Gravity and Centrifugal Casting of Light Metal Alloys Using Rapidly Produced Sand Moulds Nicholas McKenna A thesis submitted to Auckland University of Technology In fulfilment of the requirements for the degree of Masters of Engineering July 2010 School of Engineering Primary Supervisor: Dr. Sarat Singamneni ABSTRACT Traditional sand casting is a well understood method to produce metal shapes and has been used for many years in industry. It is a relatively simple method but has a significant drawback, with the requirement of a pattern to form the internal cavity. Patterns are produced at high cost through Computer Numerical Controlled machining or wood pattern making with significantly high lead times. Rapid Prototyping is seen as a solution to this problem, with the ability to produce sand moulds directly from Computer Aided Design platforms and thus eliminate the requirement of a pattern. Through layered manufacturing, the sand mould can be produced with complex internal geometry directly, minimising labour costs and involving short waiting times. While initial research was mainly concerned with the use of Selective Laser Sintering, with the advent of 3D printing, pattern-less sand moulds can be produced more easily and cheaply. With the process gaining more and more popularity, there was a need to scientifically assess the suitability of the process for sand casting as well as, establish influences of typical process parameters on significant responses. Critical mould properties, such as permeability and compressive strength, were investigated with respect to varying time and temperature of baking. To this end, mathematical models of permeability and compressive strength were developed. Also, the influence of mould material, mould coating, alloy type and pouring temperatures were investigated in static sand casting of light metals. Further work utilised the centrifugal casting process using these 3D printed moulds to establish links between process factors, such as rotational speed and cast strength using light metals. Compressive strength results for the rapidly produced materials were acceptable compared to traditional values. Permeability was however lower i than commonly used foundry sand. Results showed, nevertheless, that permeability and compressive strength were both improved by baking times and temperatures. Significant model effects were established for ZP131 and ZCast501 with respect to increased compressive strength and mould permeability. Multi-factorial experiments involving simultaneous variation of factors such as mould materials, surface coatings, alloys and pouring temperatures were conducted and static casting results in general show good as-cast mechanical properties with the factors having significant effects on surface roughness, percent elongation and hardness. Centrifugal casting of aluminium alloys initially produced below average tensile properties, due to the large presence of hydrogen porosity. However, upon degassing, much improved tensile strengths were obtained, being superior to both static casting and traditionally sand cast aluminium. Also a Magnesium alloy was successfully trialled with the centrifugal process using 3D printed moulds in spite of numerous practical difficulties. Substantial data relating to the process factors for mould materials and casting processes was produced. Analysis of factor influences facilitated optimum process configurations for the production of moulds and castings. These combinations of factors at optimum levels comprehensively showed that light metals such as aluminium and magnesium alloys could be successfully processed by rapidly produced moulds, both statically and centrifugally. ii ACKNOWLEDGEMENTS I would like to express my profound thanks to my primary supervisor, Dr Sarat Singamneni, senior lecturer, for his invaluable help and guidance through every stage of this project. I would also like to thank my secondary supervisor, Prof. Darius Singh, for his overall input and commitment in helping organise the logistics for undertaking this work. I also wish to note special thanks to the local supporting company, Centracast, for kindly allowing generous use of their casting facilities and personnel. This project was not possible without funding from the Foundation for Research, Science and Technology, which kindly granted a scholarship to the author (contract number CCWE0901), and allowed this work to be undertaken. Finally, I would also like to sincerely thank my partner Amie, mother Susan and father Colin for their continuous support, love, help, understanding and advice during the duration of this project. iii TABLE OF CONTENTS ABSTRACT .................................................................................................... i ACKNOWLEDGEMENTS ............................................................................. iii TABLE OF CONTENTS ................................................................................ iv AUTHORSHIP ............................................................................................ viii LIST OF TABLES ......................................................................................... ix LIST OF FIGURES ........................................................................................ xi ABBRIEVIATIONS ..................................................................................... xvii NOMENCLATURE .................................................................................... xviii CHAPTER 1 INTRODUCTION...................................................................... 1 1.1 Rapid Prototyping...................................................................... 1 1.1.1 Evolution of RP towards Rapid Manufacturing .............. 6 1.2 Rapid Prototyping and Casting .................................................. 8 1.2.1 Rapid Casting ............................................................... 9 1.3 Literature Review .................................................................... 11 1.4 Research Questions, Hypotheses and Project Objectives ....... 35 1.5 Methodology ........................................................................... 37 CHAPTER 2 CHARACTERISTICS OF 3D PRINTED MOULDS ................. 39 2.1 Mould Materials ....................................................................... 39 2.1.1 Basic Ingredients of the Mould Materials ..................... 39 2.2 Experimental Design for Mould Material Analysis .................... 42 2.2.1 ANOVA Calculations ................................................... 46 iv 2.3 Experimental Methodology for Testing Mould Materials .......... 50 2.3.1 Mould Compressive Strength (MCS) ........................... 51 2.3.2 Mould Permeability (MP) ............................................. 52 2.4 ZP131 Material Results and Discussion .................................. 55 2.4.1 MCS Model ................................................................. 55 2.4.2 MP Model .................................................................... 59 2.5 ZCast501 ................................................................................ 62 2.5.1 ZCast MCS ................................................................. 62 2.5.2 ZCast MP .................................................................... 66 2.5.3 A Comparison Between the Two Materials .................. 68 CHAPTER 3 CHARACTERISTICS OF STATIC CASTINGS PRODUCED IN 3D PRINTED MOULDS................................................................................ 71 3.1 Casting in Printed Moulds ....................................................... 71 3.2 Experimental Plan ................................................................... 73 3.2.1 Taguchi Response and ANOVA .................................. 76 3.2.2 Pooling of Factors ....................................................... 77 3.3 Mould Design .......................................................................... 77 3.4 Static Casting Experiments ..................................................... 79 3.4.1 Mechanical Testing of Castings ................................... 80 3.5 Static Casting Results and Discussions .................................. 83 3.5.1 Surface Roughness ..................................................... 83 3.5.2 Ultimate Tensile Strength (UTS) .................................. 89 3.5.3 Percent Elongation ...................................................... 94 3.5.4 Brinell Hardness .......................................................... 97 3.6 Metallographic Analysis......................................................... 100 3.6.1 ASTM Grain Number ................................................. 100 3.6.2 Magnesium alloy: AZ91 ............................................. 101 3.6.3 Magnesium alloy: AM-SC1 ........................................ 109 3.6.4 Aluminium: A356 ....................................................... 111 3.7 Summary .............................................................................. 121 v CHAPTER 4 CENTRIFGUAL CASTING IN RP MOULDS ........................ 122 4.1 Centrifugal Casting ................................................................ 122 4.2 Background ........................................................................... 123 4.3 Analytical Model .................................................................... 125 4.4 Results from the Analytical Model ......................................... 135 4.5 Centrifugal Casting Experimental Design and Setup ............. 137 4.5.1 Mould Design ............................................................ 140 4.5.2 Setup and Methodology ............................................ 140 4.5.3 Experimental Procedure ............................................ 142 4.6 Centrifugal Casting Results ................................................... 143 4.6.1 Surface Roughness ................................................... 145 4.6.2 Yield Strength ........................................................... 153 4.6.3 Ultimate Tensile Strength (UTS) ................................ 158 4.6.4 Percent Elongation .................................................... 164 4.7 Macro and Micro Structural Examination ............................... 167 4.7.1 Macrostructures ........................................................ 167 4.7.2 Microstructures.......................................................... 169 4.7.3 Fractography of Centrifugal Castings ........................ 175 4.8 Further Centrifugal Casting ................................................... 178 4.8.1 An Improvised Centrifugal Casting Setup .................. 178 4.8.2 Centrifugal Casting Trials with the New Setup ........... 179 4.8.3 Results of the Modified Centrifugal Casting Trials ..... 180 4.9 Summary of Centrifugal Casting Trials .................................. 186 CHAPTER 5 CONCLUSIONS .................................................................. 188 REFERENCES ........................................................................................... 191 APPENDIX A MOULD MATERIAL ........................................................... A-1 A.1 Experimental Design .............................................................. A-1 A.2 Permeability and Compressive Stress Calculations................ A-4 A.3 Raw Experimental Results From Material Testing .................. A-5 vi A.3.1 Permeability Data ....................................................... A-6 A.3.2 Compressive Strength Data ....................................... A-8 APPENDIX B STATIC CASTING INFORMATION ................................... B-1 B.1 Design of Experiments and ANOVA ....................................... B-1 B.1.1 Sum of squares .......................................................... B-2 B.2 Mechanical testing data.......................................................... B-4 B.2.1 ASTM grain size ......................................................... B-6 B.3 Microstructure Evaluation ....................................................... B-9 B.4 Castings Produced in ZP131 Moulds ................................... B-12 B.4.1 Castings Produced in ZCast Moulds ........................ B-15 B.5 Tensile Part Setup................................................................ B-18 B.6 Static Mould Design ............................................................. B-18 B.7 Alloy Constitutes .................................................................. B-19 APPENDIX C CENTRIFUGAL CASTING DATA ...................................... C-1 C.1 Experimental Design ............................................................. C-1 C.2 Mechanical Testing Data ....................................................... C-3 C.3 Etchants Used for Cast Alloys ............................................... C-5 C.4 As-Cast Macrostructures ....................................................... C-7 C.5 SEM Photographs ............................................................... C-10 C.6 Die and Mould Design Drawings ......................................... C-13 C.7 Mould Design ...................................................................... C-17 C.8 Tensile testing part .............................................................. C-18 C.9 Publications......................................................................... C-19 vii AUTHORSHIP “I hereby declare that this submission is my own work and that, to the best of my knowledge and belief, it contains no material previously published or written by another person (except where explicitly defined in the acknowledgments), nor material which to a substantial extent has been submitted for the award of any other degree or diploma of a university or other institution of higher learning” Signed: Nicholas McKenna Date: 30th July 2010 viii LIST OF TABLES Table 2.1 The ZB60 binder ingredients for the ZP131 material [49]. ............. 42 Table 2.2 ZB58 binder composition for the ZCast501 material [50]. ............. 42 Table 2.3 The Factorial part of CCD ............................................................. 44 Table 2.4 Experimental design table showing (a) the x matrix of the time and temperatures as coded variables and (b) showing the design table with the natural and coded variable combinations ....................... 45 Table 2.5 ANOVA of compressive strength model of ZP131 ........................ 56 Table 2.6 ANOVA of permeability model of ZP131 ...................................... 59 Table 2.7 ANOVA table of the MCS model ................................................... 63 Table 2.8 The ANOVA table of the permeability model ................................. 66 Table 3.1 Taguchi L9 experimental design table with natural variables ........ 75 Table 3.2 Factor and level combinations for static casting trials ................... 75 Table 3.3 ANOVA table of the surface roughness response ......................... 84 Table 3.4 Static L9 experimental design table .............................................. 88 Table 3.5 Overall ranking of factors and their level from static casting trials . 88 Table 3.6 ANOVA of UTS response ............................................................. 89 Table 3.7 ANOVA table for percent elongation response model ................... 95 Table 3.8 ANOVA of Brinell hardness response ........................................... 97 Table 4.1 Values of G developed at different speeds with respect to different radii ........................................................................................... 137 Table 4.2 Summarised factor and level combinations for the centrifugal casting trials .............................................................................. 139 Table 4.3 Centrifugal casting experimental design table ............................. 139 Table 4.4 Overall results of mechanical testing of centrifugal test bars ....... 144 Table 4.5 Contribution and significance of individual terms for the surface response model ......................................................................... 152 Table 4.6 ANOVA table showing linear, square and interaction model effects .................................................................................................. 152 Table 4.7 Breakdown of model parameters in the yield strength model ...... 157 Table 4.8 ANOVA table of the yield strength model response .................... 157 Table 4.9 Results of T-Tests conducted between each speed range. ......... 158 ix