Numerical and experimental investigation of multi-pass forming process of aluminum sheets using laser

Dissertation for receiving a master's degree in the field

Mechanical engineering, manufacturing and production

Abstract:

Laser shaping process, especially in recent years, has been widely noticed. In this process, shaping is done by irradiating the laser beam on the surface of the sheet, which is accompanied by a rapid local heating and subsequent cooling of the heated area. However, the way of temperature distribution induced by laser beam radiation in the piece determines the type of forming mechanism. Depending on the choice of the size of the parameters affecting the process, the mechanism and the way of changing the shape of the sheet are different. In this regard, three mechanisms of temperature gradient, buckling and shortening have been introduced. In line with the objectives of this thesis, the study of the effect of different parameters on the bending angle is predicted and investigated. Then the bending of metal sheets using laser beam as thermo-mechanical couple is modeled in ABAQUS finite element software and the simulation is investigated by changing parameters such as beam diameter, beam speed, sheet thickness, laser power and number of passes. The continuation of the conducted research is experimental and based on the presented experiment design, five parameters (laser power, laser beam speed, beam diameter, sheet thickness, number of passes) have been investigated as effective parameters in the process. Then, in order to evaluate, the effect of laser power, laser beam speed, sheet thickness and number of passes on the process of increasing the bending angle has been checked by means of diagrams and charts with the help of Design Expert software. The next study deals with the effect of the stop time or cooling time between the passes. The results of the investigations showed that among the mentioned variables, the number of passes and the laser power have a direct relationship with the thickness of the sheet, the diameter of the beam and the speed of the scan, respectively, the relationship between the image and the final angle of the bend. process, finite element method

1-1-Introduction

The shaping process is a production method in which, by keeping the amount of mass and the type of intermolecular bonds constant, a specific three-dimensional shape is created on the workpiece. In fact, shaping is one of the processes that do not have material removal or removal.

1-2-types of shaping processes

Shaping processes can be classified into the following types based on the DIN 8582 standard, depending on the direction of stresses applied to the part

1. Forming under compressive stresses; such as roller forming process

2. Forming under combined tensile and compressive stresses; such as deep stretching process

3. Forming under tensile stresses; such as tensile forming

4. bending Like bending with linear molds

5. Forming under shear stresses; Twisting process[1

1-2-1-bending processes

In general, conventional bending methods are divided into the following two types:

a) mechanical bending b) thermomechanical bending[2].

1-2-1-1- Mechanical bending

Among the mechanical methods, bending using U-shaped and V-shaped molds and bending using rollers can be mentioned. In this method, bending is done by a hard tool and applying external force. Also, different molds should be used to create different curves. The disadvantages of this method are:

This method is very expensive.

In high-volume production, there is a need to constantly change tools and molds.

With the increase in sheet thickness, a high-capacity press is needed.

Dimensional inaccuracy caused by the spring return phenomenon. It exists in mechanical bending.. [2]

1-2-1-2- Thermomechanical bending

In the flame bending process, which is an example of thermomechanical bending, moving an oxyacetylene flame along a straight line on the workpiece is used. As a result of thermal stress, which occurs during heating and cooling of the workpiece, plastic deformation occurs. Compared to mechanical methods, this method has no tool erosion and is therefore less expensive. The main disadvantages of this method are:

1. The torch flame cannot be focused. Therefore, the area affected by heat is large [1] and it is difficult to create accurate bends.

2. This method is not automated. Reproducibility of the process is difficult and dependent on the operator.

3. The piece should be cooled immediately after heating with a torch [2].

1-3-Laser shaping process

Laser, since its invention, has found many applications. "Materials processing by laser" refers to several industrial processes in which lasers are used to modify the shape of a part, for example by melting the workpiece and removing unnecessary parts. One of the unique features of the laser beam is its radiation intensity and focusability. These features have caused the laser to be used in processes such as bending, welding, drilling, cutting, heat treatment, alloying, etc.

In the laser forming process, the laser beam is used to create a bending angle in metal sheets as well as hard materials. Similar to the flame bending method, in this process, the workpiece is bent as a result of thermal residual stresses, instead of applying external force. Therefore, laser bending is another type of thermomechanical bending [2].

The first researches in the field of laser forming process started in the mid-1980s. This process is a non-contact process for bending and creating three-dimensional shapes in metal and non-metal parts. In this process, shaping is done by applying thermal stresses caused by laser beam radiation on the surface of the workpiece, i.e. by creating a rapid local heating and subsequent cooling of the heated area. In the heating stage, if the thermal strains in the irradiated area exceed the elastic strain of the material (it depends on the temperature and geometrical characteristics of the workpiece), the thermal strains will turn into compressive plastic strains. During the cooling stage, the part undergoes contraction and as a result, a bent angle or a deformation is created in the heated area. The laser shaping process is used in rapid prototyping as well as shape correction of parts used in the aerospace, shipbuilding, and automobile industries [2]. These parameters include the parameters of the laser beam as well as the mechanical and thermal characteristics of the material, which will be discussed in the upcoming chapters. The history of research done in the field of laser forming process is given in 0.

Abstract

The process of forming by laser, in particular in recent years has been noticed broadly. In this process, with the radiation of laser ray over the sheet surface, which is accompanied by the creation of a fast sectional heating and following that, the cooling of the heated area, forming is performed. Although, the method of the emitted temperature distribution as a result of the laser beam radiation in a piece specifies the type of forming mechanism, but, the mechanism and method of the shape change of the sheet are different depending on the size of parameters affecting the process. In this connection, three mechanisms of temperature gradient, buckling and shortening have been introduced. In line with the objectives of this thesis, the study of the effect of different parameters on the bending angle has been predicted and studied.