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1.        
Introduction

 

T

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 he low plasticity burnishing (LPB) process is
similar to other burnishing processes in which a metal object in contact with
the surface of a part is forcefully passed over the desired area. The end
result is usually a surface compressive stress and finer surface finish than
when the process was initiated. In LPB, the ball is maintained in a spherical
socket with fluid supplied to the top of the ball. Fluid pumped to the socket
applies the force and helps prevent the ball from contacting the socket directly.
A fluid film allows the ball to roll across the surface of a part and helps to
reduce the risk of the ball seizing in the socket.

LPB
processing provides residual stress results capable of approaching laser
peening. The process is constrained by geometry and may require substantial
tooling to be implemented. The process is highly dependent on maintaining the
fluid supply to the ball and can cause irrecoverable damage to parts if the
fluid quality or quantity deviates from specification.

 

2.     
Benefits of LPB

The LPB process
includes a unique and patented way of analyzing, designing, and testing
metallic components in order to develop the unique metal treatment necessary to
improve performance and reduce metal fatigue, SCC, and corrosion fatigue failures. Lambda designs a new tool for each component to
provide the best results possible and to ensure that the apparatus reaches
every inch on the component. LPB has even been shown to have the ability to
produce through-thickness compression in blades and vanes, greatly increasing
their damage tolerance over 10-fold, effectively mitigating most FOD and
reducing inspection requirements. No material is removed during this process,
even when correcting corrosion damage. The major benefit of LPB is the
improved high cycle fatigue life. An LPB treated surface is resistant to
foreign object damage and stress corrosion cracking. Shallow cracks, less than 0.010″ deep, have had their
growth arrested after being treated by LPB. 2 The LPB process can
control the plastic deformation that the material undergoes during the process.
Both the depth of compression and amount of cold work being put into the
surface of the component can be controlled. Residual compressive stresses can
be put into the surface of components with a process that is predictable and repeatable
from part to part.  The depth of
compression with LPB can be as much as 1mm (0.040″) with very low cold
work, less than 5%. In contrast, shot peening typically produces 20 % to 70%
cold work and much shallower compression. LPB can be applied to all types of
carbon and alloy steel, stainless steel, cast iron, aluminum, AISI 1040, and
nickel-based super alloys.

 

3.        
Problem Definition

Thus, the problem can be defined as “An investigation of effect of ball burnishing parameters on
fatigue behavior of Titanium Grade-V(Ti6Al4V) by optimizing
burnishing parameters through Design of
experiments”.

 

4.        
Objective

Life and reliability of machined
components or elements are greatly affected by the applied manufacturing method
and surface enhancement technique. For augmenting life of the machined component,
it is necessary to improve a surface of the component. Since all fatigue and
corrosion related failures originates from a surface produced by the
manufacturing process. it is a general procedure to introduce a layer of compressive
residual stress on the surface machined component which leads to increase
fatigue strength.

 

The objective of this work is to
investigate fatigue life aspects of Titanium Grade-V(Ti6Al4V) work material, using
Taguchi technique of design of experiments. The assessment of the fatigue
behavior on work material is done, in terms of evaluating the effects of
parameters, identifying the predominant factor amongst the selected parameters,
their order of signi?cance and setting the levels of the factors for maximizing
fatigue life.

 

5.        
Material Selection

Ti6Al4V, Ti-6Al-4V or Ti 6-4, is the most commonly used alloy.Among its many advantages, it is heat
treatable. This grade is an excellent combination of strength, corrosion resistance,
weld and fabricability. Since it is the most commonly used alloy – over 70% of
all alloy grades melted are a sub-grade of Ti6Al4V, its uses span many aerospace
airframe and engine component uses and also major non-aerospace applications in
the marine, offshore and power generation industries in particular.

Applications: Blades,
discs, rings, airframes, fasteners, components, vessels, cases, hubs, forgings,
biomedical implants. 6 

6.        
Burnishing
Test Rig and specimen fabrication

Roller burnishing test rig will be prepared. Also,
the specimen of required size of selected material will be fabricated

 

7.      Design of Experiment Technique

 The technique of defining and investigating
all possible conditions in an experiment involving multiple factors is known as
the design of experiments.

 

The Taguchi approach has been
successfully applied in several industrial organizations and has completely
changed their outlook on quality control. Taguchi espoused an excellent philosophy
for quality control in the manufacturing industries. His philosophy has far
reaching consequences, yet it is founded on three very simple and fundamental
concepts. The whole of the technology and techniques arise entirely out of
these three ideas. These concepts are:

        
i.           
Quality should be designed into the
product and not inspected into it.

      
ii.           
Quality is best achieved by minimizing
the deviation from a target. The product should be so designed that it is
immune to uncontrollable environmental factors.

    
iii.           
The cost of quality should be measured
as function of deviation from the standard and the losses should be measured system-wide.

 

Taguchi’s approach to enhance
quality in the design phase involves two steps:

        
i.           
Optimizing the design of the product
or process.

      
ii.           
 Making the design insensitive to the influence
of uncontrollable factors.

 

The concept of total loss
function employed by Dr. Taguchi has forced engineers and cost accountants to
take a serious look at the quality control practices of the past. the concept
is simple but effective. He defined the quality as the total loss imparted to
the society, from a time a product is shipped to the customer. The loss is measured
in monetary terms and includes all costs in excess of the cost of a perfect
product. The definition can be expanded to include a perfect product.

 

Application of the Taguchi
technique is accomplished in two phases:

        
i.           
Design of the experiment, which
includes determining controllable and noise factors and the levels to be
investigated, which determines the no of repetitions.

      
ii.           
Analysis of result to determine the
best possible factor combination from individual factor influences and interactions.

 

8.        
Experimentation

Surface integrity and enhancement
of any material is governed by several factors and their interactions. It is necessary to know quantitatively
about the in?uence of these factors and   their 
 interactions    on   the   response
variables. The experimental work is carried out to
investigate the effect of the different process parameters of LPB on surface
roughness of Titanium Grade-V(Ti6Al4V) work material. Typical applications of Titanium
Grade-V(Ti6Al4V), blades, discs, rings, airframes, fasteners, components,
vessels, cases, hubs, forgings, biomedical. LPB
on these material increases dislocation density near the surface, which in turn
reduces grain size. Since Titanium
Grade-V(Ti6Al4V) is more ductile, the plastic
deformation on this material will be more. Because of the effect of more cold
work, it has got finer grain size.

 

Since the bulk of the material constrains the deformed area, the deformed
zone is left in compression after the roller passes. The surface is permanently
displaced inward and no material is removed during this process. The small
amount of deformations that are associated in the LPB process, are definitely
encouraging to improve many desirable properties in a holistic approach,
without causing any undesirable side effects. Further, LPB smoothes surface
asperities and thereby improves the surface finish.

 

Actual experimentation stage includes burnishing
of fabricated specimen on CNC Lathe machine with the help of Roller burnishing
test rig.

 

9.     Surface Roughness Tester

Measuring of surface roughness parameters for burnished test
specimen is done by a Mitutoyo Surftest SJ – 400 surface roughness measuring device.
  It is also used for unburnished test
specimens. It measures Ra value. It also measures Rz value.

                      

Measurement of Surface Roughness

10.       
Surface
Hardness Tester

 

Along with surface roughness, burnishing process
enhances the hardness properties too. To measure the increment in the hardness
of the specimens, the Rockwell Hardness Tester was used

 

11.     Experimental Design Matrix

 

 

Experiments have been
carried out using Taguchi’s L9 Orthogonal Array (OA) experimental design which
consists of nine combinations of burnishing speed, burnishing feed, number of
roller passes and depth of impression of the tool. According to the design
catalogue prepared by Taguchi, L9 Orthogonal Array design of experiment has
been found suitable in the present work. It considers four process parameters
(without interaction) to be varied in three discrete levels. The experimental
design has been shown in Table 11.1 (all factors are in coded form)

 

TABLE 11.1 – Experimental Design
Matrix in Coded Form and with Actual Values

12. 
    Results for
Surface Roughness

 

Table 12.1 consists of the observed
values of surface roughness on the test specimens before and after the
experimentation. These values are tabulated below.

 

 

Table12.1
Surface Roughness Values Before and After Experimentation

Figure 12.1 shows the main effects of burnishing parameters
on the mean values of surface roughness.
Also, since smaller surface roughness is desirable, the quality characteristics
applicable in this case are smaller the
better.

FIGURE 12.1- Main Effects Plot for Means, for Surface
Roughness

For smaller the better quality
characteristics of surface roughness, the main effects plot for signal to noise
ratio is shown in figure 12.2

 FIGURE
12.2- Main Effects Plot for S/N Ratio, for Surface Roughness

By
using equation of the S/N ratios for the three levels of each control factor,
the values are computed to determine the relative significances of the
different parameters. From the analysis of the surface roughness data, it is
observed that the number of roller passes and depth of impression of the tool
play a significant role in determining the surface roughness. Furthermore, the
burnishing speed and burnishing feed are less significant parameters.

Thus, it can be
seen that the optimal burnishing performance for the surface roughness (based
on means) was obtained at A3B3C2D1.

Speed
(rpm)

Feed
(mm/rev)

No. Passes

Depth of Impression(mm)

500

0.10

2

0.5
mm

 

13.       
Results for Surface
Hardness

Table
13.1 consists of the observed values of surface roughness on the test specimens
before and after the experimentation. These values are tabulated below.

 

 

TABLE 13.1- Surface Hardness
Values Before and After Experimentation

 

Figure 13.1 shows the main effects of burnishing parameters
on the mean values of surface hardness. Also, since higher surface
hardness is desirable, the quality characteristics applicable in this case are higher the better.

 

 

FIGURE 13.1- Main Effects Plot for
Means, for Surface Hardness

 

For higher the better quality characteristics of surface hardness, the
main effects plot for signal to noise ratio is shown in figure 13.2

 

 

FIGURE
13.2- Main Effects Plot for S/N Ratio, for Surface Hardness

 

By using
equation of the S/N ratios for the three levels of each control factor, the
values are computed to determine the relative significances of the different
parameters. From the analysis of the surface hardness data, it is observed that
the burnishing feed and depth of impression of the tool play a significant role
in determining the surface hardness. Furthermore, the burnishing speed and
number of roller passes are less significant parameters.

Thus, it
can be seen that the optimal burnishing performance for the surface hardness
(based on means) was obtained at A3B3C3D2.

 

Speed
(rpm)

Feed
(mm/rev)

No.
Passes

Depth
of Impression (mm)

500

0.10

3

1.0

 

    

 

14. 
References

1        
Deepak Mahajaan, Ravindra Tajaane, “A Review
on Ball Burnishing Process”, International Journal of Scientific and
Research Publications, Volume 3, Issue 4, April 2013 ISSN 2250-3153.

2        
John T. Cammett and Paul S. Prevey, “Fatigue
Strength Restoration in Corrosion Pitted 4340 Alloy Steel Via Low Plasticity
Burnishing”, Lambda Research, Cincinnati, OH,(2006).

3        
M. Salahshoor and Y.B. Guo, “Surface
integrity of magnesium-calcium implants processed by synergistic dry cutting-finish
burnishing”, Procedia Engineering19 (2011) 288 – 293. 

4        
P. R. Prabhu, S. M. Kulkarni and S. S. Sharma, “Experimental
Investigations of Process Parameters Influence on Surface Roughness in Deep
Cold Rolling of AISI 4140 Steel”, International Journal of Research in
Engineering and Technology (IJRET), (2012) ISSN 2277 – 4378.

5        
Fathi Gharbi and Salem Sghaier and Hedi Hamdi Tarek
Benameur, “Ductility improvement of aluminum 1050A rolled sheet by a
newly designed ball burnishing tool device”, Int J Adv Manuf. Technol.
(2012) 60:87–99.

6        
Aerospace Specification Metals Inc.

http://asm.matweb.com/search/SpecificMaterial.asp?bassnum=MTP641

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