Thesis A STUDY ON THE SHAPE CORRECTION FOR STAMPED PRODUCT OF HIGH STRENGTH STEELS BY LASER IRRADIATION Thanh Hai Nguyen The Graduate School YEUNGNAM UNIVERSITY Department of Mechanical Engineering Mechanical Engineering Major Advisor: Prof. Hyun Bo Shim ABSTRACT In automotive industries, use of high strength steels (HSS) is rapidly increasing since high strength steels can meet the requirement of automotives, such as light weight of the structure, low fuel consumption, and safety of passengers, etc. Most automotive parts, including bumper part, can be manufactured by sheet metal forming, where stamping process is the 1 typical technology. The stamping of high strength steels makes difficult to develop the stamping dies due to low formability and large springback compared to low strength steels.
As a result, it demands longer period of product development than those of the stamping of conventional steels. Besides, the basic purpose of manufacturing is to create the products having the severe competition in the automotive market. Therefore, shortening the period of product development and reducing cost are strongly demanded. In order to meet both the requirement of automotive industries and the difficulties of stamping processes, a new heuristic method which is combined use of stamping and laser irradiation has been proposed in this study.
To verify the method, corresponding experiment and a series of numerical simulations, irradiation process, stamping process and springback were carried out. Springback amount can be transferred into certain angles. Based on the trend of springback angles, the desirable laser bending angles are definitely obtained. Moreover, the condition of laser irradiation in order to achieve the desirable bending angles can be derived based on the simulation analysis of laser irradiation using flat plate.
Applying the laser irradiation patterns, the springback angles were significantly compensated within acceptable tolerance. This work still remains some things being done in the future such as finding the real temperature dependent materials of high strength steels, applying laser irradiation to correct springback in the real bumper parts, ect. 2 TABLE OF CONTENTS 1.2 Springback correction methods………………………………….3 State of the Arts………………………………………………….4 Scope of Study……………………………………………………8 2.1 Coupled thermo-mechanical analysis in LS-DYNA…….2 Model development for laser bending of flat sheet……….3 High strength steel material properties…………………….4 Laser beam heat source…………………….5 Thermal boundary conditions……………….4 Effects of sizes of specimen……………………….5 Effect of eccentric laser scanning lines………………….6 Effect of power and speed…………………. Laser Irradiation Experiment of Flat Plate 3.1 Preparation of Experiment……………….2 Set up model for laser irradiation……………….3 Measurement of deformed shape……………….1 Effect of width of specimens…………………….2 Effect of length of specimens…………….3 Effect of eccentric laser scanning lines………….4 Effect of laser beam speed………….5 Effect of line energy………….
Application to Correct Real Bumper Parts 4.1 Stamping and springback analysis…………. Conclusion and Future Work 5.84 ii LIST OF FIGURES Fig. 1 Various Mechanisms of Laser Bending .42 a) Temperature Gradient Method b) Buckling Method c) Upsetting Method Fig. 2 Finite Element Mesh System for Flat Plate …………………….…43 a) Mesh System of Sheet, Viewed in XY Plane b) Mesh System of Sheet, Viewed in XZ Plane, with Scale 4:1 Compared to Viewing in XY plane Fig.3 Tensile Test at Room Temperature………………….44 a) Specimen b) Stress-Strain Curve Fig.4 Properties of Temperature Dependence………………….…………45 a) Yield Strength, Young Modulus and Poisson’s Ratio b) Heat Capacity, Thermal Conductivity and Thermal Expansion Coefficient Fig.
5 Goldak’s Heat Source Model……………………. 6 Temperature Distribution at time 9.0 sec (800W, 1m/min)………………47 a) Change of Temperature after Laser Scanning b) Temperature Distribution at the Section Cut Across Center Line Fig. 7 Effect of Length………………………. 8 Effect of Thickness…………………….
9 Effects of Scanning Position…………………….49 a) Bending Angle b) Torsion Angle Fig. 10 Effect of Cooling Time…………………………. 11 Effect of Power and Speed…………………………….…………51 iii a) Scanning Speed 1m/min b) Scanning Speed 2m/min Fig. 12 Mean Bending Angle………………………….
13 Model for Measurement Initial Shapes………………….53 a) Calculation Model for Measurement Initial Shape b) Experiment Set-up for Measurement Initial Shapes Fig. 14 Measured Initial Deflection of Blank (200mm x 60mm)……. 15 Laser Irradiation Path…………….55 a) along the Center Line b) along the Eccentric Line Fig. 16 Experiment Set-up for Laser Irradiation………….
17 Measurement of Deformed Shapes after Irradiation………….57 a) Scanning Line to Measure Deformed Shape b) Experiment Set-up for Measurement of Deformed Shapes Fig. 18 SEM Photograph after Laser Irradiation (800 W, 1m/min)….58 a) SEM Micrograph at the Scanning Line b) Microstructure around the Boundary between the Heat Affected Zone and Untransformed Zone Fig. 19 Effect of Specimen Width (Length 300mm)………….59 (a) Measured Deflection after Irradiation (b) Bending Angle Fig. 20 Effect of Specimen Length (Width 60mm)……………….
21 Effect of Eccentricity (300mm x 60mm)…………….61 (a) Bending Angle (b) Torsion Angle Fig. 22 Effect of Scanning Speed……………. 23 Effect of Line Energy…………………. 24 Stamping of Bumper Shape Part……………….…………………64 (a) Part Shape iv (b) Part Shape Parameters (c) Stamping Die Set up Fig.
25 Deformation during Springback (BHF: 20KN)…………….65 (a) X-displacement during Springback (b) Z-displacement during Springback Fig. 26 Model for calculation springback angles…………….66 a) Springback b) Required Bending Angle Fig. 27 Required Bending Angle to Compensate Springback at Various Blank Holding Force………………………………………………………. 28 Irradiation Bumper Part Model………………………….
29 Application of Laser Bending to Compensate Springback (Blank Holding Force 20KN)…………………………………………………….69 a) Finding Scanning Length b) Bending Angle Interpolation Fig. 30 Springback Laser Scanning Patterns…………………….70 a) Scanning Pattern, Speed 1m/min b) Scanning Pattern, Speed 2m/min Fig. 32 Power Time Dependence……………….72 v LIST OF TABLES Table 1 Chemical composition of HSS material………………….………73 Table 2 Preparation of Specimen to Investigate the Effects of Size…….73 Table 3 Process Parameters for Investigating the Effects of Laser Beam Speed, Power and Line Energy….74 Table 4 Specimen Parameters for Investigating the Effects of Eccentric and Diagonal Laser Scanning Line………………….74 Table 5 Parameters for Springback Correction by Simulation…………….75 vi CHAPTER 1 INTRODUCTION 1.1 Problem statement It is known that a holistically designed steel body structure that meets tough structural and crash criteria while weighing less and costing no more than typical vehicles in its class. ULSAB-AVC (Ultra Light Steel Auto Body-Advanced Vehicle Concepts) is the global steel initiative which offers steel solutions to meet society’s demands for safe, affordable, fuel efficient, environmentally responsible vehicles for the 21st century.
Since the wide applications of high strength steels (HSS) are keys to the realization of ULSAB, the use of HSS is rapidly increasing in the automobile industries. Most automotive parts, including bumper part, are produced through the stamping process. As far as the stamping is concerned, HSS makes difficult to develop the stamping process due to its low formability and significant amount of springback compared to conventional steel. Despite the technical difficulties, the shortening of development period and cost reduction are also demanded, since the automobile industries competes with global challengers.
Due to the environment surrounding the automobile industries, a hot press forming process, especially for the HSS, has been introduced. The hot press forming is a stamping process of HSS, where simultaneous stamping 1 and heat treatment are carried out during the stamping by using the heated sheets up to 920 ℃ and controlled cooling at the die. The advantages of hot press forming are such as complex forming, lower press tonnage and lower or near zero springback, etc. However the requirement of an extensive scale of investment can be the main drawbacks of the hot press forming.
Contrarily to the hot press forming, the cold stamping or conventional stamping is well developed and widely used process, except for the springback control of HSS. This study is a motivated combination of cold stamping and posterior shape correction by laser irradiation instead of the hot press forming. Due to the limitation of low formability of the HSS, the major stamping process of HSS parts is forming, where bending is dominant to minimize surface strain, rather than drawing in which significant surface straining is involved. Since the low formability problem can be ignored, remaining technical problem of HSS part stamping is to control springback.
The shape error caused by the springback can be corrected by the laser irradiation.2 Springback correction methods In metal forming, a certain amount of springback is always accompanied in greater or less degree, because of the elastic recovery of the material. Especially in stamping processes, the springback causes shape error compared to the die shape. It is influenced not only by the tensile and yield strength but also by the thickness and bending radius. Due to many advantages considered in advance, high strength steels are 2 used as dominated materials in automotive industries.
Because of larger yield strengths, parts made of HSS demonstrate more springback than parts made of mild steel. Thus, understanding, prediction, control and reduction of springback behavior have become very crucial in terms of decreasing development times. Numerous studies have attempted to determine the controlling factors in springback and find ways to overcome the drawback of using HSS in stamping processes. Gang Liu [1] has proposed a method to use the variable blank holding force to eliminate the springback error in U–shape part.
The wrinkling limit and fracture limit in the forming process has been considered to obtain the blank holding force trajectory. Nielen and his partners [2] developed an optimization method based on the die-optimality criterion for experimental approximations to minimize the difference between the simulation results and the intended design. Using geometric variables such as critical locations where tool elevation should be modified, important tool radii and starting blank geometry, a set of stamping and springback models can be created automatically. After first set of simulations, optimized values for each design variables are predicted.
The next set of simulations is automatically created using these variables. This iterative process continues until every variable are determined within a specified tolerance or until a limiting number of iterative have been completed. Jernberg [3], Dutton [4] and Lingbeek [5] have proposed a method based on the springback compensation tools. This method is divided into two stages.
The first one is to create stamping dies based on the exactly designed shape. The stamping products obtained by using these dies do not 3 completely coincide with the desired shape due to the springback appearance. The second stage is to calculate the shape difference between the springback profile and desired profile. The new stamping dies are repeatedly generated based on the difference.
The desired shapes are created exactly. It is very easy to recognize that new compensation tools must be generated for each kind of parts. Therefore, it took a long time for designing and manufacturing the compensation tools and huge cost was also needed. To overcome the limitation above, laser irradiation method was proposed in this study as a method to compensate the springback behavior.
The laser irradiation is already known as a method to form the sheet metal. Many authors [6-10] have proposed a method to form sheet metals using laser irradiation. They investigated the effects of processing parameters to the bending angles using short specimens by both simulation and experiments. The bending angles were supposed as constant values along the longitudinal direction.
The effects of first scanning and multi scanning to the bending angles were also studied. Bao [11] has investigated the effects of edge to the bending angles. The edge is found to be somewhat curved and the bending angles varied along the laser scanning path. These phenomena are termed edge effects.
Based on the survey of laser forming, it seems to be a promising method to compensate the springback behavior after stamping of sheet metals for high strength steels.3 State of the Arts Comparing to modifying tool geometry method, laser irradiation method had many advantages. There were no tools used in this method, thus, time for designing and manufacturing tools were reduced. The cost was also significantly reduced. Besides, this method was more flexible and simpler than modifying tool geometry method.
Moreover, the size of specimens had a widely changeable range while it was limited by the limitation of size of tools in modifying tool geometry. Other hand, the equipments of laser irradiation method were so simple included laser machines. First and most important thing is to understand completely the laser irradiation process.