Effect of high-pressure rolling followed by laser processing on mechanical properties, microstructure and residual stress distribution in multi-pass welds of 304L stainless steel

Multi-pass fusion welding by a filler material (wire) is normally carried out to join thick steel sections used in most engineering applications. Multiple thermal cycles from a multi-pass weld resulted in a variable distribution of residual stress field across the weld and through the thickness. Presence of tensile residual stresses can be detrimental to the integrity and the service behaviour of the welded joint. In addition to a complex distribution of residual stress state, multi-pass welds also form dendritic grain structure, which are repeatedly heated, resulting in segregation of alloying elements. In this research, microstructural refinement with modification of residual stress state was attempted by applying post-weld cold rolling followed by laser processing and then cold rolling. The residual stress was determined non-destructively by using neutron diffraction. Post-weld cold rolling followed by laser processing was carried out to induce recrystallization of the cold rolled grains. Microstructural characterisation indicates a significant grain refinement near the capping pass. However, post-weld cold rolling followed by laser processing reinstates the lock-in stress. In this study, it was demonstrated that a complete recrystallized microstructure with compressive state of stress can be formed when a further cold rolling is applied on the laser processed, recrystallized microstructure

Parameters controlling weld bead profile in conduction laser welding

In laser welding and other processes, such as cladding and additive manufacturing, the weld bead geometry (depth of penetration and weld width) can be controlled with different parameters. A common practice is to develop process parameters for a particular application based on an engineering approach using the system parameters i.e. laser power and travel speed. However, in such a case the process is optimised for a particular system only. This study is focused on understanding of the phenomena controlling the weld profile in conduction welding for a wide range of beam diameters from 0.07 mm to 5.50 mm. It has been shown that the weld bead geometry can be controlled by the spatial and temporal distribution of laser energy on the surface of workpiece, such as power density, interaction time and energy density. This means that similar depths of penetration can be achieved with various optical set-ups. It has been also found that it is more difficult to achieve pure conduction welds with small beam diameters, which are typically used in powder bed additive manufacturing, due to high conduction losses and low vaporisation threshold

Effect of beam shape and spatial energy distribution on weld bead geometry in conduction welding

The size of a projected beam onto a workpiece and its intensity distribution profile defines the response of the material to the applied laser heat. This means that not only the processing parameters, but also the optical set-up and process tools define the process and the resulting weld profile. In high power laser delivery systems the beam propagation characteristics of the laser beam can vary during processing. A change of the focal distance, for instance, alters the spot size projected on the workpiece as well as its intensity distribution. Some dynamic optical systems can also change the shape of the projected beam. Galvo-scanners induce a small distortion to the projected beam from circular to elliptical when the mirrors deflect the beam across the working domain. This continuous change of the spatial energy distribution may affect the process stability and material response locally. This work examines the influence of changing the shape of the projected beam and its energy distribution on the weld bead profile in conduction laser welding, which is also relevant to laser cladding and additive manufacture. It has been found that for the same optical set-up and system parameters, different bead profiles can be obtained with different degree of distortion of the beam profile. In addition, different intensity distribution profiles led to different penetration depths for the same nominal beam diameter and energy density due to the difference in peak intensity

Experimental investigation of productivity, specific energy consumption, and hole quality in single-pulse, percussion, and trepanning drilling of IN 718 superalloy

Laser drilling is a high-speed process that is used to produce high aspect ratio holes of various sizes for critical applications, such as cooling holes in aero-engine and gas turbine components. Hole quality is always a major concern during the laser drilling process. Apart from hole quality, cost and productivity are also the key considerations for high-value manufacturing industries. Taking into account the significance of improving material removal quantity, energy efficiency, and product quality, this study is performed in the form of an experimental investigation and multi-objective optimisation for three different laser drilling processes (single-pulse, percussion, and trepanning). A Quasi-CW fibre laser was used to produce holes in a 1 mm thick IN 718 superalloy. The impacts of significant process parameters on the material removal rate (MRR), specific energy consumption (SEC), and hole taper have been discussed based on the results collected through an experimental matrix that was designed using the Taguchi method. The novelty of this work focuses on evaluating and comparing the performance of laser drilling methods in relation to MRR, SEC, and hole quality altogether. Comparative analysis revealed single-pulse drilling as the best option for MRR and SEC as the MRR value reduces with percussion and trepanning by 99.70% and 99.87% respectively; similarly, percussion resulted in 14.20% higher SEC value while trepanning yielded a six-folds increase in SEC as compared to single-pulse drilling. Trepanning, on the other hand, outperformed the rest of the drilling processes with 71.96% better hole quality. Moreover, optimum values of parameters simultaneously minimising SEC and hole taper and maximising MRR are determined using multi-objective optimisation

Root stability in hybrid laser welding

Hybrid laser welding offers promising advantages over the traditional arc-based welding processes. The high penetration ability of lasers and the filler wire delivery of gas metal arc welding (GMAW) enable joining of thick section materials without the need of multi-pass. The output power of modern solid state lasers provides enough energy to penetrate thicknesses exceeding 20 mm in steel. However, the high aspect ratio fusion zone with the rapid solidification does not always provide beneficial conditions for achievement of good weld profiles. Distribution of the liquid metal between the top and root sides of a joint, and hence the weld profile, are determined by a complex balance between the vaporization pressure of a laser, the electromagnetic force of an arc and the surface tension of a meltpool. In this work, the stability of root profile and all aspects related to the achievement of acceptable roots in pipeline welding have been investigated. It has been found that in order to achieve a smooth root profile in deep penetration hybrid laser welding, not only a sufficient penetration force, but also a certain amount of energy need to be provided. This is required to maintain the keyhole fully developed with a steady state pressure balance throughout the thickness. It is also important to achieve sufficient temperature in the root and to provide appropriate wetting between the liquid metal and the back surface of the material. Depending on the power density and energy used, different regimes were identified with sagging of the root in the initial stage, followed by good quality root profiles and then ending on excessive melt expulsion with further increase of power density. The results suggest that if operated in the right regime, the process is very tolerant, in terms of energy and power density required for acceptable root profiles and good quality joints can be achieved

Study of fundamental parameters in hybrid laser welding

This thesis undertakes a study of laser welding in terms of basic laser material interaction parameters. This includes power density, interaction time and specific point energy. A detailed study of the correlation between the laser material interaction parameters and the observed weld bead profiles is carried out. The results show that the power density and the specific point energy control the depth of penetration, whilst the interaction time controls the weld width. These parameters uniquely characterise the response of the material to the imposed laser energy profile, which is independent of the laser system. It is demonstrated that by studying the laser welding with respect to the basic laser material interaction parameters also helps explain some phenomenological phenomena in laser welding, such as the effect of beam diameter on the weld profile. In addition a new approach for parameter selection in laser and hybrid laser welding is investigated. A phenomenological model allowing achievement of a particular laser weld on different laser systems is developed. In the proposed method the user specifies the required weld profile, according to the quality requirements and then the model provides combination of laser parameters, which lead to this particular weld on a given laser system. This approach can be potentially used to transfer laser data between different laser systems with different beam diameters. An extensive study of residual stains in laser and hybrid laser welding is carried out. Both processes are compared either at a constant total heat input or at conditions required to achieve the same depth of penetration. The results demonstrate that there is a trade-off between the fit-up tolerance and the residual stress-induced distortion. Hybrid laser welding provides better ability to bridge gaps than the laser welding, but for the price of increased residual stress and distortion. Additionally, industrial study of the sensitivity to fit-up of hybrid laser welding with high deposition rate MIG sources is carried out. This thesis is a part of NEGLAP (Next Generation Laser Processing) project sponsored by EPSRC (Engineering and Physical Sciences Research Council) and Tata Steel. The main objective is to understand the process fundamentals and exploit the usefulness of laser technology in pipe industry.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Towards cost modelling for laser drilling process

Laser drilling is a widely used non-traditional machining process, in power generation and high-value manufacturing industries, to produce components such as nozzle guide vanes, combustion chambers, fuel injection nozzles and turbine blades. The operating cost of the drilling process is one of the critical factors for companies to consider to survive in the competitive global market. This research is intended to develop the cost model for laser drilling process due to its extensive application in the aerospace sector. Cost estimation facilitates the aerospace sector economically through the identification of critical parameters which act as key cost drivers and their relationship with cost. The proposed model will benefit the designers and cost engineers to have a brief overlook of cost distribution before manufacturing the component

Does knowledge management mediate the relationship between entrepreneurial orientation and firm performance?

[EN] Purpose ¿ This study investigates the impact of entrepreneurial orientation (EO) and knowledge management (KM) on firm performance (PERF), as well as the mediating role of KM in the EO¿PERF (EOPERF relationship). In particular, this study aims to explain the impact of KM on the relationship between the EO dimensions and PERF; dimensions are risk-taking (RT), innovativeness (IN) and proactiveness (PR). Design/methodology/approach ¿ This study uses structural equation modelling and fuzzy-set qualitative comparative analysis (fsQCA) methodologies to explore target relationships. The sample consists of 150 small furniture manufacturers operating in Poland (out of 1,480 in the population). Findings ¿ The study findings show that KM partially mediates the IN¿PERF relationship. Furthermore, fsQCA reveals that KM accompanied by IN is a core condition that leads to PERF. Moreover, the absence of KM (accompanied by the absence of RT and IN) leads to the absence of PERF. In addition, the results show that all the variables examined (RT, IN, PR and KM) positively impact PERF. Originality/value ¿ This study explores the role of KM in the context of EO and its impact on PERF in the low-tech industry. The study uses simultaneously two methodologies that represent different approaches in the search for the expected relationships. The findings reveal that KM mediates the EO-PERF relationship.This work has been supported by the AGH University of Krakow (funds for the maintenance and development of the research capacity of the Faculty of Management of the AGH University of Krakow and grant received by Rafa Kusa under Excellence initiative research university for the AGH University of Krakow).Kusa, R.; Suder, M.; Duda, J.; Czakon, W.; Juárez Varón, D. (2023). Does knowledge management mediate the relationship between entrepreneurial orientation and firm performance?. Journal of Knowledge Management. 1-29. https://doi.org/10.1108/JKM-07-2023-060812

Study of the effect of inter-pass temperature on weld overlap start-stop defects and mitigation by application of laser defocusing

Laser keyhole initiation and termination-related defects, such as cracking and keyhole cavities due to keyhole collapse, are a well-known issue in laser keyhole welding of thick section steels. In longitudinal welding, run-on and run-off plates are used to avoid this problem. However, such an approach is not applicable in circumferential welding where start/stop defects remain within the workpiece. These issues can hinder industry from applying laser keyhole welding for circumferential welding applications. In this paper, the effect of inter-pass temperature on laser keyhole initiation and termination at the weld overlap start-stop region was investigated. This study has identified that defects occurring within this region were due to laser termination rather than laser initiation because of keyhole instabilities regardless of the thermal cycle. The laser termination defects were mitigated by applying a laser defocusing termination regime to reduce the keyhole depth gradually and control the closure of the keyhole

Study on effect of laser keyhole weld termination regimes and material composition on weld overlap start-stop defects

The effects of two different laser termination regimes; laser power ramp-down and laser defocusing, on the weld overlap start-stop region, have been studied on three different compositions of S355J2+N grade steel plates. The plates have varying amounts of deoxidisers, such as manganese and silicon, to allow for the effect of dissolved oxygen on melt pool dynamics and defect formation to be studied. Observations show that gas entrapment, manifesting as porosity, is the main issue in both the laser termination regimes studied, due to melt pool instabilities related to keyhole termination. However, it is more severe during laser defocusing due to the formation of porosity clusters below a specific power density and melt pool size. The percentage of dissolved oxygen and deoxidisers was found to be vital and could be correlated to the response of the melt pool during laser processing. Steel, with a higher percentage of deoxidisers, such as manganese and silicon, would combine with the dissolved oxygen in the weld pool and thereby affect the surface tension and melt pool dynamics. The effect of lower dissolved oxygen would lead to a reduction in penetration depth, but more importantly, porosity and porosity cluster formation, as observed for both laser termination regimes. Successful laser termination with defect-free weld overlaps at the start-stop region could be achieved using both termination regimes for the steel with a higher proportion of deoxidising element