Material extrusion additive manufacturing of metal parts: analysis of the performance and behaviour of parts according to process parameters

The Additive Manufacturing (AM) technology is one of the enabling technologies of the Industry 4.0. According to standard ASTM/ISO 52900, Additive Manufacturing compared to the conventional manufacturing (CM) based on subtractive (machining) or formative (casting) methods, enables to realize parts layer by layer starting from 3D model data and usin g different kinds of materials as metals, polymers or ceramics. There are seven different technologies that belong to the AM class, but one of the main commons is the Material Extrusion Additive Manufacturing (MEX). MEX technology born to process polymeric materials in form of filament or pellets. In the last years, however, its material portfolio expanded considering other materials as ceramics and metals. When metal alloys are processed with MEX it takes the name of Metal MEX . The growing interest in the Metal MEX field thanks to the simple processing equipment, the low investment costs and the low hazards for the worker and environment allowed to consider this ne w technique a potential alternative to the main Metal AM technologies as the Powder Bed Fusion or Direct Energy Deposition. In fact, these well known techniques require the use of laser or electron beam, the exposure at metal powder and high cost of investment and maintenance. Metal MEX technology is a multistep process composed by three stages , the printing, the debinding and the sintering. After each phase, the part is in a different condition: once printed the part is called “green part”, where the metal powder is dispersed in the polymeric matrix deposited during the printing stage. O nce debound , the part is called “brown part and a remaining part of the polymeric matrix is present to give strength to the part before the sintering. In this phase all polymers are removed, and the metal particles start to grow creating necking between the par ticle and the part become denser due to the fill of the voids. After sintering, the result is a “metal part” or “white part”. In each stage , the process parameters defined play a fundamental role to modify the properties of the different parts. The entire process is referred as Printing Debinding Sintering (PDS). In this context, this thesis has the aim to provide a deep study about th e performance and the behavior of the parts realized with this emerging technology. T he response of the parts in the green and sintered condition will be analysed considering the influence of the main process parameters those act during the entire PDS chain. The materials investigated are two different stainless steel: t he martensitic precipitation hardened stainless steel 17 4 PH and the austenitic stainless steel 316L . An analysis of the response of the parts realized with these two materials cover a major part of the current work of thesis. However, it is reported a preliminary study about a novel material processed trough MEX as the pure copper. In this way, a strong optimization of the printing parameters to densify the part is the main target reported in this work. In detail the thesis is structured in the following way: a brief introduction about the Additive Manufacturing and its technologies with a focus on the Metal AM technologies. I n the second chapter , the focus is on the MEX Debinding Sintering chain . The different methods to process the feedstock and the different materials processable . A focus on the debinding and sintering about the different methods, the main parameters and the effects on the part are reported. In the third chapter is a review of the state of art about 316L, 17 4 PH and copper processed via MEX. The main properties are explained and investigated are reported . In the fourth chapter the research question of the current work f thesis is reported. From the fifth chapter to the eighth chapter the experiments performed on the 316L and 17 4 PH . In the ninth chapter, a preliminary study about the feasibility to print and sinter copper parts performed at Politecnico di Bari, instead in the tenth chapter the activities performed in collaboration with the AML group of the KU Leuven during the abroad period. The last chapter repo rts the general conclusions of the c urrent work and the possible future activities.