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Module 1: Additive Manufacturing Overview

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Additive Parameter Development

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Additive Parameter Development

Hello all, in this session we will look into the factors considered for the additive parameter
development work.
(Refer Slide Time: 00:26)

First going into the details of the LPBF process as you have already learned it is a layer by
layer manufacturing process. I am not going into the details as it is already covered into the
other session. But I am going into some details which are very important for the next session
or next few slides. It involves selectively heating a thin layer of a fine metal powder with a
moving focused laser causing it to melt and then solidify.

This intense localizer heating affects not just the metal that solidifies to form the part but the
melt pool also emits a hot high-speed vapour plume that cools to form a fine mist of metal
condensate nanoparticles. In addition, larger irregular spatter particles are rejected from the
rolling melt pool. Both of these condensate and spatter will affect the laser interaction with
the powder.

And cause the defect generation. Also with its melting and re-melting of fine powder size in
micron, length and time scale lead to high cooling rates of 10
3
to 10
6
degree centigrade per
second and a very different metallurgical response to processing and solidification can be
seen. Triggering metallurgical defects which jeopardize mechanical properties.
(Refer Slide Time: 01:56)

Going into the details of the LPBF parameter here I categorized some important parameters
into different heads. These parameters need to be developed or decided during any new
development work. Parameter to the laser beam or the laser diameter, laser power, and laser
speed. Parameter categorized under the head of hatch strategy is the layer thickness, hatch
distance, stripes, stripe width, rotation angle, sky writing, and time homogenization.

Contour parameters and then recoater material and recoater design also are very important.
Build chamber parameters for example selection of the build atmosphere, gas flow rate, build
plate temperature and type of nozzle. So, these how it affects your properties?
(Refer Slide Time: 02:59)

So, these parameters can be divided into two categories. First category is the bulk parameter
and the second category is the contour parameter. These bulk parameters are directly affect
your density productivity, stability of the well, mechanical properties, and microstructure,
whereas contour parameters affect to surface quality, minimum thickness, and fatigue
properties. During the parameter development there will always be some trade off between
these different properties.

For example, if you need productivity you may need to compromise on the mechanical
properties, same with the surface quality. If you need good quality surface finish in as build
condition you need to compromise on the productivity. So, your AM should be clear while
developing the parameters. So, this is regarding the parameters.
(Refer Slide Time: 03:59)

And next slide is on the important considerations for the material selection. While selecting
the material we are considering weldability, carbon content, difference in the liquidus and
solidus temperature. Solid solution strengthening elements, percentage of gamma forming
elements in nickel alloys, coefficient of thermal expansion, thermal conductivity, thermal
diffusivity, reflectivity, absorptivity, enthalpy of fusion and specific heat capacity. So, why
we are considering this?
(Refer Slide Time: 04:34)

Because weldability is one criterion which we are considering because LPBF process is
nothing but a welding process at micron level. It is very close to the welding technology. So,
those steels having problem associated with the weld hot, cracking of weld will also face
problems during the additive manufacturing. As cooling rags are very high and magnitude is
of 10
4
to 10
5
.

So, welding with a higher cooling rate should be taken into considerations. If the material is
suitable for such a welding it is suitable for SLM as well. This is the part showing the Nickel
alloys weldability, below this red line alloys are easy to weld and well established in AM
industry. But above this redline these are some alloys which are difficult to weld due to their
cracking tendency and because of the complex metallurgy phenomena.

These are the alloys difficult to weld but not impossible. So, major work is going on these
alloys for bringing it into the AM bucket. We are also working on the CM247 alloy and is
very near to the completion of the development work.
(Refer Slide Time: 06:59)

So, next consideration is the carbon content, to process any material through SLM carbon
content should be as low as possible in the material composition. Because high carbon
content promotes the formation of hard brittle microstructure on cooling from above the
phase transformation temperature and degree of carbide precipitation increases with increase
in the carbon content.

Carbides usually sit at the grain boundaries and can lead to the crack formation due to the
differential solidification rates. Basically, it will embrittle your material to mitigate this
nowadays AM material comes with a low carbon content. For example, in conventional
SS316L, it comes with a low carbon content with the carbon content of 0.08, but the alloy
available for the AM that is SS316L comes with the carbon content of 0.03.

Here in this SS316L meaning of L is the low carbon content grid, same is with the CM247
alloy, conventional CM2 for this 247 alloy still comes with a carbon content of 0.15%, but
whereas in CM247 alloy available for the AM that is CM247LC low carbon grade it comes
with a 0.07% of carbon. So, nowadays more and more alloys are coming with the low carbon
content for the additive manufacturing.
(Refer Slide Time: 07:44)

Going to the next consideration that is percentage of gamma forming elements in nickel-
based alloys. Titanium and Aluminium elements form gamma and gamma prime and gamma

double prime. The chemical formula of the gamma prime is the Ni3 Ti or Ai. These are the
precipitates gamma prime and gamma double prime which are part of strengthening
mechanisms alloy, but with excess precipitation susceptible for cracking.

In this case processing difficulty increases with a high-volume fraction of gamma prime. In
Nickel base alloys there is a limit of Titanium and Aluminium percentage, above which the
alloy is considered difficult to build. If Aluminium and Titanium combine content is greater
than the 4.5% it is considered as a difficult to weld material. We discussed this graph in the
earlier slide. Here in the weldability slide here you can see the CM247 alloy is having 6.3%
of combined Titanium and Aluminium content.

But still can be possible to print difficulty level increases and care needs to be taken while
choosing the alloy.
(Refer Slide Time: 09:08)

Next consideration in the selection criteria is the difference in the liquidus and solidus
temperature. This needs to be considered as a powder of aluminium and molybdenum alloy
such as a ALSi12 or ALSi10 mg are relatively suitable for laser melting due to the small
difference between their liquidus and solidus temperatures compared to the high strength rot
aluminium alloys.

Here is the Aluminium Silicon phase diagram is showing in the figure you can see for ear
eutectic alloys such as ALSi10 mg have a relatively lower T which is 40 Kelvin. So, ALSi
10 mg can be possible to process with LPBF. But whereas AL6061 has a relatively large T
and around 70 Kelvin and difficult to process through the LPBF which is; further increased
by non-equilibrium solidification and fast cooling rates in a DMLS.

So, what happens if the T is higher, a large T results in a greater chance of hot dating,
because less liquid is available for the interdendritic feeding when the material reaches to the
solidus or eutectic temperature. So, we look into the interdendritic feeding. So, far we have
considered the melting aspects of the LPBF process and the effect on the part density.

Now we will look into the solidification process that is most critical to establishing the
performance characteristics of the metal component, because solidification defines
microstructure and which in turn drives mechanical properties. Many alloys are complex and
can exist in multiple phases at different temperatures and compositions and so the
solidification does not happen all at once.

Relatively little heat is lost into the neighbouring un-melted powder or via radiation into the
chamber. As a molten metal cools the outer region of the melt pool falls below the liquidus
temperature and one or more phase of the alloy will start to solidify, the remaining liquid
phases are trapped between these primary dendritic frames, only solidifying once their lower
melting points are reached.

Opposing cellular dendritic growth frames from the individual grain boundaries where the
remaining liquid phases can also accumulate. The cooling process places strain on this
cellular and grain boundary region which can results in a unwelcome porosity through a
process known as a hot tearing or solidification crack in some materials. This is a waste
where there is a large difference between the temperatures at which the different phases
solidify.

So, that is why we are considering this T difference in the T liquidus and T solidus temperature
as important selection criteria for the metal layer.
(Refer Slide Time: 12:40)

Next consideration in the list is the amount of solid solution strengthening elements. When
the atoms of the base metal that is solvent and the alloying element which is solute
completely dissolve in each other and become an integral part of the solid phase of an alloy,
the resulting phase is called as a solid solution. There are two types of the solid solution. One
is a substitutional solid solution and the second is an interstitial solid solution.

In substitutional solid solution solute atoms such as are roughly similar to the solvent atoms,
due to the similar size solute atoms occupy a vacant site in the solvent atoms. For example,
copper and zinc, copper and nickel are some examples of substitutional solid solution and in
interstitial solid solution solute atoms are much smaller than the solvent atoms.

So, they occupy interstitial positions in the solvent lattice. Carbon, Nitrogen, Hydrogen,
Oxygen, and Boron are the elements which commonly form interstitial slide solid solutions,
due to this substitution of the parent atoms or settle in the interstitial stress of the crystal
lattice of an alloy. This can lead to the localized stress generation. This is also one of the
strengthening mechanisms. But stress generation is what you need to take care during the
processing.
(Refer Slide Time: 14:17)

Next consideration is the high reflectivity and thermal conductivity. So, creating an effective
melt pool is difficult for the alloys which have high reflectivity and high thermal
conductivity, such as a Copper, Aluminium, Silver and Gold. High power lasers up to 1
kilowatt have been used to process these materials along with a different wavelength to
increase the laser absorption.

For processing of ALSi10 mg we are using the highest power almost near to 100% utilization
of laser power on our USM 290 machine, whereas other materials can be processed with only
60 to 70% of laser capacity. Materials in the powder form do scatter and interrupt the laser
light and are 2 to 7 times more efficient to absorb the laser light compared to the flat surfaces.

Because of this at least it is possible to process these materials with LBPF technology.
Otherwise it is very difficult to process these alloys because of this high reflectivity. So, these
are the some of the important consideration factors we have seen in this session which will
guide you for the right material selection for the AM and what are the difficulty levels with
the different types of the materials will get to know and maybe you will be prepared for the
same.

So, this is all about the parameter development and considerations factors for the material
selection. In the next session, we will go into the details of the common defects in LPBF and
what mitigation strategies can be used to remove these common defects on to mitigate these
common defects. Thank you for attending the session.