Volume 6 Preprint 76
Laser forming : A High Performance Alloy Component Manufacturing Technology for Corrosion Resistance in the 21st century
S Mahapatra, A S Khanna and A Gasser
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Volume 6 Paper H021
Laser forming : A High Performance Alloy Component
Manufacturing Technology for Corrosion Resistance in
the 21st century.
S Mahapatra*, A S Khanna* and A Gasser**
*Corrosion Science and Engineering, Indian Institute of Technology
Bombay, Powai, India-400076. **Fraunhofer Institute for Lasertechnik
The fabrication of metal components is the backbone of the modern
manufacturing industry. Laser forming is combination of five common
technologies: lasers, computer-aided design (CAD), computer-aided
manufacturing (CAM), and powder metallurgy. The resulting process
creates part by focusing an industrial laser beam onto a flat tool work
piece to create a molten pool of metal. A small stream of powdered alloy
is then injected into the melt pool to increase the size of the molten pool.
By moving the laser beam back and forth and tracing out a pattern
determined by a CAD, the solid metal part is built--line by line, one layer
at a time. By this method, a material having a very fine microstructure
due to rapid solidification process can be produced. In the present work,
the corrosion behaviour of the material is studied and main emphasis is
given on the high temperature oxidation, sulfidation and hot corrosion
resistance. These studies were done after characterizing the formed
component in terms of their microstructure. The mechanical properties of
the formed samples were also studied. Finally the high performance of
the formed alloys was confirmed after the above studies.
Laser forming or laser assisted direct metal deposition refers to the
additive layered manufacturing technology for building components from
a computer-aided design (CAD) model. Metal powders, injected into the
laser focal zone, are melted and then re-solidify into fully dense metal in
the wake of the moving molten pool created by the laser beam
[#ref1,2,3,4]. Fabrication proceeds by moving the work piece, thereby
building the structure line by line and layer by layer as shown in Fig 1.
During fabrication, a complex thermal history is experienced in different
regions of the component. These thermal histories include remelting and
compositions can be created within three-dimensional components to
vary the properties to match localized requirements due to the service
environment. The technology offers the designer a rapid prototyping
capability at the push of a button. Parts are deposited with a surface
roughness of 10µm, making a secondary finishing operation necessary
for some applications to achieve high accuracy and polished surface
Kreicher et al[#ref6], have processed direct metal deposition of 316SS
and Alloy 625. In 316SS full metal density, increased tensile strength
(85%) and ductility by 30% have been achieved as compared to that of the
conventionally processed material. Similar trend was observed for Alloy
625[#ref7]. On the other hand laser formed alloys produced by adding Cu
and Al in stainless steel helped in increasing thermal conductivity of the
steel but the yield strength was less than that of 316SS. This was due to
the presence of microcracks in the clad. The structure was found to break
by brittle fracture[#ref8]. Khanna et al [#ref9] studied the high
temperature corrosion behavior of the alloys formed using this method at
three different temperatures 750, 850 and 900°C. Arcella et al[#ref9], in a
similar way made several titanium components using laser forming.
The Alloy 718 powder used in the fabrication of samples was 40-60 µm
in size and had the following elemental composition (wt%).
Laser shutter, XY table, powder feeder was controlled simultaneously in
order to feed desired powder to clad the designed pattern. Once one XY
plane of a layer is cladded, a Z-axial elevator is moved up the substrate
with a predefined layer thickness. Powders were transported by tubes,
mixed in a carburetor, and then side fed into melt pool with a lateral
nozzle as shown in Fig 1. Cladding tracks are fixed.
The following analysis were used in comparing the specimens resulting
from laser forming with those conventionally prepared[#ref10,11]
Micro-hardness testing for selected samples
Microstructure and porosity evaluation for all samples
Scanning Electron Micrography of the formed samples
conventional alloy and the formed alloy
XRD pattern to show the presence of phases
Oxidation behaviour of the formed alloy and characterization of the
Sulphidation behaviour of the formed alloy and its characteristics
Mechanical properties includes tensile testing of the formed alloy
Microhardness Testing :
Overall the hardness was found to be
microhardness within the samples. These factors include locations within
the samples tested and changes in the process parameters. The interface
region has lower values for hardness in comparison to other region, lower
hardness is expected since the interface region is essentially re-melted
and re-heated atleast twice. When the underlying cladding is cladded to
build next layer, it is re-melted, annealed or heat-treated and therefore,
loses some of its hardness.
The material is hardest at the top surface and becomes softer towards the
bottom of the sample. The bottom layers were softer than top layers
because they had a longer temperature history, as layers of cladding were
added to the sample, the previous layer were heated up again giving
them time to slightly anneal each time.
Microstructure and porosity analysis:
All samples were etched
electrolytically in 5% oxalic acid. The microstructures were recorded using
an optical microscope. The same method was applied to analyze a cross-
sectional image of the formed samples to determine the amount of
porosity. However, a small percentage of porosity observed in a few
samples could be due to uneven power density distribution as the clad
gets thicker. The microstructural feature for the conventional alloy 718
and that of laserformed alloy is compared (Fig 2).
The local solidification conditions which are commonly encountered
during laser forming lead to a dendritic morphology compared to many
conventional casting process where a large fraction of equi-axed grains
are present. The solidification morphology in the clad is rather columnar.
A part made by the laser forming is composed of multiple single track
cladding passes, each cladding pass is composed of interfaces and
columnar grain regions [#ref12]. The interface region is area where the
laser re-melts and/or reheats the substrate material and begins to add
the new cladding material. This will result in heat -treating the existing
grains, allowing the grains to grow. Above the interface, the grains are
more columnar as a result of directional cooling and are aligned with the
largest temperature gradient during cooling because heat is flowing out
of the clad towards the substrate material. The grains or primary
dendrites, are long, slender and perpendicular to the interface [#ref13].
Scanning Electron Micrography/EDS of the formed samples: EDAX
results show that there is not much variation in the composition of
powder, the deposit and the conventional alloy. There is a slight variation
in composition of Al which is reduced in laser formed alloy 718. SEM
analysis showed a cast layered structure with negligible porosity and
smooth surface .
XRD Analysis: XRD analysis revealed the formation of certain phases.
The phases observed are Nb2O5(110), ZrO2(200), Cr,Ni(210), Al2O3,
Ni3Mo(121). Additional phases like Ni3(Ti,Al) were also found which is the
main phase present in superalloys as γ' precipitate.
complex compounds were observed on the surface of the laserformed
alloy on the as prepared samples, however, these precipitates did not
appear in the XRD pattern after polishing the as formed samples.
The polished samples of size 1x1cm2 were kept in a furnace maintained
at 950°C for 1000h. The plot of time of exposure v/s weight change in
gm/cm2 can be seen in Fig 3. The oxide layer surface was also seen
under scanning electron microscope to see the morphology for both the
oxide layers (Fig 4). The XRD results show presence of some oxides and
intermetallics compounds. However, the oxide layer formed on both the
alloys shows similar peaks indicating similar oxidation behaviour.
The samples after polishing up to 600 grade silicon carbide polishing
paper were kept in SO2+O2 environment at a temperature of 750°C for
600h.The sulfidation kinetics were studied and the plot of time of
exposure v/s weight change in gm/cm2 can be seen in Fig 5. The SEM
micrographs of the sample were taken to observe the scale formed
during sulphidation (Fig 6). The results show that both the conventional
as well as laser formed alloys show almost similar XRD pattern.
The tensile tests were carried out using a tensile machine of Instron
Model 1195 and the yield strength and ultimate strengths were recorded.
During the tensile testing, the stress was applied in both longitudinal as
well as transverse direction of layer formation. The yield strength and
ultimate tensile strength of the laser formed alloys (550MPa and 840MPa
corresponding values of 500MPa and 1260MPa for the annealed wrought
alloy. Percent Elongation for the laser formed and the wrought alloy was
found to be 11%. The values of the yield strength and ultimate tensile
strength can be seen from table 1. The SEM micrographs of the fractured
samples are shown in Fig 7.
With the above encouraging results, a CAD model (using Pro/Engineer
software) of the actual turbine blade was designed. The programme used
for the model was converted into .stl file and then implemented in the
process. The half way blades fabricated using the same model were cut
and analyzed for microstructure, composition and hardness which are
found to have nearly same properties as that of the simple rectangular
The above results analysis shows that laser forming is a promising
technology to develop some advanced alloys. The present work is to
establish the microstructure and composition of the laser formed alloy. It
appears that the laser formed alloy has similar structure as that of
conventional wrought alloy with some additional peaks formed as a result
of the oxidation of active alloying elements. The ability to laser form
structures in 3-D shapes directly from electronic CAD representations,
without molds or dies (i.e., with minimal non-recurring costs), has vast
implications for the costs of many complex structures. Therefore, it can
Ref1 L J. Li and J. Mazumder, 'A study of the mechanism of laser cladding
processes', Applications of Lasers in Material Processing, by E A
Metzbower and S.M Copley, ASM, 1983.
Ref2 J Gerken, H Haferkamp, H Schmidt, 'Rapid Prototying/manufacturing
of metal components by laser cladding', Symposium on Laser Material
Processing at Aachen, Germany, Page 85-90, 1995
Ref3 M. Gauman, S Henry, F. Cleton, J. D Wagniere, W Kurz, “Epitaxial
laser metal forming: analysis of microstructure formation”, Mat Sci. &
Engg, A271 pp 232-241 1999.
Ref4 A.S Khanna, Streiff, R. Critical issues in laser surface alloying to form
components, Materials Science Forum, Volume 369-372, Part 2, Pages
Ref5 J.G Conle, Marcus, H.L. Rapid prototyping and solid free form
fabrication, Transactions of the ASME. Journal of Manufacturing Science
and Engineering, Volume 119, Issue 4B, Pages 811-816, 1997
Ref6 D.M Keicher, Smugeresky, John E, 'Laser forming of Metallic
Components using Particulate Materials' , JOM Volume 49, Issue 5, May,
Pages 51- 54, 1997.
Ref7 G K Lewis, Thoma D J, Milewski J O, Nemec R B. “Near Net Shape
Processing of Metal Powders using Directed Light Fabrication”, Advanced
Materials and Technology for 21st century, J. Inst Metals, 1995 Fall Anual
Meeting, Hawaii, 13-15 Dec, 1995.
Manufacturing Processes, Vol 13, No. 4, 537-554, 1998.
Ref9 Sujata Mahapatra, A S Khanna and A gasser “Characterization,
oxidation and Sulphidation Resistance of UNS 7718 superalloy fabricated
by laser forming process, NACE International, Corrosion/2002 (USA), pp.
Ref10 E.John Smugeresky, D.M Keicher, Lasers in Surface Engineering,
Narendra B.Dahotre, Surface Engineering Series, (ASM International,
Materials Park, OH, 1, p505, 1998
Ref11 A.S Khanna and K Sridhar, Lasers in Surface Engineering, Narendra
B.Dahotre, Surface Engineering Series, (ASM International, Materials Park,
OH, 1 p395, 1998
Fig 1 The schematic of the laser forming process.
Fig 2 The microstructure of (a) Inconel 718 (Conventional) and (b) Inconel
718 (Laser formed) as seen under optical microscope.
Conventional 718 alloy
Laser formed alloy 718
Fig 3 Plot of weight change v/s time after oxidation at 950°C for 1000h.
Fig 4 SEM microgcraphs showing the oxidised samples of (a) conventional
and (b) laser formed alloys (oxidised at 950°C for 1000h)
Conventional 718 alloy
Laser formed alloy 718
Fig 5 The plot of time of exposure v/s weight change in gm/cm2 after
sulfidation at 750° for 600h.
Fig 6 SEM Micrographs showing sulfidation scale in (a) conventional and
(b) laser formed sample after 600 h of exposure in SO2+O2 environment
Fig 7 SEM micrographs showing the fractured samples from both the
direction of stresses (a) longitudinal (b) transverse.
Table 1 Mechanical Properties of the laser formed Inconel 718.
A1 (along the
A2 (Heat treated)
B2 (Heat treated)