Jonathan David Griffiths

Laser forming: overview of the controlling factors in the temperature gradient mechanism

Abstract Laser forming (LF) offers the industrial promise of controlled shaping of metallic and non-metallic components for prototyping, the correction of design shape or distortion, and precision adjustment applications. The potential process advantages include precise incremental adjustment, flexibility of application, and no mechanical ‘spring-back’ effect. To date, there has been a considerable amount of work carried out on two-dimensional LF, using multi-pass straight-line scan strategies to produce a reasonably controlled bend angle in a number of materials, including aerospace alloys. A key area, however, where there is a limited understanding, is the variation in the bend angle per pass during multi-pass temperature gradient mechanism-based LF along a single irradiation track, in particular, the decrease in the bend angle per pass after many irradiations for a given set of process parameters. Understanding this is essential if the process is to be fully controlled for a manufacturing environment. The research presented in this paper through empirical data and numerical simulation of the LF of sheet mild steel, Ti6Al4V and AA5251, by CO2 laser offers a novel coherent picture of the key influencing factors and at which point in the bend evolution each is dominant, which has not been presented before.

Laser cleaning of the output window in a laser ignition system for gas turbines

Laser ignition (LI) of both liquid fuels and gaseous combustible mixtures in gas turbines offers the potential for reduced emissions and increased reliability. During the combustion process, carbon and other by-products accumulate on the walls of the combustion chamber. For laser based ignition systems, this could potentially reduce the transmissive properties of the output window required for transmission of the laser radiation into the combustion chamber. Presented in this paper is an empirical study into the laser cleaning of an output window for the removal of accumulated carbon prior to laser ignition, with the mechanism of removal discussed.

Finite Element Modelling of the Laser Forming of AISI 1010 Steel

Laser forming offers the industrial promise of controlled shaping of metallic and non-metallic components for prototyping, correction of design shape or distortion and precision adjustment applications. In order to fulfil this promise in a manufacturing environment the process must have a high degree of controllability, which can be achieved through a better understanding of its underlying mechanisms. One area of limited understanding is that of the variation in bend angle per pass observed during multi-pass laser forming along a single irradiation track, notably the decrease in bend angle per pass after many irradiations. Finite element (FE) modelling can be used to ascertain which of the various process parameters (such as graphite burn-off, geometrical effects, variation in absorption, etc.) contribute towards this phenomenon and subsequently the magnitude of their contribution.

Towards high speed laser cutting of thin transparent polymer films

I this study, the effect of modified atmosphere (50% CO2+50% N2) and vacuum packaging on the sensory charactarestics of hot and liquid smoked rainbow trout fillets over a period of 120 d at 4oC was investigated. There groups were constituted: group Avacuum packaged and hot smoked, group Bmodified atmosphere packaged and hot smoked, group Cvacuum packaged and liquid smoked, Group Dmodified atmosphere packaged and liquid smoked. Five experienced panelists, academic staff who were trained in sensory descriptors for smoked fishes, were employed to evaluate the quality of trout fillets during storage. Rainbow trout fillets were assessed on the basis of appearance, taste, texture and odour characteristics using a 9 point descriptive scale. A score of 7-9 indicated ‘‘very good’’ quality, a score of 4.0-6.9 ‘‘good’’ quality, a score of 1.0-3.9 denoted as ‘‘spoiled’’. Sensory scores of each sample were at “good quality” after processing. Group A samples were assessed as the most acceptable products by the panellists. This study determined that vacuum packaging and hot smoke technique was better than modified atmosphere packaging and liquid smoke technique in terms of sensory properties.

Passive Control of Critical Speeds of a Rotating Shaft Using Eccentric Sleeves: Model Development

This paper considers the passive control of lateral critical speeds in high-speed rotating shafts through application of eccentric balancing sleeves. Equations of motion for a rotating flexible shaft with eccentric sleeves at the free ends are derived using the extended Hamilton Principle, considering inertial, non-constant rotating speed, Coriolis and centrifugal effects. A detailed analysis of the passive control characteristics of the eccentric sleeve mechanism and its impact on the shaft dynamics, is presented. Results of the analysis are compared with those from three-dimensional finite element simulations for 3 practical case studies. Through a comparison and evaluation of the relative differences in critical speeds from both approaches it is shown that consideration of eccentric sleeve flexibility becomes progressively more important with increasing sleeve length. The study shows that the critical speed of high-speed rotating shafts can be effectively controlled through implementation of variable mass/stiffness eccentric sleeve systems.

Goal Driven Optimization of Process Parameters for Maximum Efficiency in Laser Bending of Advanced High Strength Steels

Laser forming or bending is fast becoming an attractive option for the forming of advanced high strength steels (AHSS), due primarily to the reduced formability of AHSS when compared with conventional steels in traditional contact-based forming processes. An inherently iterative process, laser forming must be optimized for efficiency in order to compete with contact based forming processes; as such, a robust and accurate method of optimal process parameter prediction is required. In this paper, goal driven optimization is conducted, utilizing numerical simulations as the basis for the prediction of optimal process parameters for the laser bending of DP 1000 steel. A key consideration of the optimization process is the requirement for minimal micro-structural transformation in automotive grade high strength steels such as DP 1000.

Laser Forming of ERW Steel Square Tubes within Metallurgical Constraints

Laser forming (LF) is a non-contact method to shape metallic sheets and tubes by induced thermal stress without melting using a de-focused laser beam. Laser forming offers the industrial promise of controlled shaping of metallic and non-metallic components for prototyping, correction of design shape or distortion and precision adjustment applications. In order to fulfil this promise in a manufacturing environment the process must have a high degree of control, be repeatable and have a minimal impact on the material and mechanical properties of the part to be formed. In order to demonstrate the capability of the LF process a study is presented in this paper on the 3D Laser Forming of ERW steel square tubes SHS EN10305-5 E220 +CR2 (1.5x25x25mm and 1.5x50x50mm 300mm long tube) using a 1.5kW CO2 laser and industrial 5 axis gantry. Strategies have been developed for out of plane bending with specific emphasis on process throughput balanced with minimising adverse localised changes to material properties that could lead to stress concentration features in a component in service. Presented in this paper is empirical 3D LF shape data verified by a scanning laser profiler, a metallurgical study, hardness testing and a FEM model developed in Comsol Multi-Physics. The results of these studies were employed to develop optimised scan strategies for the controlled laser forming of the ERW steel square tubes within strict metallurgical constraints.

Modelling of real temporally variant beam shapes in laser materials processing

The ability to incorporate real, temporally variant intensity distributions is key to improving the accuracy of Finite Element (FE) models of laser processes. The work presented in this paper uses Laser Forming as a process example. Laser forming (LF) offers the industrial promise of controlled shaping of metallic and non-metallic components for prototyping, correction of design shape or distortion and precision adjustment applications. In order to fulfill this promise in a manufacturing environment the process must have a high degree of controllability, which can be achieved through a better understanding of its underlying mechanisms. FE modeling can be used to gain this understanding.

Laser micro-adjustment using ultra-short pulses

MEMS manufacturing requires accurate positioning and high reproducibility. Lasers can be utilised in accurate post-fabrication adjustment, allowing for manufacturing processes with relatively large tolerances. Laser micro forming (LμF) is a process for the precision adjustment, shaping or correction of distortion in micro-scale metallic components through the application of laser irradiation without the need for permanent dies or tools. The non-contact nature of the process is also useful in accessing specific micro-components within a device which may be highly sensitive to mechanical force. As such it has potential for widespread application in both the manufacturing and microelectronics industry. Presented in this work is a novel technique for thermal LμF involving picosecond duration pulses. A potential application in the micro-adjustment of MEMS scale components is also presented.MEMS manufacturing requires accurate positioning and high reproducibility. Lasers can be utilised in accurate post-fabrication adjustment, allowing for manufacturing processes with relatively large tolerances. Laser micro forming (LμF) is a process for the precision adjustment, shaping or correction of distortion in micro-scale metallic components through the application of laser irradiation without the need for permanent dies or tools. The non-contact nature of the process is also useful in accessing specific micro-components within a device which may be highly sensitive to mechanical force. As such it has potential for widespread application in both the manufacturing and microelectronics industry. Presented in this work is a novel technique for thermal LμF involving picosecond duration pulses. A potential application in the micro-adjustment of MEMS scale components is also presented.

Towards controlled 3D laser forming

2-Dimensional laser forming can currently control bend angle, with reasonably accurate results, in various materials including aerospace alloys. However, this is a different situation for 3-Dimensional laser forming. To advance this process further for realistic forming applications and for straightening and aligning operations in a manufacturing industry it is necessary to consider larger scale controlled 3D laser forming. The work presented in this paper uses a predictive and adaptive approach to control the laser forming of mild steel and aluminium sheet into a desired surface. Key to the control of the process was the development of a predictive model to give scan strategies based on a required geometry and the surface error. The forming rate and distribution of the magnitude of forming across the surface were controlled in the closed loop by the process speed. When the geometry is not formed within one pass, an incremental adaptive approach is used for subsequent passes, utilising the error between the current and desired geometry to give a new scan strategy, thus any unwanted distortion due to material variability can be accounted for and distortion control and removal is possible.