Direct and Indirect Laser Sintering of Metals
Total Page：16
File Type：pdf, Size：1020Kb
Direct and Indirect Laser Sintering of Metals By Montasser Marasy A. Dewidar A Thesis submitted in accordance with the requirements for the degree of Doctor of Philosophy The University of Leeds School of Mechanical Engineering Leeds UK May 2002 The candidate confirms that the work submitted is his own and that appropriate credit has been given where reference has been made to the work of others. Abstract 11 ABSTRACT Manufacturing functional prototypes and tools using conventional methods usually is a time consuming procedure with multiple steps. The pressure to get products to market faster has resulted in the creation of several Rapid Prototyping (RP) techniques. However, potentially one of the most important areas of Rapid Manufacturing (RM) technology lies in the field of Rapid Tooling (RT). Layer manufacture technologies are gaining increasing attention in the manufacturing sector for the production of polymer mould tooling. Layer manufacture techniques can be used in this potential manufacturing area to produce tooling either indirectly or directly, and powder metal based layer manufacture systems are considered an effective way of producing rapid tooling. Selective Laser Sintering (SLS) is one of available layer manufacture technologies. SLS is a sintering process in which shaped parts are built up layer by layer from bottom to top of powder material. A laser beam scans the powder layer, filling in the outline of each layers CAD-image, and heats the selected powder to fuse it. This work reports the results of an experimental study examining the potential of layer manufacturing processes to deliver production metal tooling for manufacture of polymer components. Characterisation of indirect selective laser sintering and direct selective laser sintering to provide the metal tooling is reported. Three main areas were addressed during the study: mechanical strength, accuracy, and build rate. Overviews of the results from the studies are presented. Two materials (RapidSteel 2.0 and special grade of high speed steel) and also two generations of SLS machines (Sinterstation 2000 and sinterstation research machine, which was constructed in Leeds) were used during this work. A cknowledgements iii A CKNO WLEDGMENTS First of all I wish to express my deepest gratitude to Dr. K. W. Dalgarno, lor his valuable guidance, supervision, advice, encouragement, and understanding throughout the research period and preparation of the thesis. I would also like to thank Professor T. H. Childs, for his help through the duration of research. I would like also to express my thanks to Egyptian Government for the sponsorship of this research and financial support. I would like to thank the members of the Manufacturing, Strength of Materials and Measurement Laboratories for their support, In particular, Mr. Phillip Wood, Mr. Abbas Ismail, Mr. Tony Wiese and Mr. Alan G. Heald. I would like to thank Dr. Todd Stewart and Mr. M. Haiba. I would like to thank my friends that I have met at University of Leeds. In addition, I would like to thank my friends outside the Leeds community for their continued support throughout the years. I would like to give special thanks to my parents, my brothers, and my sisters for their valuable emotional support. Last, but definitely not least, I would like to give very special thanks to my wife Asmaa and my lovely kids, Moamen and Mona for their work related help, understanding, patience, and emotional and endless support. Montasser M. Dewidar Leeds, May 2002 Table of Contents IV Table of Contents ABSTRACT ii ACKNOWLEDGEMENTS iii TABLE OF CONTENTS iv LIST OF FIGURES xi LIST OF TABLES xix LIST OF NOTATIONS xxi CHAPTER ONE INTROUDUCTION 1.1 Rapid Manufacturing and Tooling 1 1.2 Aims and Objectives of the Thesis 4 1.3 Thesis out Line 5 CHAPTER TWO LITERATURE REVIEW 2.1 Introduction 6 2.2 Rapid Prototyping and Tooling 9 2.2.1 Rapid Prototyping Techniques 9 2.2.1.1 Stereolithography (SLA) 9 2.2.1.2 Fused Deposition Modeling (FDM) 11 2.2.1.3 Solid Ground Curing (SGC) 12 2.2.1.4 Laminated Object Manufacturing (LOM) 13 2.2.2 Rapid Tooling 14 2.2.2.1 Direct tooling 15 2.2.2.2 Direct AIM Tooling 17 2.2.23 Ceramic powder tools 17 2.2.2.4 Laminated metal sheets 18 2.2.2.5 Accuracy and Surface Finish of Rapid Prototyping Techniques 18 2.2.2.6 Summary 20 2.3 Selective Laser Sintering 21 Table of Contents v 2.3.1 Historical Development 23 2.3.2 Indirect SLS of Metals 26 2.3.2.1 Introduction 26 2.3.2.2 The DTM RapidTool Process 28 2.3.3 Direct SLS of Metals 34 2.3.4 Accuracy of SLS parts 37 2.3.5 Process Parameters in SLS 43 2.3.5.1 Spot Diameter 43 2.3.5.2 Laser Power and Scan Speed 44 2.3.5.3 Scan Spacing 47 2.3.5.4 Layer Thickness 48 2.3.6 Atmosphere Control 49 2.3.7 Material Properties 50 2.3.7.1 Powder characteristics 50 23.1.2 Thermal properties of the powder bed 54 2.4 High Speed Steels 57 2.4.1 High Speed Steels History 57 2.4.1.1 Common High Speed Steel Materials 57 2.4.2 Powder Metallurgy of High Speed Steel 60 2.4.3 Powder Atomisation 61 2.4.3.1 Gas Atomisation 61 2.4.3.2 Water Atomisation 62 2.4.4 Sintering Mechanisms 63 2.4.4.1 Solid State Sintering 64 2.4.4.2 Liquid Phase Sintering 66 2.4.4.3 Supersolidus Liquid Phase Sintering 68 2.4.4.4 Infiltration 70 2.4.5 Mechanical Properties of Sintered High Speed Steel 72 2.4.5.1 Hardness 72 2.4.5.2 Bending strength 73 2.5 Summary 76 CHAPTER THREE COMPARISON CRITERIA FOR INDIRECT AND DIRECT SLS OF METALS 3.1 Introduction 77 3.2 Identification of Key Parameters 77 3.2.1 Mechanical Properties and Microstructure 77 Table of Contents vi 3.2.2 Accuracy and Surface Finishing 3.2.3 Lead Time, Productivity and Cost 79 3.3 Summary CHAPTER FOUR INDIRECT SELECTIVE LASER SINTERING, EXPERIMENTAL PROCEDURES 4.1 Introduction 85 4.2 Powder 86 4.2.1 Storage 86 4.2.2 Powder Apparent Density 86 4.3 Manufacture of Test Pieces 87 4.3.1 Sinterstation Processing 87 4.3.2 Furnace Cycle Procedures 89 4.4 Mechanical Properties Measurement 93 4.4.1 Tensile Test 94 4.4.2 Hardness 95 4.4.3 Density of Green Parts 96 4.4.4 Density of sintered parts 97 4.5 Accuracy and Surface Finish 97 4.5.1 Equipment 100 4.5.1.1 Coordinate Measuring Machine (CMM) 100 4.5.1.2 Surface Profilometer Form Talysurf 120L 101 4.5.1.3 Pycnometer 102 4.5.1.4 Metallography 103 4.5.1.4.1 Sample preparation 103 4.5.1.4.2 Scanning Electron Microscopy 104 4.5.2 Test Piece Geometries 105 4.5.2.1 Small feature 105 4.5.2.4 Bulk accuracy. 107 4.5.2.3 Internal channels 107 4.6 Summary 108 Table of Contents vii CHAPTER FIVE RESULTS AND DISCUSSIONS OF INDIRECT SLS 5.1. Introduction 109 5.2 Tensile Strength of RapidSteel 2.0 Material 109 5.2.1 Influence of Infiltration on the Strength and Ductility of RapidSteel 2.0 109 5.2.2 Influence of Type of Furnace on the Strength of RapidSteel 2.0 113 5.2.3 Strength of RapidSteel 2.0 Material 114 5.3 Density Study of RapidSteel 2.0 118 5.3.1. Powder Bed Density 118 5.3.2. Density of Green Parts, Brown Parts and Infiltrated Parts 118 5.3.3 Density of Green Parts at Different Laser Powers 120 5.3.3 Density of Infiltrated Parts 122 5.4 Hardness of RapidSteel 2.0 122 5.5 Accuracy of RapidSteel 2.0 Material 124 5.5.1 Small Features Accuracy 134 5.5.2 Bulk Accuracy and Surface Roughness 144 5.6 Internal channels 149 5. 7 Build Rate 150 5.8 Microstructure of RapidSteel 2.0 153 5.9 Summary 157 CHAPTER SIX DIRECT SELECTIVE LASER SINTERING, EXPERIMENTAL PROCEDURES 6.1 Introduction 159 6.2 Experimental Powder 159 6.2.1 Storage and Use of the powder 164 Table of Contents viii 6.2.2 Preparation and Handling 165 6.3 Experimental Equipments 165 6.3.1 Selective Laser Sintering Equipment 165 6.3.2 Laser and Focussing Optics 166 6.3.4 Process Chamber and Powder Handling Equipment 171 6.4 Software and the Scanning Procedure 175 6.4.1 Laser Power Calibration 176 6.5 Experimental Procedure 178 6.5.1 Scanning Conditions of Single Lines 179 6.5.2 Scanning Conditions of Single Layer and Multiple Layer Parts 180 6.5.3 Environmental Conditions 182 6.5.4 Dimensional Measurement 183 6.5.5 Mass Measurement and Density Calculations 184 6.5.6 Post-Processing 184 6.5.6.1 Vacuum Sintering Infiltration Cycle 184 6.5.6.2 Infiltration 184 6.5.7 Mechanical Testing 185 6.5.7.1 Bending Test 185 6.5.7.2 Tensile Test and Hardness 187 6.5.8 Sample Preparation for Microscopy Inspection 187 6.5.8.1 Single Layer and Multiple Layer Samples 187 6.6 Summary 188 CHAPTER SEVEN RESULTS AND DISCUSSION OF DIRECT SLS 7.1. Introduction 189 7.2 Single Line Melt Tests Using Different Scan Speed and Laser Power 189 7.2.1 The Effects of Scan Speed and Laser Power 189 7.2.2 Warping of single lines 200 7.2.3 Accuracy of Single Line at Constant Scan Speed, and Laser Power 201 Table of Contents 1X 7.3 Building Monolayers Using Different Scan Speed, Scan Spacing, and Laser Power 204 7.3.1 Defects of Monolayer Samples 210 7.3.2 Strategy of Building 213 7.3.2.1 One layer 213 7.4.3 Post-processing of Monolayers of HSS 216 7.5 Building Multiple Layer Using Different Laser Power, Scan Speed, Scan Spacing, and Layer Thickness 217 7.6 Density Study with HSS 223 7.6.3 Powder Bed Density 223 7.6.4 Density of Monolayer Samples Sintered by Direct SLS Process 224 7.5.2.1 Density of Monolayer Samples Sintered by Direct SLS Process before Infiltration 224 7.5.2.2 Density of Monolayer Samples Sintered by Direct SLS Process after Infiltration 225 7.6.5 Density of Multiple Layer Samples Sintered by Direct SLS Process 226 7.6 Mechanical Properties of HSS 229 7.6.1 Mechanical Properties of Monolayer 229 7.6.1.1 Effect of Scan Spacing on Bending Strength 229 7.6.2.2 Effect of Scan Direction