Advanced Powder Technology 26 (2015) 401–408
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Advanced Powder Technology
journal homepage: www.elsevier.com/locate/apt
Original Research Paper
Iron powder-based graded products sintered by conventional method
and by SPS
Krzysztof Zarebski a,⇑, Piotr Putyra b
a
b
Institute of Materials Engineering, Faculty of Mechanical Engineering, Cracow University of Technology, Poland
Centre for Materials Research and Sintering Technology, Institute of Advanced Manufacturing Technologies, Poland
a r t i c l e
i n f o
Article history:
Received 3 September 2014
Received in revised form 22 October 2014
Accepted 17 November 2014
Available online 29 November 2014
Keywords:
Functional graded materials (FGM)
Powder metallurgy (PM)
Spark Plasma Sintering (SPS)
Distaloy SE
a b s t r a c t
Iron powder-based graded products sintered by various methods were evaluated. Cylindrical samples
were made from the Distaloy SE powder with different carbon content in the outer layer and core.
Between the outer layer and the core, a transition zone was formed as a result of the application of special
die filling technique providing mixing of the two powder compositions. The applied technique allowed
the transition zone to be formed not only by the diffusion process during sintering, but earlier at the stage
of product shaping. Two methods of sintering after compaction were tested, i.e. pressureless sintering
and pressure-aided sintering using pulsed current, known as Spark Plasma Sintering (SPS).
Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder
Technology Japan. All rights reserved.
1. Introduction
The traditional and modern techniques of powder metallurgy
allow the manufacture of materials of nearly any composition
and a wide range of controlled characteristics and microstructure.
Suitable chemical composition enables control of final properties
in different layers of the product, according to the requirements
imposed. However, manufacture of products with characteristics
different in the outer layer and core requires the presence of transition zone formed between the materials used. The properties can
vary in any previously scheduled direction. Depending on the
existing needs and opportunities, changes in microstructure, and
hence in the material properties, can occur in different layers of
the product in a jumping mode, stepwise, or continuously (Fig. 1).
A wide range of materials with different properties are shaped
in products in which the surface layer is characterised by high
mechanical strength, thermal resistance, corrosion resistance, or
abrasive wear resistance, while completely different properties,
like high toughness and dynamic resistance, are preserved in the
core of the product. These materials are produced by traditional
and modern, developed in recent years methods like casting [1],
infiltration [2], laser [3] and other surface layers technique. A very
important group are functional materials used for all kinds of medical implants, known under the general name of biomaterials,
⇑ Corresponding author.
E-mail address: kazar@mech.pk.edu.pl (K. Zarebski).
which should have adequate strength, biocompatible and high corrosion resistance [4].
Producing the transition zone and providing the required
graded structure is relatively easy when it is formed through
diffusion process during sintering, although even in such cases
the short sintering time may not be sufficient to ensure proper
thickness of the gradient zone. This problem occurs especially
when the composition of the tested material includes elements
with different diffusion coefficients. Then, one or more intermediate layers can be produced by mixing in appropriate proportions
different materials at the stage of product shaping. Mixing can be
done on materials in a loose form or on specially prepared powders
and slurries. Different methods are used to induce the sedimentation process, e.g. centrifugal force [5]. The essential difficulty in the
technology of making graded products is the difference in physical
and thermo-mechanical properties of the applied materials which
affects the course of the entire manufacturing process, from
moulding through sintering, and in the final heat treatment or
machining ending. Conventional pressureless sintering can lead
to differential shrinkage and the formation of internal stresses.
Many researchers involved in modelling the size and evolution of
the gradient composite materials. In the paper [6] it is showed
the effect of relaxation on the formation of cracks in gradient materials. Interesting model to predict the formation, development and
final nature of cracks is presented in [7].
One of the latest technologies for shaping and sintering of
functionally graded materials is the method of SPS (Spark Plasma
Sintering) using for the consolidation of powder a combined
http://dx.doi.org/10.1016/j.apt.2014.11.010
0921-8831/Ó 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
402
K. Zarebski, P. Putyra / Advanced Powder Technology 26 (2015) 401–408
heating effect of pulsed current and pressure [8]. SPS enables the
production of materials which are difficult to be sintered by
conventional techniques, including functionally graded materials
reinforced with ceramics [9], and nanocrystals [10]. Sintering is
carried out at a lower temperature and shorter time compared to
the other sintering methods, does not cause grain growth and
allows obtained a very good physical properties of sintered materials [11]. Among the available publications can be found many
studies on different methods of producing sintered alloy based
on iron powders and their compositions. There are papers describe
physical phenomena and modelling of activated sintering processes [12] and sintering alloy steels based on iron by this method
[13]. However, there is not so many works comparing the sintering
materials obtained by various methods. In paper [14] is presented
a comparison of the mechanical properties of sintered high-speed
steel and tool steel sintered by SPS and HIP method.
This paper presents a comparison of gradient materials based
on two iron powder composition Distaloy SE with carbon addition
obtained by free sintering by SPS method. The gradient structure is
achieved by a special method of filling a die, mixing of iron powders and as a result of diffusion processes during sintering. SPS sintering was carried out with different parameters: temperature and
duration. The analysis and comparison of the physical properties,
porosity and microstructure sintered gradient materials are presented in paper. Determined possible use of die filling method
and SPS sintering process as an alternative technology for obtaining of materials with different in the surface layer and in the core.
2. Test material
Test products were made from the Distaloy SE iron powder
manufactured by Hgans SA, commonly considered an equivalent
of low-alloy structural steel, designed for items undergoing final
heat treatment directly after sintering by the method of ‘‘sinterhardening’’. Two mixtures of the powder were prepared and were
used for cylindrical samples with height-to-diameter ratio close to
unity (h=d 1), as shown in Fig. 1. Table 1 gives the chemical
composition of the powder (manufacturer’s data), as well as the
composition of respective mixtures used for the test products,
including the results of theoretical density measurements taken
by a pycnometric method. Mixture A had the starting chemical
composition of the Distaloy SE powder. Mixture B contained
0.6 wt% of carbon added in the form of synthetic graphite of the
trade name Timrex F10 PM Special Graphite symbol N-010. Both
mixtures additionally contained the Kenolybe P11 slip agent added
in an amount of 0.5 wt%; to mixture B it was added after mixing
the powder with graphite. Stirring was carried out for 2 h.
The 16 mm diameter core of the product was made from a carbon-free mixture (mixture A); the outer layer was made from a
mixture with the addition of carbon (mixture B). Assuming product
dimensions given in Fig. 2 and ensuring the required volume of
both mixtures, the average theoretical density of a composition
of mixtures A and B used for the product manufacture was determined by pycnometry at a level of qtðAþBÞ = 7.749 g/cm3.
The selected powder mixtures gave products characterised by a
hard 4 mm thick surface layer and a softer 16 mm diameter core.
The weight of the batches of mixtures was calculated assuming a
uniform density of the core and outer layer, the theoretical densities given in Table 1, and the final porosity after compaction and
sintering at a level of Hk = 0.05.
To induce the formation of a transition zone between materials
A and B still at the stage of compaction, a method for filling the die
has been developed and applied, its essence consisting in this that
the two powders are mixed at the core-outer layer interface.
According to this method, the die is filled up in two layers without
the use of pre-compaction. The powder for the core is initially
placed in a special container separating the two mixtures
(Fig. 3a). The place of the container removed prior to compaction
is occupied by a transition zone formed of the mixed powders
A + B (Fig. 3b and c). The compaction of the product is done by
two-sided pressing in a die ‘‘floating’’ on the spring, which guarantees obtaining products of a homogeneous and uniform density
(Fig. 3d).
Two methods of sintering were tested. In the first, conventional
method, products were sintered by a pressure-free technique in a
tube furnace; in the second method, sintering was done in an
HD5-type device made by FCT for the SPS type of sintering applying different process parameters. In each case, sintering was performed on products pre-compacted. Table 2 gives the sintering
process parameters applied in each of the two above mentioned
methods.
The sintering flowchart for products C11 and C13 is presented
in Fig. 4, while temperature profiles for the sintering of products
S1, S11 and S12 are shown in Fig. 5.
3. Tests performed
The physical properties of products were determined after
pressing and after sintering. Detailed metallographic studies were
carried out on polished sections of the sintered products to determine the individual microstructural constituents and pore
morphology.
The following tests were performed:
measurement of the average density of products compacted by
gravimetric method,
Fig. 1. Types of changes in the structure and properties of graded products.
Table 1
Chemical composition of Distaloy SE and the composition of powder mixtures A end B (wt%).
Mixtures code
Carbon (graphite)
Kenolybe P11 (slip agent)
A
B
0.00
0.60
0.50
0.50
Theoretical density (g/cm3)
Distaloy SE
Cu
Ni
Mo
C max.
Fe
1.50
1.50
4.00
4.00
0.50
0.50
0.01
0.01
Rest
Rest
7.801
7.708
ID
144443
Title
Ironpowder-basedgradedproductssinteredbyconventionalmethodandbySPS
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