27—THE BENDING AND CREASING OF MULTICOMPONENT VISCOELASTIC FIBRE ASSEMBLIES. PART I: GENERAL CONSIDERATION OF THE PROBLEM

The problems involved in making a mechanical analysis of bending deformations in textile fabrics are discussed. It is suggested that progress can be made by making simplifying assumptions concerning the geometry of fabric structure and deformation and using energy considerations to calculate forces and moments. Certain simple examples are given of the usefulness of this approach. A foundation is laid for the construction of a computer model that will, it is hoped, simulate the viscoelastic bending and creasing behaviour of textile fabrics from a knowledge of the bending and torsional viscoelastic properties of the constituent fibre(s) and their relative geometrical arrangement.     

22—A CRITICAL COMPARISON OF EMPIRICAL TESTS OF THE EXTENSIONAL RESILIENCE OF TEXTILE FIBRES: PART I: DESCRIPTION OF THREE TEST METHODS

The relevance of the extensional recovery of single filaments to the resilience of textiles is discussed, and tests are described that yield information at three different effectiverates of loading. The methods considered are: (i) cyclic measurements on an Instron Tensile Tester; (ii) recovery from a sustained extension; and (iii) recovery from impact. Applications of these methods will be discussed in two later papers.     

MARTENSITE TRANSFORMATION MICROSTRUCTURE OF 40Cr STEEL COMPLEXLY INDUCED BY LASER SHOCK

40Cr steel is laser quenched by the NEL-2500A rapidly axial flow CO2 laser. Then the martensite induced by laser quenched is shocked by Nd:YAG laser again. Through comparing and analyzing the appearance and size of martensite,the dislocation density in microstructure between the treated zones by laser quenched and by laser quenched plus laser shock,the following results are shown: The second martensite obtained by laser compound treatment is more finer compared with those obtained by laser quenched; In the hardened zones obtained by compound treatment,a lot of slender second twin crystal martensites are induced; A lot of more high density dislocation tangles and cellular dislocations are generated. From the transmission electron microscope (TEM) micrograph after compound treatment,there are not only long lath and short nubbly martensites arranged in cross direction,but also massive nubbly and small short nubbly martensites arranged in longitudinal direc-tion. Some martensites look like the broken blocks of quenched martensites. These new martensites are inserted transversely in the quenched martensites with large tangle. And they make quenched martensites break into pieces. Compared with the quenched martensites,the size of fresh martensites are smaller,about 0.3～0.5μm.     

Probing into diffusion bonding of laser surface treated γ-Ti–Al alloy/Ti–6Al–4V alloy

Abstract

In this work, a novel diffusion bonding technique combining the laser surface treatment (LST) with the diffusion bonding is used to join a γ-Ti–Al alloy with a Ti–6Al–4V alloy. By using the LST and subsequent heat treatment, a layer with a fine grain structure can be obtained on their surface of the two alloys. The diffusion bonding behaviour between γ-Ti–Al alloy and Ti–6Al–4V alloy with or without LST under the different bonding conditions is investigated. The result reveals that LST can improve the diffusion bonding behaviour of the two alloys, and the three point bending strength of the joints can be promoted. The sound bonding between the two alloys with the LST is achieved at 1173 K under 80 MPa in 2 h.     

Laser cladding repair of turbine blades in power plants: from research to commercialisation

Abstract

Reliable and efficient power generation is a major global issue due to both political and environmental concerns. Nevertheless, many critical components, particularly the blades of the low pressure (LP) side of power generating steam turbines, are subject to failure due to severe erosion at the leading edges. Since taking machines offline for maintenance and removal of damaged blade for repair is extremely expensive, increasing the service life of these critical components offers significant economic and political benefits. Conventional techniques to increase service life include brazing of an erosion shield at the leading edge of the turbine blade, open arc hardfacing, and cladding with erosion resistant materials using gas tungsten, manual metal or plasma transferred arc welding. The authors have been investigating since 2001 the use of laser cladding technology to deposit a high quality and erosion resistant protection shield on the leading edge of LP blades. The project has demonstrated the feasibility of in situ repair of turbine blades in trials conducted at a power station using a fibre delivered diode laser and a robot. A company, Hardwear Pty Ltd, was established in late 2005 to commercialise this technology and has to date carried out successfully several commercial contracts involving the repair of 340 LP blades.     

Methodology to improve applicability of welding simulation

Abstract

The objective of this paper is to demonstrate a new simulation technique which allows fast and automatic generation of temperature fields as input for subsequent thermomechanical welding simulation. The basic idea is to decompose the process model into an empirical part based on neural networks and a phenomenological part that describes the physical phenomena. The strength of this composite modelling approach is the automatic calibration of mathematical models against experimental data without the need for manual interference by an experienced user. As an example for typical applications in laser beam and GMA–laser hybrid welding, it is shown that even 3D heat conduction models of a low complexity can approximate measured temperature fields with a sufficient accuracy. In general, any derivation of model fitting parameters from the real process adds uncertainties to the simulation independent of the complexity of the underlying phenomenological model. The modelling technique presented hybridises empirical and phenomenological models. It reduces the model uncertainties by exploiting additional information which keeps normally hidden in the data measured when the model calibration is performed against few experimental data sets. In contrast, here the optimal model parameter set corresponding to a given process parameter is computed by means of an empirical submodel based on relatively large set of experimental data. The approach allows making a contribution to an efficient compensation of modelling inaccuracies and lack of knowledge about thermophysical material properties or boundary conditions. Two illustrating examples are provided.     

Effect of weakly ionised plasma on penetration of stainless steel weld produced with ultra high power density fibre laser

Abstract

A weakly ionised plasma can be generated in stainless steel welding with a 10 kW fibre laser beam at the ultra high power density of ～1 MW mm^–2 in Ar shielding gas. The objectives of this study are to obtain a fundamental knowledge of optical interaction between a fibre laser beam and the weakly ionised plasma, and to evaluate effects of the plasma on weld penetration. The optical interaction was investigated by the high speed video observation or the power meter measurement of another probe fibre laser beam, which passed horizontally through the weakly ionised plasma induced during bead on plate welding of a 20 mm thick type 304 plate with a 10 kW fibre laser beam of 0˙9 MW mm^–2 in power density. The probe laser observed was refracted at 0˙6 mrad angle in average, which was much lower than the 90 mrad divergence of the focused fibre laser beam. The attenuation of the probe laser was measured to be ～4%, which was not mainly caused by Inverse Bremsstrahlung but by Rayleigh scattering. Moreover, a stable laser welding process could be produced at such ultra high power density that 11˙5 mm deep penetration was obtained even if the laser peak power was modulated 1 ms periodically from 10 to 8˙5 kW. It was consequently considered that the optical interaction between the 10 kW fibre laser beam and the weakly ionised plasma was too small to exert the reduction in weld penetration.     

Characterisation of solidification cracking in pulsed Nd:YAG laser welding of 2024 aluminium alloy

Abstract

Aluminium alloy AA2024 is one of the most susceptible alloys to solidification cracking. The influence of Nd:YAG laser welding parameters on cracking severity of AA 2024 alloy was studied. Welding was performed in two modes: single spot welding and overlap spot welding. In single spot welding mode, the formation of columnar zone increases cracking severity in the fusion zone. In overlap spot welding mode, the solidification cracks were characterised as: cracks propagated from previous spot, cracks initiated from the fusion line with the previous spot, and cracks initiated from the fusion line with the base metal. It was established that in comparison there is very limited tendency for initiation of new cracks from the fusion line with the weld metal of a previous spot.     

Weld porosity in fibre laser weld of thixomolded heat resistant Mg alloys

Abstract

Porosity in fibre laser welds of two thixomolded heat resistant magnesium alloys AE42 and AS41 was investigated in detail, and porosity formation mechanism was discussed in terms of gas compositions in porosity. It is found that the area percentage of porosity in welds decreases with increasing welding speed, and can be correlated to width of weld metal. Microstructure observation and gas composition analysis in porosity show that the porosity in welds is mainly attributed to the micropores pre-existing in base metals during melting of AE42 and AS41 alloys by fibre laser welding, which are formed due to air entrapment during thixomolding process. Hydrogen rejection and Ar shielding gas entrapment are also the possible reasons for the porosity formation; however, their contribution is much smaller than that of pores in base metals. Furthermore, the addition of rare earth element may probably decrease porosity amount in the thixomolded Mg alloys and their welds.     

Bead on plate laser brazing of dual phase steel with two copper based consumables

Abstract

High strength steels with good formability properties have been developed in recent years, especially for the automotive industry. Joining these metals is however increasingly difficult as the fusion joining processes destroy the carefully constructed microstructure of the steels, resulting in less favourable mechanical properties in and around the joint. A possible solution to this problem is the use of joining processes that require less heat input; laser brazing is one such process. In this work, the brazability of a dual phase steel sheet has been investigated by means of bead on plate brazes produced with two consumables, CuSi3 and CuAl8. Two brazability diagrams are reported and high speed video images are used to explain the differences in operating conditions for these two consumables. Temperature measurements in the steel provide an indication about the temperatures reached during the joining process, which in turn explain the changes observed in the hardness of the steel.