Initially, experimental studies on laser bending of magnesium alloy M1A were carried out to evaluate the feasibility of laser bending of magnesium alloy sheets. The developed pre-displacement movement methodology can be confidently used to increase the productivity and product quality of the laser bending process.
EXPERIMENTAL STUDIES ON LASER BENDING OF MAGNESIUM ALLOY M1A
SINGLE SCAN STRAIGHT LINE LASER BENDING OF MAGNESIUM M1A ALLOY SHEETS
CURVILINEAR LASER BENDING OF MAGNESIUM M1A ALLOY SHEETS
MULTI-SCAN LASER BENDING OF MAGNESIUM M1A ALLOY SHEETS
LASER ASSISTED BENDING WITH MOVING PRE-DISPLACEMENT
CONCLUSIONS AND FUTURE SCOPE
NOMENCLATURE
PE Plastic deformation ved node S, Mises Von-Mises stress ved node TP Laser pulsbredde.
LIST OF GREEK SYMBOLS
LIST OF ABBREVIATIONS
LIST OF TABLES
INTRODUCTION
- Laser Bending Process
- Advantages and Limitations
- Applications of Laser Bending
- Motivation of the Work
- Organization of the Thesis
The motivation for conducting research in the field of laser bending of magnesium alloys has been put forward at the end of this chapter. The validations of predictions of the simulations for the respective variant of laser bending operation are presented.
LITERATURE REVIEW ON LASER BENDING PROCESS
- Scope
- Laser Bending Mechanisms
- Temperature gradient mechanism
- Buckling mechanism
- Upsetting mechanism
- Edge Effect in Laser Bending Process
- Process Parameters
- Material properties
- Geometry parameters
- External constraint parameters
- Multi-Scan Laser Bending Process
- Curvilinear Laser Bending Process
- Materials Processed by Laser Bending Process
- Lasers Used in Bending
- Effect on Mechanical and Micro-structural Properties
- Studies on microstructural properties
- Studies on mechanical properties
- Process Modeling of Laser Bending Process
- Analytical models
- Numerical models
- Soft-computing models
- Inverse Modeling and Optimization
- Process Design of Laser Bending
- Feedback Based Control in Laser Forming
- Observations and Conclusions
- Research Objectives
Thermal and mechanical properties of the workpiece affect the laser bending process (Shen and Vollertsen 2009). They found that the bending angle decreases with the increase in width (along scan line) of the section. The important studies on numerical modeling of the laser bending process are presented in the next subsection.
This chapter provided an overview of the established research work on the laser bending process.
EXPERIMENTAL STUDIES ON LASER BENDING OF MAGNESIUM ALLOY M1A
- Scope
- Overview of the Present Work
- The Need
- Experiments on Laser Bending of Magnesium Alloy M1A
- Specimen preparation
- Laser irradiation
- Bend angle measurement
- Study on Mechanical Properties of Laser Bent Magnesium Alloy M1A
- Summary
Experimental studies have been carried out to check the feasibility of laser bending of magnesium alloys. The change of the mechanical properties of the material of the work sheet due to laser radiation is also presented. Experimental studies on laser bending of magnesium alloy M1A were carried out to check the feasibility of the process for magnesium alloys.
Various details of the experimental studies on laser bending of magnesium alloy M1A are presented in the following sections. The enlarged image of the bottom surface (away from laser irradiation) of the laser bent sample is shown in Figure 3.9 (b). It can be observed that the cracks are not generated during multi-scan laser bending of.
The stress-strain behavior of the laser-scanned sample was compared to that of the base material. The hardness of the laser-irradiated samples was measured and compared with that of the base material samples. The hardness of the laser-irradiated sample is slightly higher, which may be due to compression deformation in the irradiated area.
3-D THERMO-MECHANICAL NUMERICAL SIMULATION OF LASER BENDING PROCESS USING FINITE ELEMENT METHOD
- Scope
- The Need
- Thermo-Mechanical Modeling of Laser Bending Process
- Assumptions
- Governing equations and boundary conditions
- Solution methodology
- Finite Element Formulation
- Continuum discretization
- Thermal analysis
This mathematical formulation is useful in describing the static and dynamic performance of the laser bending process. The primary mechanism of laser bending is the deformation that occurs due to stresses induced in the workpiece by laser heating. The transient temperature field created in an isotropic material due to laser beam irradiation is determined using,.
The last term of Equation (4.1) is the heat generated due to volumetric change of the work sheet and it is neglected due to its much smaller value. The measurement of major and minor axes of the laser beam was performed on an optical surface profile projector. The comparison between experimental and calculated values of the beam diameter is shown in Table 4.1.
Boundary conditions of zero displacement and zero rotation were applied to the clamped side of the worksheet. The finite element formulation of the laser bending process is coupled with heat transfer analysis and mechanical analysis. The strain and stresses also depend on the temperature distribution generated during each solution step.
Mechanical analysis
K B D B V is the global tangential stiffness matrix, [Deq] is the elasto-plastic stress-strain matrix, and also called constitutive matrix, {u} is the displacement incremental vector at the element nodes, and {F} is the global force vector, including nodal force and force caused by the thermal deformation. For coupled analysis, the basic equations of temperature (Equation 4.19) and displacement (Equation 4.26) used in sequential coupled thermomechanical analysis are combined as. This is the main governing equation for coupled thermomechanical analysis, and can be solved either by writing computer code or by using commercially available software.
In view of the time requirement in the development of the computer code, it was considered appropriate to use a commercial code with a basic understanding of the finite element method. Therefore, a numerical model for laser bending of magnesium alloy M1A was developed in commercially available finite element solver ABAQUSTM. Before the comprehensive analysis of the laser bending process was carried out, the numerical results were validated with those obtained in experiments.
Development of Numerical Model for Laser Bending of Magnesium Alloy M1A Using FEM
- Worksheet material and geometry
- Mesh model
- Solution parameters
However, the further increase in the number of elements does not significantly affect the bending angle. It can be seen that the numerical model predicted a negligible bending angle when there was only one element in the thickness direction. The outer region was discretized with one-sided coarse mesh and four equal elements were taken in the thickness direction.
In this work, an automatic (self-adaptive) time-stepping algorithm for selecting the time step increment was used to solve the thermomechanical laser bending problem. This is based on the tolerance of the maximum temperature change allowed in a time step. 0.5 mm × 0.5 mm elements were taken in the heated area, and a coarse bias mesh with a total of five elements was modeled in the outer area.
It justifies the use of fine mesh in the hot zone and one-sided coarse mesh in the outer zone. In the present work, the edge effect was determined by calculating the relative change in bending angle (RVBA) per unit length measured at five equal positions along the scan line as. The predictions of the developed numerical model are validated with those obtained in the experimental studies for different cases of laser bending such as single scan, curve, multi-scan and with moving pre-displacement.
Summary
The bending angle predicted using the numerical simulation was compared with that obtained in the experiment. It was observed that the numerical bending angle has an error of about 2.4% compared to that of the experimental bending angle. Therefore, it can be concluded that the numerical model predicts the bending angle quite accurately for the selected process condition.
It is able to predict the bending angle and edge effect based on temperature distribution, stress and strain distribution and distortions obtained during numerical calculations. A melting effect was included to make the developed model more realistic and suitable for a wide range of process conditions. The methodology for calculating the bending angle and edge effect was also discussed in detail.
The developed numerical model was validated with the experimental results and found to be in good agreement. The next chapter presents experimental validation of the developed numerical model for straight line single scan laser bending of the magnesium alloys. The bending mechanism and the effect of process parameters on bending angle and edge effect are presented in detail for laser bending of magnesium alloy.
SINGLE SCAN STRAIGHT LINE LASER BENDING OF MAGNESIUM M1A ALLOY SHEETS
- Scope
- The Need
- Experimental Validation of Numerical Simulations for Single Scan Laser Bending In the present work, a three-dimensional non-linear thermo-mechanical numerical model is
- Bending Mechanism
- Temperature distribution
- Stress distribution
- Strain distribution
- Residual stresses
- Effect of Process Parameters on Bend Angle
- Effect of laser power
- Effect of scan speed
- Effect of beam diameter
- Effect of Process Parameters on Edge Effect
- Effect of laser power
- Effect of scan speed
- Effect of beam diameter
- Summary
The peak temperature increases with the increase of laser power at both the top and bottom surfaces. It can be observed that the plastic strain at both the top and bottom surfaces decreases with the increase of scanning speed and beam diameter. It can also be observed that the difference between plastic deformation at the top and bottom surfaces decreases with the increase of scanning speed and beam diameter.
It can be observed that with a scanning speed of 2000 mm/min and 3000 mm/min, the bending angle increases with increasing laser power. Increasing the compressive strain on the bottom surface results in decreasing the bending angle with higher laser power. This leads to decreasing bending angle with increasing laser power with slow scan speed and small beam diameter.
It can be seen that the bending angle generally decreases with the increase in scan speed. It can be seen that the edge effect increases with the increase in scan speed for all sets of process conditions. This is because the peak temperature and bending angle decrease with the increase in beam diameter.
CURVILINEAR LASER BENDING OF MAGNESIUM M1A ALLOY SHEETS
- Scope
- The Need
- Numerical Simulation of Curvilinear Laser Bending Process
- Experimental Validation of Numerical Simulations
- Effect of Arc Height on Edge Effect
- Effect of Arc Height on Bend Angle
- Analysis of the Deformation Behavior: Bending Offset and Edge Displacement Based on the studies reported in the earlier chapter, it was found that the worksheet bends along
- Effect of laser power on bending offset
- Effect of scan speed on bending offset
- Effect of beam diameter on bending offset
- Effect of arc height on bending offset
- Effect of arc height on edge displacement
- Summary
The deformation of the work sheet changes with the movement of the laser beam as shown in Figure 6.2 (c) to Figure 6.2 (g). It results in offset of peak temperature outside the scan path as shown in Figure 6.24. The second reason is that the bend in the middle of the scan path tries to bend the worksheet away from the scan path.
However, in the middle of the scan path, the curvature occurred above the scan path. The deflection offset is about 11 and 10.5 mm at the end of the scan path, respectively. However, close to the scanning end position, the laser beam arrives later while the effect of plastic deformation in the middle of the scanning path starts earlier.
The bending offset is closer to the laser beam exit edge (Edge F), which is due to higher peak temperature at this edge compared to that of the scan start edge (Edge S) (Figure 6.27). As discussed earlier, the plastic deformation at the center of the scan path (for example, Point A) tends to bend the workpiece away from the scan path, and therefore the bending offset increases with the increase in arc height. It can be seen that the bending occurs in the middle of the edge (at 30 mm), when laser is irradiated along a straight line (arc height is zero).
MULTI-SCAN LASER BENDING OF MAGNESIUM M1A ALLOY SHEETS
- Scope
- The Need
- Numerical Simulations of Multi-scan Laser Bending
- Experimental Validation of Numerical Simulations
- Bending Mechanism: Effect of the Number of Scans
- Temperature distribution
- Stress distribution
- Plastic strains and distortions
- Bend Angle: Effect of Process Parameters
- Effect of laser power
- Effect of scan speed
- Effect of beam diameter
- Variation in Bend Angle
- Edge Effect
- Summary
- Scope
- The Need
- Proposed Methodology for Laser Bending with Moving Pre-displacement
- Development of Experimental Setup
- Preliminary Experimental Work
- Performance Study of the Proposed Methodology
- Numerical Simulations of Laser Assisted Bending with Moving Pre-Displacement As mentioned in Section 8.4, numerical simulations of laser assisted bending have been carried
- Experimental validation of the numerical simulations
- Effect of Process Parameters on Bend Angle
- Effect of Pre-displacement .1 Bending mechanism
- Bend angle
- Spring-back effect
- Edge effect
- Edge displacement
The bending angle was measured in the middle of the scan path as shown in Figure 3.10 (Chapter 3). It was seen that the bending angle increases with increasing number of laser scans. Effect of laser power on bending angle for predisplacement laser-assisted bending (a) at small beam diameter (b) at large beam diameter.
Effect of scan speed on bending angle for pre-movement displacement laser-assisted bending (a) at small beam diameter (b) at large beam diameter. It can be observed that the bending angle increases with increasing laser power. It can be observed that the bending angle decreases as the scan speed increases.
It can be seen that the bending angle decreases with the increase of the beam diameter. Effect of beam diameter on bending angle for laser-assisted bending with moving pre-displacement (a) at low scan speed (b) at high scan speed. This leads to an increase in bending angle with an increase in predisplacement, as shown in Figure 8.21.
From Figure 8.22 to Figure 8.24, it can be observed that the bending angle decreases with increasing scan speed. It can also be seen from Figure 8.22 to Figure 8.24 that the bending angle increases with increasing laser power.
