Direct SLS of Ceramics

From: Peter Hardro (hardro@yahoo.com)
Date: Fri Dec 18 1998 - 15:56:26 EET


RP-ML,

I have received a number of questions about my own inquiry into direct
SLS of ceramics. Here is what I know and what has been sent to me:

From: Simon Van de Crommert, DTM Representative...

"CO2 lasers work wonderfully with plastics (laser absorption rate is
around
90%) therefore we (DTM) can do plastics very well. Unfortunately
ceramics
and metals directly do not work very well (laser absorption rate is
around
10 to 20%). You either need to switch laser (e.g. a YaG laser which
has got
a different frequency/wavelength) or you coat the material with a
plastic
(as done with RapidSteel and SandForm).

Assuming a YaG laser and ceramics work very well I am personally
expecting
porosity of at least 30%. Some research with YaG lasers and direct
metals
have shown porosities of as low as 3%. If you are after full dense
ceramic
bodies you have got a problem with this porosity, so you need to
infiltrate
with ceramic slurry , dry, sinter, infiltrate, dry, sinter,
infiltrate, dry,
sinter until you are satisfied. Personally I don't think furnace
sintering
will reduce the porosity, it will only sinter the material very well (at
around 1600 deg C in a very expensive furnace) and get rid of lower
melting
point materials (such as our plastic coatings).

Porosity examples with SLS and CO2 lasers:
DuraForm polyamide: 2 or 3%
RapidSteel brown part: 45% (after furnace sintering, before bronze
infiltration)"

Personally, I believe...

Direct SLS of ceramics and metals is far more demanding than SLS of
polymers or indirect SLS. The main difficulty arises due to the
higher processing temperatures involved. The temperatures necessary
for melting metals or sintering ceramics is generally greater than
1,000 C. Here oxidation is a serious problem. To address the key
issues associated with direct SLS of ceramics and metals, namely
atmosphere control, laser power and powder bed preheat, a high
temperature selective laser sintering workstation is needed. Luckily,
one exists at the University of Texas at Austin.

This high temperature selective laser sintering workstation was custom
designed and fabricated at the University of Texas and is the only one
of its kind in the world. I believe it is equipped with two 100 Watt
Nd:YAG lasers and a 1.1 kW CO2 laser.

Currently, I believe the University of Texas is the only place that
has tried to create "dense" ceramic parts directly without furnace
operations (correct me if I am wrong) using the SLS process.

Commercially, DTM's Sinterstation 2500plus, which can be equipped with
either a 50 or 100 Watt CO2 Laser generally uses indirect SLS
processing (SLS and furnace operations) to create dense ceramic
composites. Additionally, dare I say since DTM is a sponsor of our
work, EOS has a RP system that uses SLS technology and is equipped
with a 200 W CO2 laser. However, I haven't heard anything about them
processing ceramics directly either.

As for the people at Sandia, I believe they have tried to process
ceramics directly, however, they use the LENS process which is a bit
different than SLS.

We have made much progress at URI since our humble beginning (our
research center was created earlier this year), much to the credit of
the creator and director Dr. Brent Stucker. A slightly dated
description of our center and research can be seen at:

http://www.egr.uri.edu/ime/RMC/rm.html

It describes our goals, work in progress, facilities, membership
agreement and current members. For more information please contact
Dr. Stucker at:

STUCKER@EGR.URI.EDU

I personally am conducting research in developing a material system
for indirect SLS of composites for tooling in die-casting.
Previously, Dr. Stucker has had much success with composite materials
for EDM electrode applications. Other research areas include tooling
for injection molding and machining of advanced composites.

Peter J. Hardro
Research Assistant
University of Rhode Island
HARDRO@YAHOO.COM

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