The purpose of this study is to determine as precisely the influence of friction in the compaction of powders, and "conversion" of it, increasing densification.
The process is intended to be used in the compaction of hard deformable materials, which increased to approximately 90% densification required very high compaction pressures. These pressures are high, significant wear of the tools produce the compaction and thereby increasing production costs of finished parts from metal powders

Ionuţ MARIAN

Liviu NISTOR

Luciana RUS

Marius TINTELECAN

Monica SAS-BOCA

The Annals of “Dunarea de Jos” University of Galati Fascicle IX Metallurgy and Materials Science

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197 6
 
 
THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2011, ISSN 1453 – 083X 
 
 
 
 
THE INFLUENCE OF FRICTION FORCES IN METAL POWDERS 
COMPACTION PROCESS OF STAINLESS STEEL 316L 
 
Monica SAS-BOCA, Luciana RUS, Marius TINTELECAN, 
Ionuţ MARIAN, Liviu NISTOR 
Technical University of Cluj-Napoca 
e-mail: Monica.Sas.Boca@ipm.utcluj.ro  
 
ABSTRACT 
 
The purpose of this study is to determine as precisely the influence of friction 
in the compaction of powders, and "conversion" of it, increasing densification.  
The process is intended to be used in the compaction of hard deformable 
materials, which increased to approximately 90% densification required very high 
compaction pressures. These pressures are high, significant wear of the tools 
produce the compaction and thereby increasing production costs of finished parts 
from metal powders. 
 
KEYWORDS: friction forces, “conversion”, compaction, metal powder 
 
1. Introduction 
 
Technological processes that use pressure to 
achieve plastic deformation state are common element 
of friction forces. These forces can be active or 
resistive character depending on the context of their 
occurrence as: desirable or undesirable phenomenon 
in engineering. 
Whatever the process of compaction, it involves 
a complex: factors, material and process variables. 
The ability to understand all the factors that influence 
individual particles are in a compact area of 
fundamental interest both for carrying out theoretical 
models and practical applications of these models. 
One of the factors specific to the compaction process 
is friction [2]. 
If for materials "full" (cast, forged, rolled), the 
force of friction depends on the type of contact (ie. a 
single type of contact throughout the deformation) 
and it causes one type of motion (sliding, rolling, the 
spin or the impact or combinations thereof), where 
powder densification of P/M can be no succession or 
simultaneously all types of motion caused by contact 
kinematics, during the process [3]. 
Types of friction in different stages of 
densification of powders are: 
- Friction that occurs between two or more 
powder particles in contact between them, called 
static friction - friction contact (τ ). It is given by the 
normal tension ( nσ ) on the contact surface (Fig. 1) 
and coefficient of friction (μ ). 
- Friction that occurs when starting the pressing, 
the particles tumble while some others besides 
occupying vacancies - rolling friction (Fig. 1 b). 
At this stage the powder particles are not 
deformed, the contact between them resumed at one 
point. As a result of the existence of gaps, because of 
its filling, the particles will move to these areas (least 
resistance) under the action of deformation tools. 
Friction is in turn influenced by: the relative 
kinematics of contact, geometrical surface, the nature 
of the materials that form the coupling, the chemical 
composition of the body deformed and the 
deformation tool, the presence of impurities and a 
third coupling material, forming pressure (degree of 
deformation), deformation temperature, the relative 
speed of sliding contact surface. 
Rolling friction is a complex phenomenon and is 
dependent on mechanical factors, physical and 
chemical. This type of friction can be classified: 
- Rolling with high shear forces; 
- Rolling with small tangential forces of friction 
is called "roll off".  
- Friction that occurs at the start of deformation 
of particles, the elastic deformation - we call 
intermittent friction ("stick-slip") figure 1 c) [3, 4]. 
Particles have no where to move, can only rotate and 
deform (elastic deformation), thereby increasing the 
contact area between particles and between particles 
and walls tool. 
- Friction that occurs between particles during 
the compaction effect, in which plastic deformation 
predominates.  
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2011, ISSN 1453 – 083X 
 
 
At this stage the particles are in intimate contact 
(Fig. 1 d), so the entire surface of the particle is in 
contact with other particles or walls of the pressing 
tool. We call this type of friction sliding friction.  
The same type of friction is found in contact 
with the walls of the powder forming tool at this stage 
of densification. 
 
  
a) b) 
  
c) d) 
Fig. 1. Powder compaction: 
a) powders are brought into contact by filling the die, b) early stage of deformation there is still 
pressing the powder particles, the particles stick to each other, spinning and tumbling vacancies 
c) compaction beginning: the emergence elastic strain at the contact between particles,  
d) effective compaction, material flow occurrence at the contact between particle and particle-
surface tools. 
 
- Ejection friction of green compacts process. 
This type of friction occurs between green compact 
and forming tools. In this phase may bring about a 
new compaction is the result of an improvement 
densification, or surface quality of green compacts. 
This friction is similar to the friction between blank 
and tools for solid materials.  
The direction of this frictional force is the 
opposite movement of the material. Friction between 
particles of powders and between powder and tooling 
plays a major role during densification of PM parts. 
As a result there is a densification of green edges 
compacts. 
 
2. Experimental procedure 
 
Material. In this paper was used stainless steel 
powder 316L produced by gas atomization, with a 
particle size < 120 μm.  
Cold compaction with assisted friction. Before 
compaction the powder was mixed with zinc stearate 
in a ratio of 0.5%.  
Green compacts have been carried out on an 
tensile-compression test installation (Heckert) with 
maximum force of 200 kN, driven by a hydraulic 
cylinder located at the bottom.  
The proposed process consists in moving the 
mold during the pressing, die movement in the same 
direction with a given speed (fig. 2). As a result, the 
frictional forces acting in the same direction as the 
punch achieving a better distribution of powder flow 
during compaction process. 
Activation of friction [1, 5], plastic deformation 
processes, create a peripheral flow which causes the 
material in the central and peripheral axial-flow 
direction (deformation) (fig. 2,b)). Material from 
these two regions are moving simultaneously in the 
mold resulting in reduction of strains and increase 
their uniformity, strain, working speed and the degree 
of deformation. Thus, this procedure creates 
additional opportunities to reduce deformation due to 
flow uniformity and standardization of their right 
under certain conditions.  
Sintering. Green compacts were sintered in a 
vacuum furnace room is equipped with an electric 
heater and temperature control, so they were 
maintained in vacuum (10-4 bar) at a temperature of 
1200 °C for 60 minutes. 
 
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FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2011, ISSN 1453 – 083X 
 
 
 
 
a) b) 
Fig. 2. Friction assisted compaction of 316L stainless steel powder 
a) density distribution, b) compaction process Vm> Vp 
 
3. Results and discussion 
 
Cylindrical green compacts were made of 6.5 
grams and 10.5 grams with classical friction assisted 
compaction at a pressure of 600 MPa and 700 MPa. 
The inner diameter of the die is 11.12 mm, compacts 
obtained with varying heights depending on the 
weight, pressure and compaction process.
 
 
Table 1. Densities obtained in the process of compaction 
 
No Compaction process Pressure Weight Green density Density 
  [MPa] [gr] [gr/cm3] [gr/cm3] 
1 600 6.29 6.49 
2 
with assisted 
friction 700 6.50 6.67 
3 600 6.60 6.76 
4 
classical 
700 
10.5 
6.76 6.92 
5 600 6.45 6.61 
6 
with assisted 
friction 700 6.60 6.69 
7 600 6.60 6.79 
8 
classical 
700 
6.5 
6.76 6.97 
 
Densities were obtained after sintering fig. 3 
between 82% and 89%, in line with the literature [4] 
(to 1240°C value obtained is 90% of theoretical 
density), taking into account the conditions of 
sintering.  
Density gradient was determined by removing 
the turning of 1 millimeter of material in radial 
section. Height density distribution reported in parts 
was determined by Archimedes method. Comparison 
of density gradient was performed for both processes 
at different compaction pressures (600 MPa and 700 
MPa).  
As can be seen, in fig. 3 a) and c) we obtained a 
uniform density distribution for samples of 10.5 
grams with assisted compaction achieved by friction, 
but with lower values 1% ÷3% for samples compacted 
classic. The results obtained on samples of 6.5 grams 
comply with the trend to uniform density, which leads 
to the conclusion that the activation of friction 
produces densities with 1% ÷ 3% lower, but earn 
more by getting a better gradient. As a consequence, 
the fill height of the die is no longer conditioned to 3x 
height of compact. These results are applicable for 
parts with large lengths. 
In what follows (fig. 4) presents microstructures 
in the process of sintering compacts in a vacuum. 
Metallographic samples were prepared as follows: 
longitudinal were polished with abrasive paper of 
500-1200 grit, then polished with alumina and were 
etched with royal water (HCl 33%, 66% HNO3). 
It is noted that the material was sintered well. 
The resulting structure is similar to a molten metal, is 
observed grains (crystallites - white areas) with 
polygonal boundaries clearly outlined. The 
microstructures presented for both classical and 
assisted compaction processes are very similar. In 
both, approaches can be distinguished macles in 
specific grain austenitic steels and fine carbides (small 
black dots on grains), due to alloying elements, the 
basic blend of grain weight. 
 
V
VV
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FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2011, ISSN 1453 – 083X 
 
 
10.5 gr 6.5 gr 
6.00
6.20
6.40
6.60
6.80
7.00
7.20
7.40
8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00
Height [mm]
D
en
si
ty
 [g
/c
m
^3
]
 
6.00
6.20
6.40
6.60
6.80
7.00
7.20
6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50
Height [mm]
D
en
si
ty
 [g
r/c
m
^3
]
 
a) b) 
6.60
6.65
6.70
6.75
6.80
6.85
6.90
6.95
8.00 9.00 10.00 11.00 12.00 13.00 14.00 15.00 16.00
Height  [mm]
D
en
si
ty
 [g
r/c
m
^3
]
 
6.70
6.75
6.80
6.85
6.90
6.95
7.00
6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50
Height  [mm]
D
en
si
ty
 [g
r/c
m
^3
]
 
c) d) 
 classical compaction  friction assisted compaction 
Fig. 3. Density gradient of stainless steel powder, 316L 
a), b) pressure at 600 MPa; c), d) pressure at 700 MPa. 
 
So, if compact 6.5 grams, fig. 4, 5 a), b) and for 
those of 10.5 grams, fig. 4, 5 c), d), we see a greater 
number of grains in the microstructures obtained at 
compaction with friction assisted fig. 4 b), d). This is 
due to the presence of friction assisted, given that the 
same conditions were observed in compaction.
 
  
a) b) 
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2011, ISSN 1453 – 083X 
 
 
  
c) d) 
Fig. 4. Microstructures of 316L stainless steel samples compacted at pressure of 600 MPa: 
a) classical compaction of 6.5 g samples, b) with friction assisted compaction of 6.5 g samples, 
c) classic compaction samples of 10.5 g d) compaction with friction assisted samples 10.5 gr. 
 
It can be seen in the microstructure, the presence 
of an amount of pores (dark areas), this fact it is 
specific sintered structures. Porosity decreases 
according to the method of compaction, compaction 
pressure and sintering temperature.  
The explanation for the existence of an 
increased amount of pores, or pore size may result 
from the dependence of compaction speed. A too 
rapid compaction (low height for samples) can be 
achieved without the grain to find the most 
advantageous positions of green compact volume. 
The boundaries between grains are well defined 
and can be seen present carbides in the grain.
 
 
  
a) b) 
  
c) d) 
Fig. 5. Microstructures of 316L stainless steel samples compacted at pressure of 700 MPa: a) 
classical compaction of 6.5 g samples, b) with friction assisted compaction of 6.5 g samples; c) 
classical compactness samples of 10.5 g d) compaction with friction assisted samples of 10.5 grams. 
 
Conclusions 
 
It was comparatively studied in this paper, two 
methods of densification of powders, classic - 
bilaterally and with friction assisted. Differences 
between classical and assisted friction compaction is 
relatively low, due to the small size of samples. The 
compact with longer lengths could be more relevant 
results.  
The main difference between the two processes 
is given by the influence of frictional forces between 
particles, powders and tools. 
 
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FO N D ATĂ
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THE ANNALS OF “DUNAREA DE JOS” UNIVERSITY OF GALATI. 
FASCICLE IX. METALLURGY AND MATERIALS SCIENCE 
N0. 2 – 2011, ISSN 1453 – 083X 
 
 
Advantages:  
• uniform density distribution of the 
green compact height 
• removal of the neutral zone 
• low density in the case of the classical-
bilateral compaction 
• unconditional height filling of compact 
height 
• activation of friction between the 
container and the powder effecting in 
the reduction of the pressing force 
applied to the die 
• minimum implementation costs in 
industry, concerning only the 
movement of the mobile device for 
driving the mold 
• long time suitability of friction assisted 
compaction for resistive deformable 
materials. 
 
 
Disadvantages: 
• compact density reduction by 1% ÷ 3%  
• implementation costs of the appropriate 
process  
• location of the training devices on the 
existing press. 
 
References 
 
[1]. V. Lazăr, L. Nistor, I. M. Sas-Boca Optimizarea parametrilor 
de compactizare-sinterizare în procesarea pulberilor din oţel 
inoxidabil, Metalurgia nr. 5-2008, pag 36-49. 
[2]. Sh. Keshavarz, A.R. Khoei, A.R. Khaloo Contact friction 
simulation in powder compaction process based on the penalty 
approach, Materials and Design 29 (2008) 1199–1211. 
[3]. A. Khoei Computational Plasticity in Powder Forming 
Processes, Great Britain, 2005. 
[4]. N. Kurgan, R. Varol, Mechanical properties of P/M 316L 
stainless steel materials, Powder Technology 201 (2010) 242–247. 
[5]. I.M Sas-Boca., T. Canta, D. Frunza Pressing Powder with 
Friction Assisted Compaction for HS Tool Steels, Proceedings of 3-
rt Internatioan Conference on Powder Metallurgy, RoPM 2005, pag 
640-648.  
 
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The Influence of Friction Forces in Metal Powders Compaction Process of Stainless Steel 316L

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The Influence of Friction Forces in Metal Powders Compaction Process of Stainless Steel 316L

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