This thesis, in the pursue of a systematical approach on the cause and effects in the Selective Laser Melting process, represents the most important tests and experiments performed up to date with a focus on process and quality optimization.
Laser Beam Melting or also known with its more frequently used reference as Selective Laser Melting is a relatively new and promising 3d manufacturing technology capable of further improving the current mass production methods. Due to being new and not completely explored as a manufacturing method, it required a lot of work to overcome problems such as high process-related build-up rates while maintaining high product quality through high relative density and minimization of defects and irregularities within the structure of the fabricated parts. Solving such a major problem qualifies this new technology for adaptation into series production with exceptional product quality due to the nature of SLM. The work done so far is adequate and indicates that SLM has evolved through years of hard work from a number of researchers and scientists. Information on the studies and experiments conducted so far are vastly accessible, but a compilation of these for an overall and complete insight of the process was not available.
Table of Contents
1. Introduction
1.1 Motivation
1.2 Objective
1.3 State of the Art
1.4 Structure of the Thesis
2. Theoretical Preface
2.1 Laser Additive Manufacturing and its role in the Industry
2.1.1 Introduction
2.1.2 Laser as a machine tool
2.1.3 Nature of laser processing in AM
2.2 Laser Beam Melting
2.2.1 Background
2.2.2 Insides of SLM
2.2.3 Working Principle
2.2.4 Applications
3. Processing Parameters of LBM
3.1 Overview
3.1.1 Environment and Atmosphere related
3.1.2 Powder Composition
3.1.3 Powder granulometry and deposition
3.1.4 Powder-bed traits
3.1.5 Laser Characteristics
3.1.6 Building - Processing Parameters
3.1.7 Constants
3.2 Classification of Parameters
3.2.1 The Fishbone Diagram
3.2.2 Overall Classification
4. Parametric Analysis
4.1 Parameter Screening
4.2 Laser scanning speed
4.2.1 Single track formation of SS grade 904L powder
4.2.2 Instability of the molten pool at low scanning velocity
4.2.3 Setting of scanning parameters for optimal results for different metal powders
4.2.4 Effects of laser power and scanning speed on material’s structure
4.2.5 Porosity related to scanning speed in TiAl6V4
4.2.6 Productivity improvement of SLM process for aluminum alloys through scanning speed alteration
4.3 Laser Power
4.3.1 Influence of laser power on density and microstructure
4.3.2 Porosity related to laser power in TiAl6V4
4.3.3 Mechanical Properties
4.4 Scan spacing
4.4.1 Effects of hatch distance on surface morphology
4.4.2 Effects on designed internal structure
4.4.3 Porosity related effects in TiAl6V4 powder
4.4.4 Productivity increase through hatch distance variation
4.5 Scanning strategy
4.5.1 Parallel Scanning
4.5.2 Spiral Scanning
4.5.3 Paintbrush Scanning
4.5.4 Chessboard Scanning
4.6 Building strategy
4.6.1 Hatch angle
4.6.2 Building direction
4.6.3 Overhanging Structures
4.7 Layer thickness
4.7.1 Influence of layer thickness on single track formation
4.7.2 Effects of layer thicknesses on microstructure and performance
5. Conclusions
Objective and Research Focus
The primary objective of this thesis is to provide an analytical overview of the relevant process characteristics of the Laser Beam Melting (LBM) / Selective Laser Melting (SLM) method, with a specific focus on identifying and categorizing influential process parameters that determine the final quality of additively manufactured parts.
- Systematic classification of over 50 process variables affecting SLM quality.
- Use of cause-and-effect diagrams (Fishbone) to visualize process dependencies.
- Screening of the most critical parameters: scanning speed, laser power, hatch distance, scanning strategy, building strategy, and layer thickness.
- Analysis of material-specific responses (e.g., SS 904L, TiAl6V4, AlSi10Mg) to varying process inputs to optimize density and microstructure.
- Evaluation of productivity improvements through the optimization of scanning parameters.
Extract from the Book
4.2.2 Instability of the molten pool at low scanning velocity
For the investigation of the instability of the melt pool at low scanning speeds, the authors [3] used stainless steel grade 316L powder. The sintered tracks were formed under a laser power with a range from 12.5 up to 50W at varied scanning speeds from 0.02 up to 0.22m/s with steps of 0.04m/s in between for a layer thickness of 50μm.
It was noted in their study that under an increased energy input per unit length (P/V) while processing at a rather high laser power and low scanning speed, the melt volume increased, whereas the melt viscosity decreased. In such case, the melt hydrodynamics driven by the Marangoni effect [61 – 64], play an important role and the sintered tracks attain an irregular form.
At low values of laser power and for low scanning speeds, the energy provided is not sufficient enough to melt the substrate, thus causing the stabilizing effect of the contact zone (infiltration into substrate) to vanish. When the energy is adequate to keep steady the boiling and evaporation of the molten powder, the vapor recoil pressure results into distorted and irregular sintered tracks as also seen in Figure 12 (left) [3]. Finally, further reducing the laser power will cause drops formation within the sintered tracks (Figure 12, right).
Closing, the investigation of the authors [3] regarding single track formation of SS grade 904L powder as well the instability of SS grade 316L’s molten pool showed that the process is characterized by a threshold with stability zones of continuous tracks and instability zones. The latter appeared at low scanning speeds as distortions and irregularities, while with increased scanning speeds balling effect was present.
Summary of Chapters
1. Introduction: Presents the motivation and objective of the thesis, focusing on the need for a systematized overview of LBM/SLM process parameters to achieve quality optimization.
2. Theoretical Preface: Defines the role of Laser Additive Manufacturing in industry and explains the fundamental working principles and applications of Selective Laser Melting (SLM).
3. Processing Parameters of LBM: Provides a comprehensive, categorized inventory of over 100 parameters, distinguishing between environmental, material, and controllable processing settings, and introduces classification via the Fishbone diagram.
4. Parametric Analysis: Conducts a detailed investigation into the six most critical parameters identified during screening (laser speed, power, spacing, strategy, building strategy, and layer thickness) and their impact on part quality.
5. Conclusions: Synthesizes the experimental findings, confirming that precise control of key parameters is essential for high-density production and that further research into powder characteristics and overhanging structure optimization is required.
Keywords
Selective Laser Melting, Laser Beam Melting, Additive Manufacturing, Process Parameters, Quality Optimization, Parameter Screening, Molten Pool, Scanning Speed, Laser Power, Porosity, Microstructure, Hatch Distance, Building Strategy, Fishbone Diagram, Series Production.
Frequently Asked Questions
What is the core focus of this bachelor thesis?
The thesis focuses on investigating the cause-and-effect relationships between various process parameters and the final quality of components produced via the Selective Laser Melting (SLM) process.
Which key parameters are identified as most influential in the SLM process?
After an extensive screening process, the author identifies six dominant factors: laser scanning speed, laser power, scan spacing (hatch distance), scanning strategy, building strategy, and layer thickness.
What is the primary objective regarding process optimization?
The goal is to provide a complete understanding of how these parameters influence process stability, part density, and surface integrity to enable high-quality, repeatable manufacturing.
Which scientific methods are utilized to structure the analysis?
The author performs an extensive literature review and uses quality management tools, specifically the Ishikawa or "Fishbone" diagram, to classify the vast number of SLM variables based on their impact.
What does the main body of the work cover?
The main body performs a detailed parametric analysis of the six identified factors, investigating their specific effects on material properties, surface morphology, and the formation of defects like porosity, cracking, or the "balling" phenomenon.
How can this work be characterized by its keywords?
The work is characterized by terms related to additive engineering, such as Selective Laser Melting, process parameter optimization, molten pool dynamics, and metallurgical quality metrics like density and microhardness.
How does the author define the "Balling" phenomenon?
The thesis describes balling as the formation of discrete spheres of molten metal rather than continuous tracks, caused by excessive energy or surface tension gradients, which significantly degrades component quality.
What role does the "Fishbone" diagram play in the research?
The diagram serves as a pedagogical and analytical tool to categorize the complex network of over 100 process variables into logical groups such as environment, powder composition, and laser characteristics for clearer decision-making.
Does the thesis provide specific recommendations for different materials?
Yes, the thesis analyzes experimental data for various materials including stainless steels (SS 304L, 316L, 904L), TiAl6V4, and AlSi10Mg, providing insights into optimal laser power and scanning speed ranges for each.
- Arbeit zitieren
- Stelios Kokorotsikos (Autor:in), 2017, Investigation of Cause and Effects in the Laser Beam Melting (LBM) Process, München, GRIN Verlag, https://www.grin.com/document/1418735
