisorption. SAXS profiles of ordered mesoporous silica were measured at selected equilibrium points alongside a dibromomethane (CH2Br2) adsorption isotherm at 293 K. SBA-15 was the adsorbent of choice because it consists of a regular 2D hexagonal array of cylindrical mesopores that gives rise to Bragg reflections in the small-angle regime. CH2Br2 was selected as a contrast-matching fluid because it has almost the same electron density as silica. We obtained high-quality data comparable to those resulting from experiments performed in synchrotron light sources which produce intense beams of x-rays and support advanced instrumentation for high-resolution diffraction and SAXS studies. The Bragg peaks of the pore lattice are clearly visible for the evacuated sample and at the early stages of the adsorption process. The intensity decrease and the elimination of the Bragg peaks for the saturated sample suggest that an almost perfect contrast matching was achieved. A model has been used for monitoring the fluid condensation and evaporation regime in SBA-15 by taking into account both the Bragg scattering and the diffuse scattering for spatially random pore filling. The results show the absence of spatial correlations between filled pores suggesting random pore filling.

On the compressive behavior of an FDM Steward Platform part

Abstract Acrylonitrile–butadiene–styrene (ABS) is commonly used material in the fused deposition modeling (FDM) process. In this work, ABS and ABS plus parts were built with different building parameters and they were tested according to the ASTM D695 standard. Compression strength results were compared to stock ABS material values. The fracture surfaces of selected specimens were examined under a Scanning Electron Microscope (SEM), to determine the failure mode of the filament strands. Following this a Steward Platform part was tested under compression in a tensile testing machine. The experimental results were employed to develop a finite element model of the Steward Platform part, in order to determine the maximum force the part can withstand. The Finite Element Model results were in good agreement with the values measured in the Steward Platform part compressive tests, demonstrating that the model developed is reliable. In these experiments, it was found that ABS parts build with a larger layer thickness showed lower compressive strength, which ABS plus did not show. ABS specimens on average developed about half the compressive strength of the ABS plus specimens, while the ABS plus specimens showed lower compressive strength values than stock ABS material.

Modelling a Knee Ligament Repair Device

Ligaments in the knee that connect the femur to the tibia are often torn during a sudden twisting motion, which results in instability in the knee. In such cases ligament repair surgery may be an effective treatment where the ligament involved is replaced with a piece of healthy tendon which is grafted into place to hold the knee joint together. A novel device for use in knee ligament repair surgery has been designed, which aims to reduce damage to the ligament grafts used and to minimize post-surgery complications. To evaluate the efficacy of the design the device has been modeled as it would be subjected to static forces while joining the bones together.

Impact of surface state in probeless friction stir spot welding of an Al–Li alloy

ABSTRACT Sheets of AA2198 alloy with various surface conditions were welded with probeless friction stir spot welding (P-FSSW). Results show that the oxide layer on the original lap-weld surface produces continuously distributed oxide impurities at the interface of the P-FSSWed joint with a large amount of voids. The visual flow at the interface provides a persuasive explanation of local preferential abrasion. Following surface grinding, local abrasion increases with surface roughness and results in the dispersion of voids and oxides, which contributes to the improvement of metallurgical connection. The corresponding mechanical strength of the P-FSSWed joints shows a relatively significant increase, while the fracture mode remains affected by the hook defect regardless of the surface state.

Evaluation of an intact, an ACL-deficient, and a reconstructed human knee joint finite element model

The human knee joint has a three-dimensional geometry with multiple body articulations that produce complex mechanical responses under loads that occur in everyday life and sports activities. Understanding the complex mechanical interactions of these load-bearing structures is of use when the treatment of relevant diseases is evaluated and assisting devices are designed. The anterior cruciate ligament (ACL) in the knee is one of four main ligaments that connects the femur to the tibia and is often torn during sudden twisting motions, resulting in knee instability. The objective of this work is to study the mechanical behavior of the human knee joint and evaluate the differences in its response for three different states, i.e., intact, ACL-deficient, and surgically treated (reconstructed) knee. The finite element models corresponding to these states were developed. For the reconstructed model, a novel repair device was developed and patented by the author in previous work. Static load cases were applied, as have already been presented in a previous work, in order to compare the calculated results produced by the two models the ACL-deficient and the surgically reconstructed knee joint, under the exact same loading conditions. Displacements were calculated in different directions for the load cases studied and were found to be very close to those from previous modeling work and were in good agreement with experimental data presented in literature. The developed finite element model for both the intact and the ACL-deficient human knee joint is a reliable tool to study the kinematics of the human knee, as results of this study show. In addition, the reconstructed human knee joint model had kinematic behavior similar to the intact knee joint, showing that such reconstruction devices can restore human knee stability to an adequate extent.