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.     

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.     

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.     

Mechanical strength of chip scale package with various underfills under accelerated thermal condition

Abstract

The bending strength of chip scale package (CSP) with various underfills was evaluated under thermal shock condition. The maximum bending strength and deflection of package decreased with increasing number of thermal shock cycles. The bending strength of package increased with increasing glass transition temperature T _g and damping property of the underfill, while the bending deflection decreased with increasing T _g and damping property of the underfill.     

Development of porous 316L stainless steel with controllable microcellular features using selective laser melting

Abstract

The regularly shaped parallel pore gas armed (GASAR) stainless steel porous material with a homogeneous size distribution of unusually micrometer scaled pores (2 μm in average) was successfully prepared using selective laser melting process, by adding 0·10 wt-% gas generating materials in the form of H₃BO₃ and KBF₄. The adjustment of pore morphology, pore direction, and porosity was realised by changing material combinations (such as the content of additive materials) and processing conditions (such as the scan speed of laser beam).     

Laser surface alloying of 1045 AISI steel using Ni–CrB2 powder

Abstract

Laser surface alloying is a process whose purpose is to improve the surface properties by incorporating alloying elements into the surface. The advantages of using laser for surface treatment are: formation of a non-equilibrium or amorphous phase as well as homogenisation and refinement of the microstructure, all without affecting the substrate properties. Powder (50 wt-%Ni–50 wt-%CrB₂) was injected into a melt pool created by a CW–CO₂ laser on AISI1045 steel plates. In order to alloy the entire surface, the sample was scanned at scan speeds in the range of 600–6000 mm min^–1 and the laser power was in the range of 1750–2500 W. The powder feed rate was 1·6 g min^–1, the laser beam was 2 mm in diameter, with 60% overlap between successive laser paths. Metallographic cross-sections were made of the samples. For each sample the following properties were characterised: layer depth, microhardness (HV), layer microstructure and composition. It has been found that the scan speed and the laser power affect the depth of the melt pool, the microstructure, the hardness and the treated layer composition. The laser boronised surface exhibits better wear resistance than D2 tool steel hardened to 59 ± 1 HRC. This will be discussed based on numerical analysis of the laser/material interaction.     

Microstructure of cobalt based alloy coatings produced by laser deposit welding

Abstract

Coatings of cobalt based alloy HMSP 2528, deposited on a nickel based superalloy using a transverse flow CO₂ laser, are investigated. Sections of the coatings have been examined to reveal their microstructure and phase composition using an Olympus PME–3 type optical microscope, scanning electron microscope, transmission electron microscope, and X-ray diffractometer. The results showed that primary phase (γ-Co) and eutectic (γ-Co + Cr₂₃C₆) were present in the coatings. Rapid directional solidification was found at the interface, where the growth direction was perpendicular to the interface. Multidirectional growth and growth parallel to the scanning direction were also found in the central region and near the top surface, respectively. At the remelted boundary between two tracks, a novel microstructure was found in which the crystallisation direction of the first track was transmitted to the second track. As more tracks were deposited, coarsening dendrites could be found at the interfaces.