INVESTIGATION OF MECHANICAL BEHAVIOUR IN FORMING OF SINTERED COPPER-15_TUNGSTEN NANO POWDER COMPOSITE by iaemedu

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									   INTERNATIONAL JOURNAL OF PRODUCTION (IJPTM), ISSN 0976 6383
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  (Print), ISSN 0976 – 6391 (Online) Volume 3, Issue 1, January- December (2012), © IAEME

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ISSN 0976 - 6391 (Online)
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  INVESTIGATION OF MECHANICAL BEHAVIOUR IN FORMING OF
  SINTERED COPPER-15%TUNGSTEN NANO POWDER COMPOSITE

                      N.Vijayponraj1, Dr.G.Kalivarthan2, Vetrivel.S.C3
                1
                 Research Scholar, CMJ University, Meghalaya, Shillong, India
       2
      Principal/Dept of Mech. Engg, PSN Institute of Technology and Science, Tirunelveli,
                    Tamilnadu, India, Supervisor CMJ University, Shillong.
   3
     Asso.Prof/Dept. of Mech. Engg, PET Engineering College, Tirunelveli, Tami nadu, India,
                 E mail: vijay_ponraj@yahoo.com , Sakthi_eswar@yahoo.com

  ABSTRACT

         Experimental investigations are performed in order to predict the mechanism of
  deformation and densification behaviour during cold upset forming operation on sintered Cu-
  15%W Nano composite. High-energy mechanical milling was used to produce Cu and W
  Nano powder composites. Cylindrical preforms with initial theoretical density of 85%
  possessing three different aspect ratios of 0.40, 0.60 and 0.80 were prepared using a die and
  punch assembly with a hydraulic press. The preforms are sintered in an electric muffle
  furnace at 650°C, and subsequently the furnace was cooled. Cold deformation experiments
  are conducted in incremental deformation steps. The relationships between various
  parameters are evaluated
  Keywords: Metal-Matrix Composites (MMCs), Nano-structures, Preform, Mechanical
  properties, Sintered copper-15% Tungsten

  1.       INTRODUCTION

        Composite material is the combination of two or more materials which are having
  different phases and the properties superior to the base materials. Composite materials are
  produced by casting and powder metallurgy methods. By means of casting methods,
  composite materials reinforced by dispersion particles [1, 2], platelets [3], non-continuous
  (short) fibres and continuous (long) fibres [4-5] as well as composite materials with hybrid
  reinforcement composed of particles and fibres [6] are produced by powder metallurgy
  methods, composite materials reinforced by dispersion particles [7-9], platelets [13], non-
  continuous fibres [10-11] and continuous fibres [12] are manufactured.
        High-energy mechanical ball milling can be used to produce nano powders from
  micron size. When mechanical milling is used in the traditional sense, the process does not
  generate a new material, since the microstructure of the powder does not change. Particulate
  composites are prepared by mixing reinforcement and matrix material in powder form and

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International Journal of Production Technology and Management (IJPTM), ISSN 0976 – 6383
(Print), ISSN 0976 – 6391 (Online) Volume 3, Issue 1, January- December (2012), © IAEME

cold pressing to near theoretical density. An important aspect of composites is their
fabricability to near-net shapes. To develop and improve near-net shape forming process the
study on analyzing the fabrication process is necessary as well as experimental study on
fabrication of composites.
       Most of the authors were reported the deformation behaviour of various powder
composites in micro and nano particle sizes except Cu-W. The material properties of the
sintered compacts were ascertained from the uniaxial compression test with 15% Tungsten
with copper three aspect ratios 0.40, 0.60 and 0.80. Graphite was used as surface preform
lubricant. From this study, it was also attempted to establish the relationship between the
fractional theoretical density, the Poisson’s ratio and other parameters namely the stress,
strain and strain factor under Cu-10%W preform compositions.

Table 1 Characteristics of powder
                                     (a) Copper Powder
    Test                  IS 5461             ASTM B-417           ASTM B-         ASTM E-
  Standard                                                            213             194
  Property         Sieve analysis, %              Apparent         Flow rate         Acid
               +75µm +45µm -45µm                density (g/cc)     Sec(50g-1)      Insoluble
EC/86 Grade      0.40      5.40      94.20          1.56               Nil            Nil

                                    (b) Tungsten Powder
         Characteristics                       Test Standard                     Value
     Sieve analysis : - 45 µm                  ASTM D-185                        99.00
  Average Particle Size, Fisher                ASTM B-330                        3.90
             Number
 Oxygen Content (Hydrogen Loss)                 ASTM E-159                       1.87
        Other Impurities                           AAS                           0.22
              Purity                                                             97.90


2. EXPERIMENTAL DETAILS

2.1 Materials and characteristics

     Copper and Tungsten powders were used in the present investigation. These powders
were purchased from M/s. Metal Powder Company (P) Ltd., Tirumangalam, Madurai,
Tamilnadu, India. Electrolytic copper and atomised tungsten were obtained with 100% and
99.00% purity respectively. The characteristic of the copper and Tungsten powder is shown
in Table 1 (a-b). The individual powders were pulverized in a high energy ball mill (Fritsch,
Germany - Pulverisette - 6) for four hours after that it was mixed on weight basis with 5%
Tungsten and rest Copper powder. These composite powders were pulverized in a high
energy ball mill and after 10 hours milling, the obtained particle size was approximately
below 400nm. SEM was used for evaluation of morphological changes of the particles after
milling and is shown in Fig. 1(a-c). Fig. 1(a) shows the SEM image of the Cu particles at
9500X magnification and has a structure of a cluster of tiny particles and like small flattened
flake particles due to severe plastic deformation of copper, micro-welding and fracture of the
large flakes due to typical mechanical milling. Fig. 1(b - c) shows the SEM image of the W
powders at 6000X and 2000X magnification respectively, It is in the formation of flattened
particles with pancake structure. Fig. 1(d) shows the SEM image of the Cu-10%W Powder

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International Journal of Production Technology and Management (IJPTM), ISSN 0976 – 6383
                                                      January
(Print), ISSN 0976 – 6391 (Online) Volume 3, Issue 1, January- December (2012), © IAEME

                                                                         Cu-10%W
Composite at 8000X particles. It shows the morphological changes of Cu 10%W powder
                                   significant
mixture after 10 hours milling. No significant difference between the Cu morphology in the
composite and the monolithic W powder is observed at low milling times; that means the fine
W elements distributed throughout the Cu matrix and represents the particle size in the range
between 200-400nm.




                       (a)                                             (b)
      .
   Fig. 1 SEM micrograph after ball milling (a) Cu powder 9,500 X (b) W powder 6,000X

2.2. Compacting

       Cylindrical compacts of 20 mm diameter with an aspect ratio of 0.40, 0.60 and 0.80
                                                           mi         10%
were prepared. The compacts were prepared using ball milled Cu-10% W composite. The
composite powders were compacted by using suitable punch and die set assembly on a
Universal Testing machine having 1 MN capacity. Compacting pressure was applied
                                                               was
gradually and it was 1.2 GPa for three aspect ratios. Graphite was used to lubricate the punch,
die and the butt. When preparing the compacts, the initial density and aspect ratio were
maintained by precisely controlling the mass and accurately monitoring the compacting
pressure employed.

2.3 Sintering
                         action,
        After the compaction, the compacts were immediately taken out from die set
assembly and loaded into the furnace for sintering. To prevent oxidization, the green
compacts were initially covered with inert argon atmosphere in the furnace. The sintering was
carried out in an inert gas circulated electric muffle furnace at 650°C for a holding period of
one hour. As soon as the sintering schedule was over, the sintered preforms were cooled
inside the furnace itself to the room temperature. After the completion of sintering, t     the
preforms were cleaned by using a fine wire brush.

2.4 Cold Deformation Experiments
        Deformation experiments were carried out by using flat faced dies and a hydraulically
                                                                                    m
operated compression testing machine of having 1MN capacity. The flat dies were machined
and tempered. Flat faces of the dies were ground after heat treatment in a grinding machine,
in order to obtain the final dimensions and surface quality and its hardness was measured as
91 HRB after tempering. Graphite was well applied as lubricant on the ends of preforms and
contacting surfaces of flat dies, which created a situation for almost frictionless ideal
deformation. In general, each compact was subjected to an incremental compressive loading
                                         visible
in steps of 50kN until the appearance of visible cracks on the free surface. Immediately, after
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International Journal of Production Technology and Management (IJPTM), ISSN 0976 – 6383
(Print), ISSN 0976 – 6391 (Online) Volume 3, Issue 1, January- December (2012), © IAEME

the completion of each step of loading, the height, the contact diameters at the top and
bottom, the bulged diameter and the density were measured for each of the deformed
preforms. The density measurements were carried out using Archimedes principle.
Experimental measurements were also used to calculate the various parameters namely the
stresses, the Poisson’s ratio, density ratio and the strain. The Theoretical approach to
calculate the above parameters are discussed in detail under ref. [13].

3. RESULTS AND DISCUSSION

3.1. Deformation Behaviour of Preforms
       The results of deformation behaviour of 85% initial fractional density, Cu-15%W
composite preforms are discussed in this section. The following stress strain relationship
concerning the plastic deformation is discussed in detail for the various aspect ratios with
graphite lubricants

3.1.1. Various stresses and axial strain
        Using the simple theory of plasticity, the axial stress (σz), the hoop stress (σθ) and the
hydrostatic stress (σm) were calculated and plotted against the axial strain (εz) for different
aspect ratios in Figures 2(a –c) for the Cu-15%W composite preforms. The hoop stress is
tensile in nature because during compressive loading the bulged diameter expands.
Meanwhile, for any deformation level, the increase in the hoop stress due to loading is very
low compared to that of the axial stress. However, the value of the hydrostatic stress is much
less than the other stresses, namely the axial stress (σz) and the hoop stress (σθ) and it is also
compressive in nature, at different strain level. For all preforms, the axial and the hoop stress
level also increases for a given aspect ratio. The values of the axial stress, the hoop stress and
the hydrostatic stress are high for the lower aspect ratio (0.40) than higher aspect ratios
(0.60and 0.80) preforms.


                       600

                       400                                        Initial Fractional Density 0.85
                                                                            Cu -15%W
                                                                          Aspect Ratio:0.8
                       200                                             Lubricator: Graphite
           Stess,Mpa




                         0
                                                                           Axial Stress
                              0   0.5              1        1.5
                                                                           Hoop Stress
                       -200
                                                                           Hydrostatic Stress
                       -400
                                                                                  (a)


                       -600
                                    Axial Strain




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International Journal of Production Technology and Management (IJPTM), ISSN 0976 – 6383
(Print), ISSN 0976 – 6391 (Online) Volume 3, Issue 1, January- December (2012), © IAEME


                        600

                        400                                        Initial Fractional Density 0.85
                                                                             Cu -15%W
                                                                           Aspect Ratio:0.6
           Stess,Mpa    200                                             Lubricator: Graphite



                          0
                                                                            Axial Stress
                               0   0.5              1        1.5
                                                                            Hoop Stress
                        -200
                                                                            Hydrostatic Stress
                        -400

                        -600                                                        (b)
                                     Axial Strain




                        600

                        400                                         Initial Fractional Density 0.85
                                                                              Cu -15%W
                                                                            Aspect Ratio:0.4
                        200                                              Lubricator: Graphite
           Stress,Mpa




                          0                                                 Axial Stress
                               0   0.5              1        1.5            Hoop Stress
                        -200
                                                                            Hydrostatic Stress
                        -400                                                      (C)


                        -600
                                     Axial Strain



Fig. 2.The Variation of stresses with respect to axial strain for various aspect ratios (a) 0.80
(b) 0.60 (c) 0.40
3.1.2 Axial stress and axial strain
        Fig.3 shows the effect of aspect ratio on the characteristic features of the axial stress
(σz) against the axial strain (εz) for a given initial fractional density of 0.85 of Cu-15%W
composite. These plots are similar in nature irrespective of the aspect ratio values. It is
observed that the true axial stress increases rapidly as the true axial strain is increased,
followed by a gradual increase in the true axial stress with further increase in the axial strain.
Further, it is found that the 0.80 preform improved load bearing capacity compared to that for
other aspect ratios, while the initial fractional density remains constant.




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International Journal of Production Technology and Management (IJPTM), ISSN 0976 – 6383
(Print), ISSN 0976 – 6391 (Online) Volume 3, Issue 1, January- December (2012), © IAEME

                                                                 Initial Fractional Density 0.85
                                                                           Cu -15%W

                                      1000
                                       800




                 Axial Stress
                                       600
                                                                                                          Aspect ratio 0.4
                                       400
                                                                                                          Aspect ratio 0.6
                                       200
                                                                                                          Aspect ratio 0.8
                                          0
                                              0             0.5                  1                 1.5
                                                                 Axial Strain

                                                  Fig.3 The relationship between σz and εz

3.2 Densification Behaviour of preforms
        In densification behaviour of preforms the following relationships are discussed in
detail by varying the initial aspect ratio provided the initial fractional density value is kept
constant for Cu-15%W composites.
                                   ρf 
            • Fractional density  ρ  and strain parameter
                                        
                                                               e (ε z − ε θ )
                                   th 
3.2.1. Fractional density and strain parameter

       Figure 4 has been drawn to establish the relationship between the fractional density
value  f  of the deforming preform and the value of e (ε z − ε θ ) in accordance with the
        ρ
      
      ρ  
       th 
theoretically-derived relationship [16] for sintered P/M preforms during cold axial
deformation. The plots show the existence of power law relationship between the fractional
theoretical density  ρ f  and e (ε z
                         
                                                            −ε   θ   )
                                                                         . However the lower aspect ratio shows better
                         
                                        ρ th 
fractional density values because it has lesser porosity.
                                      1.04
                                      1.02                                                                Cu -15%W
                 Fractional Density




                                         1
                                      0.98
                                      0.96                                                                 Aspect ratio 0.4
                                      0.94
                                                                                                           Aspect ratio 0.6
                                      0.92
                                                                                                           Aspect ratio 0.8
                                       0.9
                                      0.88
                                              0             0.5                  1                  1.5
                                                            Strain Parameter


               Fig.4 The relationship between Fractional Density and Strain parameter


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International Journal of Production Technology and Management (IJPTM), ISSN 0976 – 6383
(Print), ISSN 0976 – 6391 (Online) Volume 3, Issue 1, January- December (2012), © IAEME

4. CONCLUSION

    The study has been carried out of cold upset forming of sintered Cu-15%W nano
composites performs. The basic conclusions that can be drawn from the present
investigations are as follows.
    The axial stress has an increased trend with higher level of deformation till failure. This is
because, at the time of increasing the load, the pores in the preforms are flattened and contact
surfaces are increased. So the load –bearing capacity increasing steeply by increase of strain.
    Lower aspect ratio exhibits improved densification and load –bearing capacity compared
to that of higher aspect ratio performs densify uniformly and quite easily due to rapid load
transfer resulting in extensive work-hardening.

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