evaluation of different monitoring methods of laser additive manufacturing of stainless steel
TRANSCRIPT
Evaluation of Different Monitoring Methods of Laser Additive
Manufacturing of Stainless Steel
Marika Hirvimäki1,a, Matti Manninen1,b, Antti Lehti1,c, Ari Happonen2,d,
Antti Salminen1,3,e and Olli Nyrhilä4,f 1Lappeenranta University of Technology, Tuotantokatu 2, 53850 Lappeenranta, Finland
2Lappeenranta University of Technology, Skinnarilankatu 34, 53850 Lappeenranta, Finland
3Machine Technology Centre Turku Ltd, Lemminkäisenkatu 28, 20520 Turku, Finland
4EOS Finland Oy, Lemminkäisenkatu 36, 20520 Turku, Finland
Keywords: Laser additive manufacturing (LAM), sintering, active monitoring, on-line monitoring, process development, process control, quality control.
Abstract. Different monitoring methods for the laser additive manufacturing process were studied in
this study. Possibilities and downfalls of three different methods were compared to each other to
define their applicability in future on-line and adaptive monitoring use in LAM processes. The
material used on all the LAM process tests was EOS StainlessSteel PH1 in fine powder form. In this
study, e.g. parameters like scanning speed, layer thickness and hatch space were tested. Based on the
results of this study, the pyrometer seems to be more easily adaptable to continuous monitoring than
the spectrometer or systems based on active illumination imaging system. It seems that the pyrometer
is a promising method for quality control. The ability to control quality through on-line measurements
can be further utilized in future e.g. for on-line quality control and dynamic process control, i.e. the
ability to change and correct parameters on the fly.
Introduction
Laser additive manufacturing (LAM) is a process that allows creation of complex 3D parts by melting
and solidifying metallic, plastic or ceramic powder material layer by layer with laser beam. Nowadays
range of materials available has widened and the high automation level has made laser additive
manufacturing a feasible process for manufacturing. Accuracy of processing depends on the
dimensions of the powder particles. [1,2,3,4]
Laser additive manufacturing includes a number of different kinds of process parameters, like laser
power, laser beam scanning speed and hatch spacing. Key issue to all of these parameters is that they
are affected by energy input to work piece via laser beam. Final properties of the LAM product, such
as geometrical (accuracy etc.) and mechanical properties (density, strength etc.), are particularly
dependent on energy input and heat transfer from the treated layer to surrounding material. [5,6] Thus,
ability to control energy input in LAM process is very important for product quality and it is examined
also in this study. Monitoring of the LAM process is studied to get deeper understanding of the
on-going process and phenomena during it. Characterization and understanding of these processes
can also help to further develop them and to produce basic knowledge of the processes required for
process development.
Aim of this study was to examine and compare the possibilities of different laser monitoring
methods of laser additive manufacturing process. Typical monitoring methods generally used on-line
are different acoustic (sensors detecting shock or stress waves) or radiation emission sensors (e.g.
infrared and high-speed cameras), space charge systems (e.g. plasma charge sensors), or laser beam
based monitoring systems. [4] In this study the used monitoring methods were spectrometer, active
Advanced Materials Research Vol. 651 (2013) pp 812-819Online available since 2013/Jan/25 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/AMR.651.812
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illumination imaging system and pyrometer. These monitoring methods were chosen based on the
long history of monitoring laser welding of metals and polymers and laser beam interaction with
paper material at Lappeenranta University of Technology. [7,8,9,10,11,12] All three methods are
distinctively different; spectrometer detects intensity of emitted light over a defined wavelength
range, pyrometer monitors thermal radiation, and active illumination system is used to take images
from bright sources only at the illumination wavelength. By comparing three different monitoring
methods, a valuable evaluation of the advantages and disadvantages of different monitoring
approaches can be obtained, especially pertaining to on-line monitoring. Study covers preliminary
monitoring of the effect of parameter values on basic LAM process to evaluate usability of each
method.
Experimental procedure
The material used in all tests was EOS StainlessSteel PH1 in fine powder form. It can be used in a
variety of medical, aerospace and other engineering applications. The chemical composition and the
physical properties of the powder are shown in Table 1.
Table 1. Chemical composition and physical properties of the powder used in this study. Chemical composition Physical properties
Element Fraction Element Fraction Element Fraction Minimum
recommended layer
thickness
Particle size Minimum wall
thickness
Surface roughness after
shot peening and
polishing
Fe < 80 % Ni < 6 % C < 0.1 % 20 µm 20 – 50 µm
±0.2%
0.3-0.4 mm Ra 2.5 – 4.5 µm,
Ry 15 – 40 µm,
Rz up to < 0.5 µm
Mn < 1 % Nb < 0.5 % Cr < 20 %
Mo < 0.5 % Si < 1% Cu < 6 %
Following equipment was used to monitor laser additive manufacturing process:
• Ocean Optics HR2000+ -spectrometer, (wavelength range 190-650 nm)
• Thyssen Laser-Technik Temperature-Control-System -pyrometer (wavelength ranges
1200-1400 nm and 1400-1700 nm)
• Cavilux HF illumination system by Cavitar with Baumer camera and dichroic window
Ocean Optics HR2000+ is a spectrometer that uses CCD-array-detector and it is responsive to
wavelengths from 200 to 600 nm. This means that it works both in the lower part of spectrum in the
ultraviolet area, and at the spectrum range visible for human eye. Spectrometer can be used to detect
emission spectrum of a light source by measuring emission intensity on a specific wavelength
interval. Integration time of spectrometer is adjustable. Integration time means the time CCD-array is
exposed to the light for one image, and, with HR2000+, it ranges from 1 ms to 65 s.
Temperature-Control-System (TCS) pyrometer can be used for on-line temperature measurement
and controlling of laser power. TCS consists of measuring optics with photodiode sensor, optical fiber
and industrial PC with appropriate software. Pyrometer measures two ranges between 1200 nm and
1400 nm and between 1400 nm and 1700 nm. The pyrometer has round-shaped detection area, which
can be focused. It measures the average temperature within the whole detection area.
The active illumination system by Cavitar is designed to take videos of laser processes influenced
by bright light. The whole system consists of a control unit, an illumination system, control software
and a CCD-camera. Operating principle consists of a use of single wavelength light (diode laser) as
the illumination source. It is used to illuminate the process and majority of the radiation emitted by
process is filtered out with an optical filter to enable video capture of a high-brightness target at the
wavelength of the diode laser. The camera used in this study is capable of speed up to 200 fps. The
diode laser produces laser beam of 810 nm, power up to 500 W and high or ultra-high speed pulsing.
Illumination optics is attached to the LAM unit via optical fiber.
Monitoring with spectrometer and pyrometer was done at EOS Finland premises. Principle of the
spectrometer study set-up is shown in Fig. 1A. The experimental set up for pyrometer tests was
similar. The modified research machine EOSINT M 280 consists of a 200 W fiber laser, a LAM
Advanced Materials Research Vol. 651 813
chamber, a scanner and a computer. The computer controls the scanner, application of powder layer
by layer and measuring of the oxygen level of the LAM chamber. When stainless steel powder was
used, the chamber was filled up with nitrogen to decrease the oxygen level of the chamber atmosphere
to avoid oxidation of stainless steel.
LAM process was monitored with active illumination imaging system at the Laboratory of Laser
Materials Processing at Lappeenranta University. During the tests, pyrometer was used in parallel
with the illumination imaging system, to get comparable results between these two measurement
methods. Fig. 1B presents that set-up.
A) B)
Fig 1. A) Monitoring setup for spectrometer tests. Setup was similar with pyrometer. B) Set-up with
active illumination system and pyrometer.
LAM set-up consisted of a 200 W fiber laser, a scanner guided by a computer, a simple LAM
chamber and a powder unit. The gas-proof LAM chamber was filled up with nitrogen gas and the
powder unit was mounted inside the chamber. Stainless steel powder was spread on to the powder
unit manually by a metallic roller and melting was done only on one layer surface to keep the overall
system as simple as possible. The parameters applied in this research are summarized in Table 2.
Table 2. Experimental parameters used in this study. Focal point position was on the surface. Spectrometer Active illumination imaging
system and pyrometer
Pyrometer
Laser power, W 120 and 150 100-200 80-160
Scanning speed, mm/s 1000 80-800 800-3000
Layer thickness, µm 20 Parts were made on powder 20
Hatch space, mm 0.03, 0.05, 0.07 0.1 0.05
Hatch pattern
Stripe hatch Triangle hatch
As a comparative unit between different methods, energy input was used to describe amount of
laser energy (in joules) provided to one unit of laser processed length (in meters) as Eq. 1 shows.
v
P Q =
where: Q= energy input, P= laser power, v= scanning speed. (1)
Results and discussion
In this chapter results of the experiments are given separately for each monitoring method. The effect
of process parameters on the radiation intensity was investigated with the spectrometer. Pyrometer
was used to measure the changes in temperature profiles with different parameters. The active
illumination system was used to get visual information about the effect of different parameters.
814 Engineering Materials and Application
Spectrometer. The spectrometer measurement expresses intensity of emitted radiation from LAM
process as function of process time and wavelength of emitted light (see Fig. 2). In all of the
spectrometer tests the integration time was 200 ms.
It can be seen from Fig. 2A that time periods when the laser was off (during the powder spreading
between different layers), there was a considerable decrease of intensity level. When the powder layer
was spread, the laser started and the intensity level was again increased to that of typical for
processing. The flat noise floor shows that in the measurement results there is not much error caused
by the lighting conditions outside of the process. The relatively smooth three-dimensional charts
indicate that the intensity stays at a same level through the whole LAM process.
Fig. 2B illustrates the intensity level of emitted light by time and emitted wavelength when triangle
hatch and hatch spacing of 0.05 mm was used. It can be noticed from Fig. 2B that there is slight
variation between different layers and also some variation during laser additive manufacturing of one
layer. Compared to Fig. 2A, it can be seen that the Fig. 2B shows slight dropping of intensity in the
middle of the layers. This cooler area is probably caused by change of the hatching side (see triangle
hatch in table 2).
A) B)
Fig 2. A: Changes of intensity level of emitted light by time and emitted wavelength during LAM
process. Striped hatch spacing 0.05 mm, scanning speed 1000 mm/s, laser power 120W. B: Changes
of intensity level of emitted light by time and emitted wavelength during LAM process. Scanning
speed 1000 mm/s, laser power 120 W.
Pyrometer. Temperature distribution during laser additive manufacturing was measured with the
pyrometer. One test series was conducted using constant speed of 1000 mm/s, and another using
constant power of 120 W. 0.05 mm wide striped hatch was used. Fig. 3 presents the comparison of
results of these tests as energy input on temperature of the sample. Y-axis shows average temperature
calculated from the pyrometer temperature data by using temperature values over 580 °C.
Fig 3. Averaged temperatures as a function of energy input during LAM process. Scanning speed
600-3000 mm/s, laser power ~80-160 W.
Advanced Materials Research Vol. 651 815
When energy input of 100-150 J/m were used with constant power, temperature decreased even
when energy input increased. This phenomenon could be the result of uneven cooling rate or could be
caused by unstable behavior of process properties. The LAM process seems to be more sensitive to
changes of scanning speed than to changes of laser power.
Fig. 4A presents changes of temperature when different laser power values and constant scanning
speed was used. Laser power values were varied between 80 W and 160 W and respectively, energy
input varied between 80 J/m and 160 J/m. Scanning speed was preset to constant value of 1000 mm/s.
Each individual line represents manufacturing of one layer.
It can be observed from Fig. 4A that the temperature of laser manufactured work piece varied
between 800°C and 1000°C during all measurement cycles. The integration time of the pyrometer is
evidently short and rapid temperature changes can be detected.
A steep decrease to minimum value of temperature can be observed in the beginning of each LAM
process event. This minimum is reached at ~0.4 seconds. The deep slope starts when the first hatch
pattern is done and the scanner moves to start another scanning pattern. While the scanner moves, the
laser is turned off and the part cools down. Afterwards the temperature starts to rise again, as the
second pattern starts.
When average temperature differences obtained with laser power values of 80-120 W (∆T=25 oC)
are compared to temperature difference between laser power values of 120-160 W (∆T=60 oC),
difference between temperatures is significant. In both situations, the addition of laser power is 40 W
but the temperature increases more when laser power values of 120-160 W are used. This is due to
higher absorption of a hot object; when temperature increases on the surface of the part, laser energy
absorption is higher.
Fig. 4B shows temperature differences of LAM process when different scanning speeds were
used. Scanning speed was varied between 600 mm/s and 3000 mm/s, and respectively energy input
varied from 40 J/m to 200 J/m. Laser power was kept constant at 120 W. Each line represents the
LAM process cycle of one layer.
A) B)
Fig 4. A) Temperature differences in LAM as a function of time by using different laser power values.
Monitored with pyrometer, scanning speed 1000 mm/s. B) Temperature differences in LAM as a
function of time by using different energy input values. Monitored with pyrometer.
As Fig. 4B reveals, temperature difference between energy input of 40 J/m and 200 J/m is
approximately 150 °C. Temperature changes are not significant in the lower range (600 – 1200 mm/s)
and the process is very stable. But when higher speeds (>1200 mm/s) are used, temperature variation
is larger, as expected, due to shorter processing time. The most extreme temperature variations are
caused by use of triangle hatch. Between the two halves of the pattern the work piece cools down.
With higher energy input the temperature tends to stabilize between 850 and 950 °C due to lower
cooling rate and with shorter process times (up to 2 s) the temperature is heavily dependent on the
scanning speed, varying between 650 and 875 °C.
816 Engineering Materials and Application
Active illumination imaging system. Table 3 shows images obtained when LAM was monitored
with the active illumination imaging system. Only one layer was manufactured and the pyrometer
results were taken simultaneously with active illumination. Usually phenomena easily seen in the
Table 3 are hidden, because the LAM process itself emits so much light that the conventional
photographic methods are not able to capture the phenomena observed with the illumination system.
Table 3. Images taken with the imaging system. Pyrometer results taken simultaneously. Energy
input,
J/m
When process starts Middle of the process End of process Pyrometer measurement
250
500
1000
2000
5000
From the pyrometer results in Table 3 can be seen that the temperature increases first rapidly and,
after that, decreases until it starts to rise again. This phenomenon is caused by contour processing of
the square edge around the area to be processed in the beginning of LAM process. After processing of
the contour, temperature starts to increase and peaks before the process stabilizes. This can be seen as
a strong increase of temperature and it is noticeable almost in every measurement. Only exception is
the sample where energy input was relatively small 250 J/m. Temperature increase at the beginning of
LAM process could be related to the better absorption of untouched and non-fusible loose powder.
Also part of the laser energy is probably lost because the beam cannot penetrate between bonded
particles.
Advanced Materials Research Vol. 651 817
When energy input of 250 J/m is used, the average process temperature varies a lot between
700-1100 ⁰C. This might be caused by lower energy input. Although the laser beam goes through the
scanning pattern which consists of adjacent parallel lines, the energy input might be too low to melt
material properly at some areas of the sample. That partial melting could be detected with the
pyrometer and seen as rapid and strong fluctuations of temperature. When energy input of 500 J/m is
used, temperature variation is much smaller (1000-1100 ⁰C). As energy input is increased between
1000-5000 J/m, the variation between observed average process temperatures decreases and also
average process temperature reaches a constant value of 1200 ⁰C.
It can also be concluded from Table 3 that formation of small balls with energy input range of
0-500 J/m is caused by strong fluctuation of the average process temperature. When the process is
unstable as the variation of temperature indicates, quality of the work piece is also uneven. This is an
important observation because in some cases uneven or porous result is preferable, for example in
internal structures to reduce weight of a component.
When energy input of 5000 J/m is used, it can be seen from Table 3 that the amplitude of the rapid
temperature changes during the process starts to increase again. It can be concluded that optimum
energy input to be used for this grain size (20-50 µm) is between 2000-5000 J/m when variation of
average process temperature is considered, and also when as dense as possible structure is required.
This same conclusion can also be drawn if images captured with active illumination imaging system
are analyzed. It can be concluded that the phenomena seen in LAM process with pyrometer
corresponds well with the phenomena seen in images of the active illumination imaging system. This
indicates that reading pyrometer measurements in “real-time” might give possibilities to control the
LAM process adaptively, inside the build process, to enhance the quality of the end product.
Summary
Aim of this study was to examine and compare the possibilities of three monitoring methods with
laser additive manufacturing of stainless steel powder. Active monitoring of additive manufacturing
process is important for two main reasons: Firstly, because the manufacturing process is slow, it is
crucial to notice immediately if the process starts to fail so that the entire manufacturing cycle is not
wasted; and secondly, monitoring of the process gives valuable basic data about the process which
can be used in process development. The methods used in this study were spectrometer, pyrometer
and active illumination imaging system. Monitoring can be further utilized in the future for example
for on-line quality control and dynamic process control, i.e. the ability to change and correct
parameters on the fly.
Advantages of the spectrometer in monitoring of LAM process are: wavelength range from
ultraviolet to near infrared light (200-600 nm), emission intensity of the whole wavelength interval
can be seen at the same time, the method can be used to find single emissivity peaks, and has a good
resolution (~0.035 nm). Disadvantages are: limited wavelength range that depends on grating and
entrance slit selections (usually 200-600 nm or 600-1100 nm), sensitivity to disturbance, fairly slow
integration time, and large amount of data.
Disadvantages of the active illumination system are: data amount is high due to high number of
raw images that are stored in the computer memory, which also limits the length of the video;
equipment is expensive (few times more than the other methods); and positioning of the system is
time consuming because you have to aim both the camera and the illuminating laser precisely at the
same spot. However, the method is capable of examining several phenomena (cracks, shrinkage,
bending, etc.), its results can be immediately read whereas other methods require extensive data
analysis and the image quality is very good. It can be used to obtain more specific information from
the melt pool movement and phenomena behind cooling. Understanding of these aspects could lead to
better design of LAM supports in terms of heat transfer which is an important role of the supports.
Lastly, it seems that IT industry is still able to keep up with the Moore’s law at least for the next 5-10
years, which would mean that the limitation with high data amount might not apply in the near future.
This would allow e.g. possibility to apply this method to high-speed cameras that generate much more
data but allow investigation of very transient phenomena.
818 Engineering Materials and Application
Pyrometer has many advantages: integration time (~2.5 ms) is short, very high temperatures (~1500 ˚C) can be monitored, collected data amount is reasonable and the lighting conditions do not have a significant adverse effect on the process. Disadvantages of pyrometer are: size of the equipment (fairly large, but can likely be equipped with a long optical cable so that the bulkier control unit is farther away) and measures only temperature of the process. Changes in the temperature follow scanning speed and laser power accurately. When taking all the process advocating factors into account, it might be possible to use pyrometer for monitoring and also for process control. The laser power could be changed by a loop-back system during the LAM process based on the pyrometer temperature measurements.
For the further studies use of pyrometer seems promising as a quality control measure. Active illumination system also provides valuable visual information about the process. Using these two methods simultaneously to study heating and cooling of the work piece could be one possible way to, for example, validate the design of supports. Support design is one of the most important aspects of LAM because it affects not only the process itself but also post processing.
Corresponding Author
Name: Marika Hirvimäki, Email: [email protected], Mobile phone: +358 40 5742379
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Engineering Materials and Application 10.4028/www.scientific.net/AMR.651 Evaluation of Different Monitoring Methods of Laser Additive Manufacturing of Stainless Steel 10.4028/www.scientific.net/AMR.651.812
DOI References
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http://dx.doi.org/10.1007/978-1-4471-3752-8