Microstructure of MS1 Maraging Steel in 3D-Printed Products after Semi-Solid Processing.
Rubesova, Katerina ; Jenicek, Stepan ; Kana, Josef 等
Microstructure of MS1 Maraging Steel in 3D-Printed Products after Semi-Solid Processing.
1. Introduction
Although the principle of thixoforming has been known since the
early 1970s, the process is still ranked among unconventional
metalworking methods. It relies on the thixotropic behaviour which
occurs in materials heated into the semi-solid state, i.e. to a
temperature between their solidus and liquidus. Thixoforming belongs to
precision casting and forming methods. During the forming process, shear
forces act on the partially melted material and cause its viscosity to
decrease significantly. This effect is known as thixotropy [1]. The
liquid fraction in the workpiece being formed is normally between 10 and
60% [2]. The technique was initially developed mainly for low-melting
materials: aluminium, magnesium and tin alloys [3]. Thanks to intensive
research, commercial machines for the thixoforming of low-melting alloys
are already (though scarcely) in use. Yet, when it comes to the
thixoforming of steels, basic research is lagging behind due to
technological complexity of the issue. It is unfortunate, considering
the vast potential of this technology. Nevertheless, semi-solid forming
enables difficult-to-form materials to be worked by means of a single
forming operation. In addition, more complex and intricate shapes can be
produced by this technique than by conventional forming routes. In
addition, unconventional microstructures can be produced by thixoforming
even in ordinary materials.
The research into the use of steels has made less progress than
with low-melting alloys because it faced substantial technological
difficulties. Among them are the high process temperatures, narrow
temperature intervals, and the ensuing need for strict control of the
temperature field [4]. The only detailed description of this process
available so far in literature concerns high-alloy steels. Up to now, no
comprehensive research into the behaviour of low-alloy medium-carbon and
low-carbon steels has been undertaken--which is the focus of the present
experiment.
Its objective was to explore the capabilities of mini-thixoforming
of 3D-printed steel workpieces, and obtain the desired type of
microstructure by the process. Mini-thixoforming enables near- net-shape
products to be obtained. These are geometrically accurate and require
only minimal subsequent finishing for the final surface quality and
geometric accuracy, unlike the conventional formed products.
Mini-thixoforming therefore offers material, energy, and financial
savings.
2. Experimental programme
For this investigation, the low-carbon MS1 tool steel in the form
of 3D-printed feedstock was chosen as the experimental material (Tab.
1). It is one of the popular materials for 3D printing. It is a
precipitation-hardenable steel whose properties after conventional
treatment have been thoroughly mapped. Maraging steels belong to the
strongest homogeneous steel materials ever. They are alloyed with
cobalt, molybdenum, titanium, and aluminium. Their unusual properties
make them candidates for heavy-duty parts operating predominantly under
dynamic loading. Their semi-solid processing parameters window is very
narrow due to their chemical composition. This makes the choice of
suitable thixoforming temperature all the more difficult.
In order to identify suitable thixoforming parameters, the physical
and chemical properties of the material and their profiles throughout
the entire heating process must be known. one of the key characteristics
relevant to semi-solid processing is the width of the interval between
the solidus and liquidus. The interval recommended specifically for
minithixoforming is 10-30% [5, 6]. Finding these parameters
experimentally is time-consuming. In this case, the first step involved
calculations which were completed using the JMatPro software. As the
chemistry of this steel is extremely complex, the results should be
considered with caution, not to mention the fact that the actual process
is very rapid. The calculated theoretical onset of melting is at
1395[degrees]C. The material becomes completely melted at 1445
[degrees]C (Fig. 1). Hence, the freezing range width is approximately
50[degrees]C. According to the calculation, the theoretical interval
that is usable for semi-solid forming is 1408-1420[degrees]C. The
advantage of this material is that its melting curve is smooth without
any major curvature. This is favourable for the temperature field
control. The narrow processing parameters window and the high
temperature pose major difficulties for the process.
The experiments were carried out in special mini-thixoforming
equipment which had been designed for handling and processing a very
small amount of material in partially melted state (Fig. 2). It relies
on high-frequency electrical resistance heating for precise temperature
control and generation of the required temperature fields in the
workpiece [7]. The actual mini-thixoforming operation consisted in
lateral extrusion into a mould (Fig. 3).
The blanks for the experiment were made of the MS1 Maraging Steel
by 3D printing. The total length of the blank was 48 mm, of which the
length of the effectively worked part was 38 mm. The diameter was 6 mm.
Both ends of the blank were conical with the apex angle of 28[degrees].
Their purpose was to centre the blank in the die and provide contact
surfaces for energy transfer for heating.
The blanks and the resulting semi-solid processed workpieces were
examined under a microscope in both longitudinal and transverse
directions. Since the input material for printing was powder, the entire
technology chain that governed the microstructural evolution was
documented by metallographic techniques.
3. Discusion of results
The input material for making the blanks was the commercial powder
supplied by the manufacturer of the 3D printing equipment. Its particle
size was 5-50 [micro]m (Fig. 4).
The 3D printing process was based on laser melting and provided a
dense structure of parallel buildup layers with shallow penetration
between individual laser paths. In order to acquire excellent
properties, maraging steels must be subsequently precipitation-hardened
by means of heat treatment. The heat treatment of the buildup therefore
provides mainly precipitation of particles of intermediate phases. A
major difficulty in the metallographic analysis of these materials is
the difference in the etch response of their various states. Nital
etching effectively reveals individual laser beam passes and the sharp
boundaries between layers of melted metal. In addition, it clearly shows
the widths of individual melted regions and the directions of laser
passes (Fig. 5a,b). The drawback of this etching reagent is that it
inadequately reveals the actual microstructure of the solidified metal.
For this reason, an etchant of a mixture of HCl and [H.sub.3]P[O.sub.4]
acids (Etchant 2) was used. It effectively reveals the martensitic
microstructure of this steel in its non-hardened condition (Fig. 6a,b).
At this stage of treatment, the hardness of the workpiece was 360 HV10.
The next operation was stress relieving. As expected, the
workpieces showed no considerable microstructural changes, apart from
the desired reduction in stress. Considerable changes occurred after the
hardening step: uniformly-distributed fine precipitates of intermediate
phases have formed in the martensitic structure (Fig. 7a,b).
The final step of the process chain was mini-thixoforming. It
comprised heating to 1440[degrees]C, holding for approximately 10
seconds, and lateral extrusion of the workpiece material into the mould
held at room temperature. The heating time was 68 seconds. The forming
stroke finished in 0.3 seconds and was followed by rapid solidification
thanks to the contact with the surface of the metal die cavity.
An analysis of the product has shown that when appropriate process
conditions are used, maraging steel can develop a microstructure that is
characteristic of products made by semi-solid processing. Such a
structure consists of polyhedral islands of 20-40 [micro]m size embedded
in a uniform closed network. The fact that it evolves from semi-solid
state is confirmed by a characteristic defect which was found by
accident in the melted and solidified material between polyhedral
islands (Fig. 8). As the region of the semi-solid state available for
processing this maraging steel is very narrow, melting also occurred
within some of the polyhedral islands. The growth of such melted spots
had no immediate constraints which is why they are more or less
spherical in shape and have no contact with the network which separates
the polyhedral islands. During this process, the thixotropic behaviour
was not manifested strongly. Therefore, this material can be considered
to be difficult to minithixoform. The next research efforts will focus
on the redistribution of chemical elements and on the evolution of
microstructural constituents in this process.
4. Conclusion
A maraging steel was chosen for this mini-thixoforming process. The
objective was to test a combination of 3D printing and semi-solid
processing. A simple experiment involving lateral extrusion provided
information on complex microstructural evolution in the technology chain
considered. Suitable and achievable processing parameters have been
found which can provide the desired structural state upon semi-solid
processing, rapid solidification, and cooling. Given the high processing
temperature and the narrow temperature interval, the process became a
great technological challenge. Nevertheless, it was completed
successfully. Despite that, current findings suggest that at these
parameters, the thixotropic property of this material is not favourable.
Unless more suitable parameters are found, this material cannot be used
in this 3D printing-thixoforming process chain.
DOI: 10.2507/27th.daaam.proceedings.070
5. Acknowledgements
This paper presents results achieved under the project LO1502
Development of Regional Technological Institute. The project is
subsidised by the Ministry of Education of the Czech Republic from
specific resources of the state budget for research and development.
6. References
[1] Fleming M.C. Behavior of Metal Alloys in the Semisolid State
(1991). Metall Trans A, 22: 957-981.
[2] Thixoforming: Semi-solid Metal Processing. Edited by HIRT G.
and KOPP R., pp 1-27, Copyright 2009 WILEY-VCH Verlag GmbH & Co.
KGaA, Weinheim ISBN: 978-3-527-3220 4-6.
[3] Puttgen W., Bleck W., Hirt G., Shimahara H., Thixoforming of
Steels--A Status Report, Advanced Engineering Materiale 9, (2007), No. 4
234-245.
[4] Vancura F., Vorel I., Pilecek V., Masek B. Material
Technological Modeling of Thermomechanical Processing of Die Forging of
Microalloyed Steel, Kovarenstvi, 2015, ISSN 1213-9289.
[5] W. H. Bauer and E.A. Collins, in F.R. Eirich (Ed.), Rheology:
Theory and Applications, Vol. 4, (1967) Academic Press, New York, ch. 8.
[6] Wolf, A., Baur, J., Gullo, G.C. Thixoforging, (2012), Available
from: http://www.cct-
bw.de/veroeffentlichung_pdf/WoBa%20Massiv01%20english.pdf.
[7] Ronesova, A., Masek, B., Stankova, H., Stadler, C. Patent CZ c.
299758-- Method of handling and its shaping at a temperature between
solid and liquid, (3. 10. 2008).
This Publication has to be referred as: Rubesova, K[aterina];
Jemcek, S[tepan]; Kana, J[osef] & Zetkova, I[vana] (2016).
Microstructure of MSI Maraging Steel in 3D-Printed Products After Semi-
Solid Processing, Proceedings of the 27th DAAAM International Symposium,
pp.0467-0472, B. Katalinic (Ed.), Published by DAAAM International, ISBN
978-3-902734-08-2, ISSN 1726-9679, Vienna, Austria
Caption: Fig. 1. Calculated feasible forming interval
Caption: Fig. 2. Typical product manufactured by Mini-thixoforming
and the tool in forming machine
Caption: Fig. 3.--Principle of Process
Caption: Fig. 4--Maraging steel powder before printing REM
Caption: Fig. 5--Laser melted microstructure after D3 printing
Caption: Fig. 6a--optical micrograph, etched with a mixture of HCl
and [H.sub.3]P[O.sub.4] acids
Caption: Fig. 6b--REM micrograph, etched with a mixture of HCl and
[H.sub.3]P[O.sub.4] acids
Caption: Fig. 7--Microstructure after hardening
Caption: Fig. 8. Micrograph of maraging steel after semisolid
processing
Table 1. Chemical composition of the MS1 Maraging steel
measured by chemical analysis
Ni Co Mo Ti Al
17-19 8.5-9.5 4.5-5.2 0.6-0.8 0.05-0.015
Ni Cr, Cu C Mn, Si
17-19 [less than or [less than or [less than or
equal to] 0.5 equal to] 0.3 equal to] 0.1
Ni P, S Fe
17-19 [less than or balance
equal to] 0.01
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