Surface Finishing and Corrosion Resistance of 3D Printed Maraging Steel PAPER WITHIN Product development and Materials Engineering AUTHOR: Yinan Shao TUTOR: Donya Ahmadkhaniha JÖNKÖPING May 2020 Postadress: Besöksadress: Telefon: Box 1026 Gjuterigatan 5 036-10 10 00 (vx) 551 11 Jönköping This exam work has been carried out at the School of Engineering in Jönköping in the subject area product development and materials engineering. The work is a part of the two-year Master of Science programme. The authors take full responsibility for opinions, conclusions and findings presented. Examiner: Caterina Zanella Supervisor: Donya Ahmadkhaniha Scope: 30 credits Date: 2020/5/19 Postadress: Besöksadress: Telefon: Box 1026 Gjuterigatan 5 036-10 10 00 (vx) 551 11 Jönköping Abstract Abstract 3D printing, also known as additive manufacturing (AM), has got rapidly developed since 1987. Compared with conventional manufacturing methods, 3D printing provides some advantages such as increasing material utilization and less waste of material. Maraging steel provides good strength and toughness without losing ductility, which has been used for the 3D printing technique. Selective laser melting (SLM) is one of the 3D printing methods, which is mostly used for metal and alloy powder. In this thesis, selective laser melting will be used for maraging steel. 3D printing maraging steel is a new material, the research about the properties of 3D printing maraging steel is still ongoing. Corrosion resistance is one of the most important properties of maraging steel due to the high cost of corrosion. So this thesis will focus on the corrosion behavior of 3D printing maraging steel. The purpose of this thesis was to find the best heat treatment condition for high corrosion resistance and to find the relationship between microstructure and corrosion behavior of maraging steel. In this thesis, several kinds of maraging steel samples with different heat treatment conditions were used. SLM, SLM austenized&quenched, SLM aged, conventional austenized&quenched, and conventional aged. Besides, two kinds of solutions were produced, NaOH (pH=11.5) and Na2SO4 (pH=6.5). To observe the microstructure, an optical microscope was used. The grain size is different between SLM and conventional samples, and also different between the samples with different heat treatment conditions. The potentiodynamic polarization method was used to measuring the corrosion behavior. SLM samples have much lower current density, and the passivation potential and the corrosion rate are similar compared with conventional samples. But due to the lack of further experiments, the relationship between corrosion behavior could be affected by the combined effect of several factors. Keywords Surface finishing, corrosion resistance, selective laser melting, microstructure, maraging steel. 1 Contents Contents 1 Introduction .......................................................................................................... 4 1.1 Background ................................................................................................ 4 1.2 Purpose and research questions ............................................................... 4 1.3 Delimitations .............................................................................................. 4 1.4 Outline ........................................................................................................ 5 2 Theoretical background ........................................................................................ 6 2.1 3D printing ...................................................................................................... 6 2.2 Selective laser melting ................................................................................... 6 2.3 Maraging steel ................................................................................................ 8 2.3.1 Heat treatment ........................................................................................... 8 2.3.1.1 Austenitizing .......................................................................................... 8 2.3.1.2 Quenching ............................................................................................ 9 2.3.1.3 Aging ................................................................................................... 10 2.3.1.4 Tempering ............................................................................................ 11 2.3.2 Martensite transformation ......................................................................... 11 2.3.3 Microstructure ........................................................................................... 11 2.4 Corrosion ...................................................................................................... 12 2.4.1 Corrosion types ....................................................................................... 13 2.4.1.1 General or uniform corrosion .............................................................. 13 2.4.1.2 Pitting .................................................................................................. 13 2.4.1.3 Crevice corrosion ................................................................................ 14 2.4.1.4 Intergranular corrosion ........................................................................ 15 2.4.1.5 Galvanic corrosion .............................................................................. 15 2.4.2 Passivation .............................................................................................. 16 2.4.3 Corrosion test .......................................................................................... 16 2.4.3.1 Potentiodynamic polarization test ....................................................... 16 2.4.4 Corrosion of 3D printed samples ............................................................. 18 2.4.5 Corrosion of maraging steel .................................................................... 18 3 Method and implementation ............................................................................... 20 3.1 Preparation of samples ................................................................................ 20 3.1.1 Chemical composition ............................................................................. 20 3.1.2 Process and post-treatment .................................................................... 20 3.1.3 Schematic diagram of samples ............................................................... 21 3.2 Experiments ................................................................................................. 21 3.2.1 Porosity measurement ............................................................................. 21 3.2.2 Mounting .................................................................................................. 22 3.2.3 Grinding ................................................................................................... 24 3.2.4 Polishing .................................................................................................. 24 3.2.5 Etching ..................................................................................................... 25 3.2.6 Microstructure obseration ........................................................................ 25 3.2.7 Polarization .............................................................................................. 26 2 Contents 4 Findings and analysis ......................................................................................... 28 4.1 Porosity ........................................................................................................ 28 4.2 Metallography after etching .......................................................................... 28 4.2.1 Metallography of SLM samples ............................................................... 28 4.2.2 Metallography of Conventional samples .................................................. 31 4.3 Metallography before polarization ................................................................ 34 4.4 Polarization .................................................................................................. 35 4.4.1 Polarization in Na2SO4 solution (pH=6.5) ................................................ 35 4.4.2 Polarization in NaOH solution (pH=11.5) ................................................. 41 5 Discussion and conclusions ............................................................................... 43 5.1 Discussion of method ................................................................................... 43 5.1.1 Porosity measurement .............................................................................. 43 5.1.2 Heat treatment ......................................................................................... 43 5.1.3 Polarization .............................................................................................. 43 5.2 Discussion of findings .................................................................................. 44 5.2.1 Metallography .......................................................................................... 44 5.2.2 Polarization .............................................................................................. 45 5.3 Conclusion ................................................................................................... 46 5.4 Future work .................................................................................................