Optimising metal powders for additive manufacturing

special feature
Optimising
metal powders
for additive
manufacturing
Powder rheometry can be used to ensure that metal
powders for additive manufacturing deliver the
performance and consistency required in demanding
applications. Jamie Clayton, Operations Director of
Freeman Technology Ltd and Rob Deffley, R&D Manager
of LPW Technology Ltd, explain.
A
s additive manufacturing
(AM) becomes increasingly
established across a number
of industrial sectors, efforts
continue to exploit its wider potential
for the efficient production of complex,
precisely dimensioned components.
Currently AM is widely used in prototyping, to produce components with
‘form and fit’. Moving to the point of
being able to engineer functionality into
a component requires exacting control
of the process, of the performance of
the AM machine and of the powders
used. The manufacture of high performance components for applications in the
biomedical, aerospace and automotive
industries, exemplifies the need for precise and consistent control across the
production process.
This article describes how LPW
Technology Ltd, a global supplier of
metal powders to the AM industry, is
using the FT4 Powder Rheometer®,
a universal powder tester from
Freeman Technology, to develop a
new understanding of how metal
powders perform in AM machines.
Experimental data show how the
measurement of multiple powder
14
MPR September/October 2014
properties, most especially dynamic
flow properties, is helpful in optimising
the use and re-use of metal powders,
helping to improve lifecycle management right through from virgin powder
to final waste.
Metal powders for AM
AM offers a number of advantages
over traditional approaches to manufacturing. It enables the fast, efficient
construction of intricate, customised
components to close tolerances, and
allows the use of materials that may
be hazardous or difficult to machine.
Increasingly AM machines are being
used to make components with builtin functionality. Such items include
prosthetic implants with controlled
porosity to encourage bone regrowth
and efficient cooling systems for aerospace applications that require complex
microfluidic channels.
Figure 1: Laser Metal Deposition feeds a concentrated stream of powder into a selectively
targeted ‘melt pool’ on the surface of the work piece.
0026-0657/14 ©2014 Elsevier Ltd. All rights reserved
special feature
Manufacturing for such demanding
applications requires detailed understanding and control of the characteristics of the metal powders being used.
There is a need to closely match the
properties of the powder to the application and the specific machine being
employed, for example, and, for many
applications, to safeguard a highly consistent, certified supply chain. Lifecycle
management of the powder is also
critical, raising the question of whether
metal powder recovered from the process can be re-used without compromising the product quality.
“AM offers a number
of advantages over
traditional
approaches”
For companies working with or
s upplying AM powders, bulk powder
analysis is therefore a fundamental
capability. Analytical techniques that
characterise the powders in ways that
directly correlate with their performance in AM machines are essential,
for development work, day-to-day
quality control and powder lifecycle
management.
Choosing a new
instrument for powder
testing
LPW Technology employs a number
of techniques to characterise metal
powders comprehensively. These
include methods for the determination
of chemical composition and a range
of sophisticated physical measurement techniques such as laser diffraction particle sizing and morphological
analysis. Such methods provide valuable information about the particles
in a metal powder, however there is a
complementary need to characterise
bulk powder behaviour, most especially
flowability.
Historically the company has relied
on relatively basic bulk powder measurement techniques such as tapped density, angle of repose and Hall flow rate
to meet this requirement for flowability
metal-powder.net
Understanding how powders behave in
AM machines
There are a number of techniques
used within AM machines, each of
which subjects metal powders to different flow and stress regimes. This
makes it crucial to match powder characteristics for each specific application. During Laser Metal Deposition
(LMD) for example, powder contained
in a carrier gas stream flows continuously through the annulus of a nozzle
onto the surface of the work piece
(see Figure 1). A laser beam forms a
melt pool on the working surface into
which the powder is fed in a controlled stream. The powder melts to form
a deposit that is fusion bonded to the
substrate, giving the finished component the properties of the parent
metal. Both the laser and the nozzle
from which the powder is delivered are
controlled using a Computer Numerical
Control (CNC) robot or gantry system,
creating a fully automated process that
eliminates any requirement for experienced metal workers or welders.
Clearly, a consistent flow of powder
to the work piece is essential for reliable, continuous LMD. The powder is
flowing under gravity in a low-stress,
highly-aerated state, so quantifying
flowability characteristics under these
conditions is necessary in order to
provide information that is relevant.
However, away from the working area
the metal powder is stored in a feed
hopper. Here it is subject to the consolidating load of its own weight,
meaning that flow properties in this
type of a moderate to high shear stress
environment are also influential in
terms of overall process performance.
In addition, the ability of the powder
to release the carrier gas as it is drawn
into the melt pool will influence the
precision with which powder can be
deposited and, as a result, the quality
of the finished component.
In contrast, the flow regime for
metal powders in a Selective Laser
Melting (SLM) machine is quite different (see Figure 2). Here, a thin layer of
metal powder is dispensed across the
surface of the component being built,
which is constructed on a retractable
platform. A roller or scraper wipes the
deposited powder across the working
area to create a very thin layer of uniformly distributed powder. Typically
around 20-50 µm in depth, this is
selectively melted by a laser, fusing
it to the growing component, as in
LMD. The platform is then fractionally
lowered and another layer of powder
is wiped across. This cyclical process
is repeated many times until the component is fully built.
As with LMD, the performance of
the powder under moderate stress is
relevant since it is similarly stored in,
and dispensed from, a feed hopper.
However, for SLM applications there
is a need to understand how the powder flows under the forcing conditions
that apply as it is spread across the
bed. The ease with which the powder
releases air is also a factor since the
powder must form a uniform, homogeneous layer with no air pockets, to
ensure a finished product of consistent, defined quality. Both permeability data, which are indicative of how
easily air can flow through a powder,
and flowability measurements, are
highly relevant.
data. Angle of repose is one of the oldest and simplest methods for measuring
powder flow and classifies flowability
on the basis of the angle at which a pile
of powder settles when poured from a
vessel. Tapped density methods on the
other hand provide a classification of
powder flowability from measurements
of the change in bulk density induced
by uni-directional tapping. Hall flow
rate tests determine flowability by
measuring the rate at which a powder
flows through a calibrated orifice.
Each of these three tests is carried
out to ASTM standards and provides
useful insight into powder characteristics and they all remain in use by
LPW Technology. However in 2013
the company decided that it needed to
invest in more sophisticated powder
September/October 2014 MPR
15
special feature
This assessment turned out to be
correct and today the company routinely uses the full functionality of the FT4
Powder Rheometer, measuring shear,
dynamic and bulk powder properties.
All measured properties provide valuable insight into the how the powder will
behave in an AM machine but dynamic
properties are proving particularly helpful for defining flowability under the
low stress conditions that apply in most
parts of the AM process.
Focusing on dynamic
testing
Figure 2: Selective Laser Melting uses a scraper, rake or roller (‘Levelling System’) to spread
a thin layer of powder across a target area where a component is built up layer by layer.
Figure 3: Dynamic powder testing determines the flowability of a powder from measurements
of the axial and rotational forces acting on a rotating blade.
testing equipment because these tests
alone were insufficient to rationalise
the differences in AM performance
observed using different powders.
Powder batches with apparently
identical specifications could not be
relied upon to behave identically in a
machine. This was especially evident in
newer machines which work at faster
rates and are more demanding in terms
of powder performance. In addition,
for certain powders some of the tests
became inoperable. Hall flow rate for
example cannot reliably be measured
for powders that are not free-flowing.
A more sensitive, universally applicable
tester was required.
The choice of which powder tester to
invest in eventually came down to two
options: a shear cell or an FT4 Powder
Rheometer. The shear cell required a
16
MPR September/October 2014
lower initial investment but the research
team concluded that it was more suitable for silo applications and hopper
design than for testing powders under
all the conditions encountered in AM
machines. The FT4 Powder Rheometer
offered dynamic testing and bulk property measurement as well as shear cell
analysis, allowing testing under a broad
range of conditions, including direct
quantification of the impact of air on
powder flowability. This multi-faceted,
more relevant testing capability was
a deciding factor in the decision to
purchase. Working at the forefront of
engineering new powders for innovative
manufacturing, LPW Technology felt
that having access to multiple test protocols would bring value over the long
term, especially as powder testing is a
defining aspect of its business.
Dynamic powder testing measures powders in motion, rather than in a static
state. Dynamic powder properties are
determined from measurements of the
axial and rotational forces acting on a
blade as it is precisely rotated through a
powder sample (Figure 3). These properties include Basic Flowability Energy
(BFE), which is a measure of confined
flow properties in a low stress powder,
and Specific Energy (SE), which quantifies the unconfined flow properties of a
powder in a low stress state. SE is particularly relevant for predicting powder
flow in low stress applications, such as
gravitational dispensing.
Dynamic testing is repeatable,
reproducible and offers high sensitivity, which means it can detect subtle
differences between samples that are
otherwise similar in many respects. The
technique’s ability to measure powder
behaviour under conditions of low
stress clearly differentiates it from shear
cell testing and makes dynamic powder
testing particularly suitable for simulation of the AM process. The case study
below illustrates one of the ways in
which dynamic testing is being used to
improve manufacturing efficiency.
Case study:
Investigating the
feasibility of metal
powder re-use
A major multinational asked LPW
Technology to carry out an experimental study to compare the properties of
‘used’ and virgin powder to support
metal-powder.net
special feature
efforts to implement a successful re-use
strategy for its AM powders. Powder
bed and laser deposition technology
both require the use of significant
amounts of powder, not all of which
becomes part of the finished component. Powder re-use offers the potential
to reduce both raw materials costs and
overall levels of waste significantly.
However, re-use requires careful assessment of the extent to which powders
are altered by passing through AM
machines and whether further processing is possible without compromising
the quality of the finished component.
The key objectives of the study were to
determine if critical characteristics of
the used powder differed from those of
the virgin material and if so what strategies might be successful in returning
the powder to a condition that would
enable its re-use.
“Identification and
consistent supply of
powders able to meet
the exacting demands
of these machines”
Dynamic testing of the virgin
and used material was carried out
to determine any difference in flowability (see Figure 4). A standard FT4
test methodology for measuring BFE
was employed, involving seven repeat
measurements at a blade tip speed of
100 mm/s. Between each individual
measurement a conditioning procedure
ensures that the powder is tested in a
steady state and produces results that
are highly repeatable [1].
Comparing the results for the virgin
and used powders shows that processing has significantly increased the flow
energy of the powder. This indicates
that the used powder would not flow as
freely as the virgin material and consequently is less likely to successfully perform in the process. Powder exiting an
AM machine may contain splatter from
the melt pool, in the form of larger
particles, or may have changed chemically, picking up surface contaminants
for example. Experiments were therefore undertaken to determine whether
metal-powder.net
Figure 4: Flowability measurements show that used powders have poor flowability relative to
the new material. Blending helps to produce a material that can be re-used with confidence.
sieving the used powder would return
it to a state where its flow energy was
acceptable. Here sieving improved powder flowability but did not return it to
the original flow energy values measured for the virgin material.
Further experiments were then conducted to see if the used and virgin
powders could be blended together to
form an acceptable feed for subsequent
processing. A ratio of 75% virgin to
25% used powder produced a flowability most similar to that of the fresh
powder. The 25% used to 75% virgin
blend also exhibited relatively good
performance. The 50:50 blend had the
highest BFE of all the blended samples,
which indicates that flowability does
not change linearly with respect to the
volume of fresh powder present.
These results highlight the ability of
dynamic testing to detect subtle changes
in powders that are of direct relevance
to their performance in AM machines.
As a result dynamic testing can support
successful optimisation and lifecycle
management of metal powders for AM,
in a way that other powder flow testers
cannot.
the development of high-speed, precision machinery, and on the identification and consistent supply of powders
able to meet the exacting demands of
these machines. Increasingly the focus
is turning to the powders themselves
and how they can be optimised in an
intelligent and reliable way. Powder
characterisation has a vital role to play
in supporting this process and testing
techniques that can reliably measure properties that correlate directly
with AM performance are essential.
Experience suggests that multifaceted powder characterisation based
on dynamic, shear and bulk property
measurement is a productive strategy
for generating the information required
to drive progress.
Developing processes
for the future
Further information
The extent to which AM will shape
the industrial landscape depends on
Reference
1. Freeman R. “Measuring the flow
properties of consolidated, conditioned and aerated powders – A
comparative study using a powder
rheometer and a rotational shear
cell”, Powder Technology 174 (2007)
25–33.
LPW Technology Ltd;
www.lpwtechnology.com
Freeman Technology Ltd;
www.freemantech.co.uk
September/October 2014 MPR
17

Download Report