summary of paper

8/8/2019 Summary of Paper

http://slidepdf.com/reader/full/summary-of-paper 1/5

The lowest temperature at which a 100-percent beta phase can exist is called the beta transus; this can

range from 700° C (1,300° F) to as high as 1,050° C (1,900° F), depending on alloy composition.

Summary

1. Paper 1 : THE EFFECT OF FORGING ON THE PROPERTIES OF PARTICULATE SiC-

REINFORCED ALUMINIUM-ALLOY COMPOSITES : By Ismail, Umit and Kazim

History:

1) Aluminum is widely preferred as base metal in production of composite materials;

with their low density, wide range, favorable thermal properties, heat treatment

capability and processing flexibility.

Mechanical Properties of the MMC sample

Yield strength:

2) The yield strength in all the specimens in the as-cast and in the forged states

increases with the addition of up to about 17 vol% SiCp.

3) It decreases after the additions of SiCp above this amount.

4) With application of forging, the yield strength curves are pushed upwards,

exhibiting much heavier values than those of the as cast materials.

5) After one step forging in samples of 9 vol% SiCp the observed yield strength is lower

than the values obtained after two step forging.

6) For all other composites with over 9 vol % reinforcement , the behavior is reversed

,the yield strength values obtained after one step forging are higher than those

obtained after two step forging .

Tensile Strength:

7) The curve for as-cast sample shows that tensile strength is unaltered with addition

up to 13 vol% reinforcement, but it is reduced with addition exceeding 13 vol%.8) The samples containing above 17% reinforcement exhibit decreasing values of

tensile strength for as-cast samples

9) In forged composite specimens containing below 13% reinforcement, values of

tensile strength observed after two-step forging are lower than those of the as-cast

material, but with further additions of reinforcement the tensile strength increases.

10) The samples after one step forging possess the highest values of tensile strength.

Ductility:

11) With increasing reinforcement content, decreased values of ductility is observed in

all-cast and forged materials, but for the same reinforcement content the forged

samples possess much better quality.

12) The level of ductility is lowest in as-cast samples and below 5%.

13) Application of forging lifts up the ductility level of the composites considerably,

resulting in five times more in ductility level than the as-cast samples.





14) Application of pressure during operations like squeeze casting, rolling ,forging,

extrusion ,improves ductility ;Because there is :

a) decrease in porosity content

b) improvement of particle matrix bond

c) Refinement of matrix structure.

Micro-structures:

15) The microstructures of hot forged materials posses reduced number of the particles

of eutectic silicon and other phases, indicating that particle coarsening has occurred

to a certain extent (common in annealing and hot working).

16) It is noticeable that after forging some particles exhibit interfacial de-bonding and

cracking cracking, while almost no cracking damage can be seen in the as-cast

composites.

17) The as-cast samples possess higher level of porosity and when the SiCp content

exceeds 17 vol% the porosity values increases steeply.

18) According to Rozak , the porosity can be reduced by plastic working and it can be

eliminated if the amount of the applied deformation is over 90%.

Elastic Modulus:

19) The elastic modulus of the composites increases with the reinforcement content. Srivatsan

and Matingly stated that the elastic modulus of the composites with 30 vol% reinforcement

was 61 % higher than that of 2124 matrix alloy.(e.g.)

20) In general , the elastic modulus of the discontinuous reinforcement MMCs cannot be

predicted by the isostrain rule of mixtures

21) A different formulation for predicting the elastic modulus of the composites was proposed

by Halpin and Tsai and the following proposed equations are frequently employed for this

type of material:

Ec = Em (1+2sqVp) /(1- qVp) ---------------------(1)

q = ( Ep/Em -1) / (Ep/Ep/Em +2s) ----------------(2)

Where Ec , Em and Ep are the elastic constants of the composite ,matrix material and thereinforcing particle respectively , s is the aspect ratio and Vp is the volume fraction of

the reinforcement .

To employ this equation, Wang and Zhang; and Lagace and Lloyd used the

values of s=1 and ESiCp = 400 GPa . Em values are calculated experimentally. The

experimental and calculated values of the elastic modulus are taken as the function of

SiCp volume fraction.





22) The dominating factors controlling the elastic modulus in these materials are:

a. Volume fraction of the reinforcement

b. The aspect ratio of the reinforcement and

c. The load-transfer capability of the interface.

But the presence of porosity and micro cracks would reduce the modulus.

Yield Strength:

23) The change in yield strength as a function of SiCp volume fraction for the forged and

as-cast specimens, indicates that in all of tested specimens the indicates that in all

of the tested specimens the yield strength increases with the reinforcement contentup to 17 vol % but starts decreasing in the specimens with over 17 vol % SiCp.

24) The application of forging increases the yield strength and an overall improvement

of about 80% in yield strength of the specimen of the matrix alloy and composites is

obtained. Harrigen demonstrated hot rolling reduction above 80% to billets of SiCp

reinforced 6061 alloys improved the SiC distribution and increased strength and

ductility of material.

25) Llyod reported that increasing the SiCp content increases the yield strength of the

composite, but some of the reported results do not agree with the above

observations.

26) Davidson found that yield strength of a composite with 15 vol% SiCp in Al 2014matrix (345 MPa) was found to be lower than that of the base alloy (415 MPa). The

reason was that the matrix material found to be inhomogeneous, with unevenly

dispersed SiC p and relatively large inter-metallic particles widely distributed in the

matrix. Therefore the varying results in strength values can be attributed to

difference in processing and quality of material.

27) Addition of the particulate SiCp into aluminum alloy matrix increases the yield

strength which is in agreement with the previous observations, but this increase in

yield is limited to upto 17 vol% SiCp additions.

28) Zhong reported that in Al-Mg (5083)-alloy based SiCp composites the addition of

the oxidized SiCp decreases the strength as a result of interfacial reaction whereMgO layers form at the particlematrix interface due to Mg present in matrix.

Depletion of Mg in matrix will induce lower strength values as a consequence of

reduced solute strengthening. But depletion in strength ,although high SiCp

particulate, cannot be explained by depletion of Mg only, it can partly attributed to

formation of Mg oxides and Mg2Si at the interface reducing cohesion between the

reinforcing particles and the matrix.





29) The porosity in the as-cast component increases with increasing reinforcement

volume fraction and the increase becomes abrupt after adding SiCp above 17 vol%.

30) Achieving strength in a composite is possible if a strong interfacial bond between

the matrix and reinforcement is achieved.

31) Hot Forging has two effects on the interface between matrix and reinforcement:

a. Positive effect: Applied deformation does not disturbs the SiC/matrix

bond, rather it may improve the bond by affecting the interfacial

compounds and by covering metal across the surface of the

reinforcement particles.

b. Negative effect: the forming process may cause local tearing and easy

formation of cracks at interface due to strain incompatibilities.

Depending upon the above effects, strength may either increase or decrease in a MMC.

Conclusion:

y Fair distribution of SiCp with clusters of smaller SiCp and presence of porosity in

as-Cast samples.

y With application of forging the clusters of SiCp disappears and porosity level is

reduced to very low levels.

y Elastic modulus is primarily controlled by: a) volume fraction of reinforcement,

b) porosity content. Elastic modulus increases with increase in volume fraction

of SiCp which can be predicted. Upto 17 vol% it increases and then it decreases

on further addition of reinforcement.

y Application of forging yield strength and tensile strength increases by 100 and

40%. y The ductility of composites is decreased with increasing amount of SiCp. Forging

improves ductility and the elongation fracture increases above 10% in 17 vol%

SiCp.

summary of paper

Documents