Visual Inspection of Laser Beam Melting Processes

See our publications and the published slides for conference talks.

Laser Beam Melting (LBM) systems create solid bodies layer-wise from metal powder (“3D printing”). The original 3D CAD drawing of the part is sliced into layers which define the part geometry in two dimensions. The build process is iterative: first, a thin layer of metal powder, 20 μm to 100 μm in height, is deposited on the build platform. Next, a laser follows the part geometry in the current slice and melts powder into solid material. The build platform is lowered and all steps are repeated to produce the part (Fig. 1). Laser Beam Melting is also known as (vendor-specific): Laser Forming, Selective Laser Melting (SLM), Laser-Cusing, Electron Beam Melting (EBM) and Direct Metal Laser Sintering (DMLS) (c. f. VDI standard 3404).

Laser Beam Melting Process
Process cycles of Laser Beam Melting. 1) Powder deposition, 2) laser exposure, 3) lowering of build platform

LBM systems are suitable for prototyping and small volume production and are utilized in a variety of fields, such as automotive or tooling. Today, despite its advantageous properties, LBM systems are rarely used in areas with high security requirements (aerospace, medical applications) due to the lack of quality control for parts built with LBM. Nondestructive testing is limited to external measurements and computer tomography, which is expensive, time consuming, and unable to examine parts thicker than 20 mm.

To enable widespread application of Laser Beam Melting processes in high security areas like aerospace or medical technologies, extensive quality control and process documentation is necessary.

Overview of Research Topics

Partners

Prof. Gerd Witt and Dipl.-Ing. Stefan Kleszczynski
Institute of Product Engineering at University of Duisburg-Essen

Contact

Joschka zur Jacobsmühlen, Tel.: +49 (241) 80-27974

Funding

Federal Ministry for Economic Affairs and Energy (BMWi) via the German Federation of Industrial Research Associations (AiF), IGF 17042 N, 2011-2013

Publications

See also research topics for an overview of selected publications.

2019

Joschka zur Jacobsmühlen
Image-based methods for inspection of laser beam melting processes

2018

Joschka zur Jacobsmühlen, Jan Achterhold, Stefan Kleszczynski, Gerd Witt and Dorit Merhof
In-situ Measurement of Part Geometries in Layer Images from Laser Beam Melting Processes
In: Progress in Additive Manufacturing

2017

Joschka zur Jacobsmühlen, Stefan Kleszczynski, Gerd Witt and Dorit Merhof
Compound Quality Assessment in Laser Beam Melting Processes Using Layer Images
In: IEEE International Instrumentation and Measurement Technology Conference (I2MTC)

2016

Joschka zur Jacobsmühlen, Stefan Kleszczynski, Alexander Ladewig, Gerd Witt, Dorit Merhof
In-Situ Surface Roughness Measurement of Laser Beam Melted Parts – a Feasibility Study of Layer Image Analysis
In: Fraunhofer Direct Digital Manufacturing Conference (DDMC)

2016

Joschka zur Jacobsmühlen, Jan Achterhold, Stefan Kleszczynski, Gerd Witt and Dorit Merhof
Robust Calibration Marker Detection in Powder Bed Images from Laser Beam Melting Processes
In: IEEE International Conference on Industrial Technology (ICIT)

2015

Joschka zur Jacobsmühlen, Stefan Kleszczynski, Gerd Witt and Dorit Merhof
Detection of Elevated Regions in Surface Images from Laser Beam Melting Processes
In: 41st Annual Conference of the IEEE Industrial Electronics Society

2015

Stefan Kleszczynski, Alexander Ladewig, Katrin Friedberger, Joschka zur Jacobsmühlen, Dorit Merhof, Gerd Witt
Position Dependency of Surface Roughness in Parts from Laser Beam Melting Systems
In: Proceedings of the 26th International Solid Free Form Fabrication (SFF) Symposium

2015

Joschka zur Jacobsmühlen, Stefan Kleszczynski, Gerd Witt and Dorit Merhof
Elevated Region Area Measurement for Quantitative Analysis of Laser Beam Melting Process Stability
In: 26th International Solid Free Form Fabrication (SFF) Symposium

 
2014

Joschka zur Jacobsmühlen, Stefan Kleszczynski, Gerd Witt and Dorit Merhof
Robustness Analysis of Imaging System for Inspection of Laser Beam Melting Systems
In: IEEE 19th Conference on Emerging Technologies and Factory Automation (ETFA)

 
2014

Stefan Kleszczynski, Joschka zur Jacobsmühlen, Bernd Reinarz, Jan T. Sehrt, Gerd Witt and Dorit Merhof
Improving Process Stability of Laser Beam Melting Systems
In: Fraunhofer Direct Digital Manufacturing Conference (DDMC)

 
2013

Stefan Kleszczynski, Joschka zur Jacobsmühlen, Jan T. Sehrt and Gerd Witt
Mechanical Properties of Laser Beam Melting Components Depending on Various Process Errors
In: Digital Product and Process Development Systems

 
2013

Joschka zur Jacobsmühlen and Stefan Kleszczynski
High Resolution Imaging for DMLS Inspection
In: EOS International User Meeting 2013

 
2013

Stefan Kleszczynski, Joschka zur Jacobsmühlen, Jan T. Sehrt and Gerd Witt
Einfluss von Prozessfehlern auf die mechanisch-technologischen Eigenschaften strahlgeschmolzener Bauteile
In: Tagungsband Rapid.Tech 2013

 
2013

Joschka zur Jacobsmühlen, Stefan Kleszczynski, Dorian Schneider and Gerd Witt
High Resolution Imaging for Inspection of Laser Beam Melting Systems
In: IEEE International Instrumentation and Measurement Technology Conference (I2MTC) 2013

 
2012

Stefan Kleszczynski, Joschka zur Jacobsmühlen, Jan T. Sehrt and Gerd Witt
Error Detection in Laser Beam Melting Systems by High Resolution Imaging
In: Proceedings of the Twenty Third Annual International Solid Freeform Fabrication Symposium

Slides for Conference Publications

2017 Compound Quality Assessment in Laser Beam Melting Processes Using Layer Images
IEEE International Instrumentation and Measurement Technology Conference (I2MTC)
2016 Robust Calibration Marker Detection in Powder Bed Images from Laser Beam Melting Processes
IEEE International Conference on Industrial Technology (ICIT)
2016 In-Situ Surface Roughness Measurement of Laser Beam Melted Parts – a Feasibility Study of Layer Image Analysis
Fraunhofer Direct Digital Manufacturing Conference (DDMC)
2015 Detection of Elevated Regions in Surface Images from Laser Beam Melting Processes
41st Annual Conference of the IEEE Industrial Electronics Society (IECON)
2015 Elevated Region Area Measurement for Quantitative Analysis of Laser Beam Melting Process Stability
26th International Solid Free Form Fabrication (SFF) Symposium
2014 Robustness Analysis of Imaging System for Inspection of Laser Beam Melting Systems
Emerging Technologies and Factory Automation (ETFA), 2014 IEEE 19th Conference on
2013 High Resolution Imaging for Inspection of Laser Beam Melting Systems
IEEE International Instrumentation and Measurement Technology Conference (I2MTC) 2013

Research Topics

Image Acquisition of Powder Bed and Exposure Results

The requirements for the imaging systems are very demanding: it has to acquire images of structures of 50 – 100 µm size from great distance, as the camera cannot be placed inside the build chamber due to process atmosphere and laser radiation. The required resolution at microscopic level is achieved using a 29 megapixel industrial CCD camera (SVS29050 by SVS-VISTEK GmbH, Germany).

Camera setup in front of LBM machine (EOSINT M 270, EOS GmbH, Germany). The modular tube construction enables flexible positioning with adjustable height, position, and distance from door. A geared head allows three-axis rotation of camera and camera lens to align it with the build platform.
Camera setup in front of LBM machine (EOSINT M 270, EOS GmbH, Germany). The modular tube construction enables flexible positioning with adjustable height, position, and distance from door. A geared head allows three-axis rotation of camera and camera lens to align it with the build platform.
Lighting setup
Light setup inside process chamber
Screenshot of layer image analysis software
Screenshot of layer image analysis software. The selected region can be compared to previous/successive layers. Regions can be saved for documentation purposes. Click to enlarge,

Build Job Documentation

We have developed a data format for build job documentation based on the open Hierarchical Data Format (HDF5). It enables storage of layer images with associated metadata (imaging setup, machine state, image type), process parameters and reference part geometries (from CAD model). The HDF5 format is a self-describing container format, which does not impose any limits on the number or size of data objects. It is supported by many scientific applications (e.g. MATLAB) and programming languages (Python, C++,
Java).

 
2013

Joschka zur Jacobsmühlen, Stefan Kleszczynski, Dorian Schneider and Gerd Witt
High Resolution Imaging for Inspection of Laser Beam Melting Systems
In: IEEE International Instrumentation and Measurement Technology Conference (I2MTC) 2013

The external camera position enables retrofitting of existing LBM systems and/or flexible positioning but causes perspective distortions which have to be corrected before analyzing layer images. We compute a homography from four circular markers which are “drawn” into the powder bed by the machine’s laser and detected in the acquired images. To deal with different image quality, we have developed a robust method for the automatic detection of calibration markers in powder bed images. The homography estimation minimizes the shape error between transformed reference marker shapes and detected elliptical markers yielding an image with corect aspect ratio and minimal distortions.

2016

Joschka zur Jacobsmühlen, Jan Achterhold, Stefan Kleszczynski, Gerd Witt and Dorit Merhof
Robust Calibration Marker Detection in Powder Bed Images from Laser Beam Melting Processes
In: IEEE International Conference on Industrial Technology (ICIT)

Process Errors and Error Categorization

Layer images enable the detection of common process errors, we have analyzed different error types and categorized them based on effect (reduced process stability or part quality), type and cause.

Error Categorization
Typical process errors in laser beam melting categorized by influence, type
and cause. Click to enlarge.

 
2012

Stefan Kleszczynski, Joschka zur Jacobsmühlen, Jan T. Sehrt and Gerd Witt
Error Detection in Laser Beam Melting Systems by High Resolution Imaging
In: Proceedings of the Twenty Third Annual International Solid Freeform Fabrication Symposium

The effect of provoked process errors on mechanical part properties and associated layer appearance was examined to find a link between in-process layer images and post-process part performance.

Tensile strength
Tensile strength compared to calculated volume energy density. Click to enlarge.

 
2013

Stefan Kleszczynski, Joschka zur Jacobsmühlen, Jan T. Sehrt and Gerd Witt
Mechanical Properties of Laser Beam Melting Components Depending on Various Process Errors
In: Digital Product and Process Development Systems

During batch production of multiple identical parts in a single build job, we observed parts with deviating surface roughness in certain areas, which all faced away from the laser. This suggests a dependency of surface roughness on the part position in the build chamber. In this work we systematically reproduce and analyze this effect.

2015

Stefan Kleszczynski, Alexander Ladewig, Katrin Friedberger, Joschka zur Jacobsmühlen, Dorit Merhof, Gerd Witt
Position Dependency of Surface Roughness in Parts from Laser Beam Melting Systems
In: Proceedings of the 26th International Solid Free Form Fabrication (SFF) Symposium

Detection and Analysis of Elevated Regions in Powder Bed Images

Elevated regions pose a major risk to process stability as collisions between recoater mechanism and elevated part regions may cause damage to the part and/or the recoater blade. Jammings of the recoater mechanism may cause process breakdowns.

Elevated Regions
Elevated part regions in powder bed and recoater blade. Highlighted regions will collide with recoater blade, causing damage to part and/or blade.

Layer images of the powder bed enable detection of elevated regions before they become dangerous. We have developed an image-based detection method for elevated regions, which segments regions inside the powder bed image, which are not covered by metal powder.

 
2013

Joschka zur Jacobsmühlen, Stefan Kleszczynski, Dorian Schneider and Gerd Witt
High Resolution Imaging for Inspection of Laser Beam Melting Systems
In: IEEE International Instrumentation and Measurement Technology Conference (I2MTC) 2013

Analysis of detection results and correlation between accelerometer measurements and image-based measurements was examined in

 
2014

Stefan Kleszczynski, Joschka zur Jacobsmühlen, Bernd Reinarz, Jan T. Sehrt, Gerd Witt and Dorit Merhof
Improving Process Stability of Laser Beam Melting Systems
In: Fraunhofer Direct Digital Manufacturing Conference (DDMC)

Elevation Detection
Example of elevated area extracted from powder bed image and associated source layer image with highlighted detections. Click to enlarge.

Quick evaluation of an entire build job and comparison of single parts (with and without support structures) is enabled by quantitative visualization of elevation analysis results. Ranking of parts by relative elevated area over all layers enables quick identification of stable process parameter settings, e.g. in the design phase of a new production process. You can view the slides of the conference presentation or refer to the following paper:

2015

Joschka zur Jacobsmühlen, Stefan Kleszczynski, Gerd Witt and Dorit Merhof
Elevated Region Area Measurement for Quantitative Analysis of Laser Beam Melting Process Stability
In: 26th International Solid Free Form Fabrication (SFF) Symposium

Analysis of overhanging geometry in x/z plot of elevated area. Left: part geometry with decreasing overhang angle for increasing z. Right: x/z plot of elevated area the starting z position and thereby the overhang angle of increasing elevations can be identified by analyzing the right part boundary.
Analysis of overhanging geometry in x/z plot of elevated area. Left: part geometry
with decreasing overhang angle for increasing z. Right: x/z plot of elevated area the
starting z position and thereby the overhang angle of increasing elevations can be identified by
analyzing the right part boundary. Click to enlarge.

Detection of elevated regions is also feasible by using image descriptors computed on the melt result image. In contrast to aforementioned methods, the powder bed image is not used, here. We have evaluated different gradient-based descriptors which detect changes in the orientation of laser scan lines and trained classifiers to label image regions as “normal” or “elevated”.

2015

Joschka zur Jacobsmühlen, Stefan Kleszczynski, Gerd Witt and Dorit Merhof
Detection of Elevated Regions in Surface Images from Laser Beam Melting Processes
In: 41st Annual Conference of the IEEE Industrial Electronics Society