Forming sheet metal by laser-induced thermal stresses (laser forming) has been extensively studied and considered to have high possibility for rapid prototyping and other flexible manufacturing. Laser forming process does not require hard tooling or external forces. Therefore it has many advantages, such as no springback, high process flexibility, production of complex shapes and the formation of very small parts. This process has great potential for rapid prototyping and alignment procedures.
Over recent years laser forming techniques have been extensively investigated. Most of them have focused on analyzing the phenomena of the forming process and the effect of the process parameters. However, to apply the laser forming process to the field of industries, strategies to determine a set of process parameters and appropriate control technologies should be proposed. This procedure is called as solving inverse problem in the laser forming process. The objective of this paper is to develop a method which is reliable and easy to apply, to solve the inverse problem in the laser forming process.
Firstly, 2D laser forming for spline curve is investigated. To find the bending points and each bending angle at the bending points, which is the first step in solving the inverse problem in laser forming, two different methods are proposed by using geometrical information: first, the distance between the target surface and sheet metal is used as a criterion for making a bending point; then the angle between tangent lines is used for the surface. The algorithm for making bending points and bending angles is simple; it did not consume much time and it gave a good performance. The accuracy of the results can be adjusted freely by changing the offset value of the algorithms. Experimental investigations are performed by using the linear relationship between the bending angle and the line energy.
In addition to this, a feedback control method which is reliable and easy to apply is used. First, statistical computer simulation and an adaptive gain method are used for feedback control of single line laser forming. This method shows good effectiveness and accuracy, but the small remaining errors which are inevitable in the laser forming process, makes accumulating effect so that the overall shape has some deviation from the target. So, the feedback control algorithms for 2D free curve laser forming are proposed. These algorithms use recalculating methods by using the error of previous step. The results of our experiments shows greatly enhanced accuracy compared with those of the previous study.
The same geometrical approaches for solving inverse problem are applied 3D Bezier surface. Forming points at the edge of the surface are made by 2D algorithms, and then forming lines are composed by connecting those forming points. Plane patches consist of these forming lines. After making plane patches the amount of bending angle and shrinkage at each forming line is extracted during planar development procedure. This planar development uses the concept of imaginary spring to optimize the result. The process parameters such as laser power and travel speed are determined by FEM database calculated by ABAQUS. Experiments for 3D surface are conducted by using 100W CW Ytterbium fiber laser and 3-axis CNC table. The experimental setup is controlled by a PC with LabView program. The results with the target shapes of pillow and saddle show better accuracy compared to the previous research output, and also consume shorter calculating time.
금형을 제작하지 않고 박판을 성형 하는 방법은 크게 접촉식과 비접촉식으로 나눌 수 있다. 접촉식 방법은 프레스나 롤러 등을 이용하여 외압에 의해 재료에 스트레스를 가하여 변형을 일으키는 방법으로, 금형을 이용한 박판 성형과 그 메커니즘이 동일하다. 비접촉식 방법은 박판에 국부적으로 열을 가해 생긴 열응력(thermal stress)으로 변형을 일으키는 방법으로 물리적인 제약이나 공구의 마모가 없을 뿐만 아니라, 스프링백 현상이 없기 때문에 공정 설계가 비교적 간편하고, 정밀도가 우수한 장점이 있다[1]. 비접촉식 방법에 적용되는 열원으로는 가스화염(gas flame), 아크 플라즈마(arc plasma), 레이저 등이 이용되고 있다. 가스화염은 재료의 두께가 비교적 두꺼운 선박제조에 있어서 곡변형 공정에 주로 사용되고 있는데, 가스 화염과 아크 플라즈마의 경우에는 국부적인 가열이 비교적 어렵고, 재료의 두께 방향으로의 온도차 발생이 용이하지 않기 때문에 부가적인 수냉 시스템이 도입되어야 하는 단점이 있다. 이에 반해 레이저는 에너지 조절이 용이하고, 광학계를 이용한 조작이 가능한 데다가, 국부적인 순간 가열이 용이하기 때문에 부가적인 냉각 시스템이 필요치 않아 정밀 박판 성형에 이용하는 연구가 많이 진행되고 있다. 본 연구에서는 주어진 형상을 입력정보로 하여 레이저 성형 공정 변수를 찾아내는 역문제 해결법을 제안하고, 2차원과 3차원 형상에 대해서 적용하였다. 또한 되먹임 제어를 통한 정밀도 향상 방법을 제안하고 실험으로 증명하였다.
