Gas Tungsten Arc Welding (GTAW) is a commonly used welding process for welding Titanium materials. Welding of titanium and its alloys poses several intricacies to the designer as they are prone to oxidation phenomenon. To overcome this contamination, a relatively new type of shielding arrangement is experimented. The proposed design and arrangement have been employed for joining commercially pure titanium sheets with variations in the GTAW process parameters namely the welding current and travel speed. Bead on plate (BoP) trials were conducted on thin sheets of 2 mm thickness by varying the process parameters. Subsequently, the macro structure images were captured. Based on these results, the process parameters are chosen for carrying out full penetration butt joints on 1.6 mm and 2 mm thick titanium sheets. The influences of these parameters of GTAW on the microstructure, mechanical properties and surface morphology at the fractured locations of the welded joints are examined. The microstructural properties of base metal, heat affected zone and fusion zone are analyzed through optical microscopy. The welded joints showed an ultimate tensile strength of about 383 MPa with 15.7% elongation. The hardness value at fusion zone and base metal are typically observed to be 191 and 153 HV-0.5, respectively. X-ray diffraction study is conducted to examine the chemical composition in the parent metal and fusion zone of the weld. Fractured surface is examined using Scanning Electron Microscopy which revealed dimple kind of rupture present at the fractured surfaces owing to insufficient or excessive heat with slight impurities that prevents the accomplishment of stronger micro-level weld integrity.

Some studies on mechanical properties and microstructural characterization of automated TIG welding of thin commercially pure titanium sheets

This paper presents a methodology to evaluate the performance of carbide compacting die using graph theoretic approach (GTA). Factors affecting the die performance and their interactions are analysed by developing a mathematical model using digraph and matrix method. Permanent function or die performance index is obtained from the matrix model developed from the digraphs. This permanent function/index value compares and ranks the factors affecting the die performance. It helps in selection of optimum process parameters during die manufacturing. Hence, process output errors such as dimensional inaccuracy, large surface craters, deep recast layers, etc. will be minimised during die manufacturing which helps to achieve better die performance. In present illustration, factors affecting the performance of carbide compacting die are grouped into five main factors namely work material, machine tool, tool electrode, geometry of die and machining operation. GTA methodology reveals that the machine tool has highest index value. Therefore, it is the most influencing factor affecting the die performance. In case of die material low cobalt concentration and small grain size yields good surface finish, while in machine tool low discharge energy (i.e. low values of peak current, pulse-on time, servo voltage and high value of pulse-off time) and high dielectric flow rate yields good surface finish and, hence, favours the good die performance. In case of die geometry, large work piece thickness and small taper angles results in lesser geometrical deviations and hence helps to achieve better die performance.

Digraph and matrix method for the performance evaluation of carbide compacting die manufactured by wire EDM

The paper describes the application of single mode high power fiber laser (HPFL) for the welding of nickel based superalloy Inconel 625. Butt joints of Inconel 625 sheets 0,8 mm thick were laser welded without an additional material. The influence of laser welding parameters on weld quality and mechanical properties of test joints was studied. The quality and mechanical properties of the joints were determined by means of tensile and bending tests, and micro hardness tests, and also metallographic examinations. The results showed that a proper selection of laser welding parameters provides non-porous, fully-penetrated welds with the aspect ratio up to 2.0. The minimum heat input required to achieve full penetration butt welded joints with no defect was found to be 6 J/mm. The yield strength and ultimate tensile strength of the joints are essentially equivalent to that for the base material.

Fiber laser welding of nickel based superalloy Inconel 625

Machining the EN X160CrMoV12 tool steel by electro-discharge machining (EDM) process generates significant modifications of microgeometrical, microstructural and mechanical properties of the upper layers of the machined components. In this paper, the role of these modifications in controlling the stability, under cyclic loading, of the propagation of the crack networks generated by EDM is discussed. High cycle fatigue tests (2 × 106 cycles) show that the presence of these cracks in brittle layers, i.e. white layer, quenched the martensitic layer, and a field of tensile residual stresses (+750 MPa) results in a loss of 34% of endurance limit comparatively with the endurance evaluated for the milled state that generates crack-free surfaces. It is shown, in this work, that the detrimental effect of these crack networks can be controlled by putting in compression the upper layers of the EDM surfaces. Indeed the application of wire brushing to EDM surfaces generates compressive residual stresses (≈−100 MPa) that stabilise the crack networks propagation and therefore restores to the EDM surfaces their endurance limit value corresponding to the milled state. Moreover, removing the crack networks by polishing generates a stabilised residual stress value of ≈−130 MPa. This results in an improvement rate of about 70% of the endurance limit comparatively with the EDM state and of 26% in comparison to the milled state. These rates could be further increased by the application of the wire brushing process to the polished surfaces that reached 75% and 30% comparatively to the EDM and milling states respectively. In this case, a stabilised surface residual stress of about ≈−150 MPa was measured on the specimen surfaces.

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Effects of finishing processes on the fatigue life improvements of electro-machined surfaces of tool steel

Bead-on-plate welding of zircaloy-4 (a reactive material) plates was conducted using electron beam according to central composite design of experiments. Its predictive models were developed in the form of knowledge-based systems in both forward and reverse directions using neural networks. Input parameters considered for this welding of reactive metals were accelerating voltage, beam current and weld speed. The responses of the welding process were measured in terms of bead width, depth of penetration and micro-hardness. Forward mapping of the welding process was conducted using regression analysis, back-propagation neural network (BPNN), genetic algorithm-tuned neural network (GANN) and particle swarm optimization algorithm-tuned neural network (PSONN). Reverse mapping of this process was also carried out using the BPNN, GANN and PSONN-based approaches. Neural network-based approaches could model this welding process of reactive material in both forward and reverse directions efficiently, which is required for the automation of the same. The performance of the neural network models was found to be data-dependent. The BPNN could outperform the other two approaches for most of the cases but not all in both the forward and reverse mappings.

B.H. Choi

Papers

The effect of welding conditions according to mechanical properties of pure titanium