Journal of Minerals and Materials Characterization and Engineering, 2013, 1, 358-362
Published Online November 2013 (http://www.scirp.org/journal/jmmce)
http://dx.doi.org/10.4236/jmmce.2013.16055
Open Access JMMCE
Predictability of Hardness of the Heat Affect Zone in
Aluminum Weldments Cooled in Palm Oil Based on
Hardness of Similarly Cooled Mild Steel and Cast Iron
Weldments
Chukwuka Nwoye1*, Stanley Nwakpa1, Agbo Alfred2, Chidume Nwambu1, Uche Obialor3
1Department of Metallurgical and Materials Engineering, Nnamdi Azikiwe University, Awka, Nigeria
2Department of Metallurgical and Materials Engineering, Enugu State University of Science & Technology, Enugu, Nigeria
3Department of Mechanical Engineering, Abia Polythecnic Aba, Nigeria
Received September 8, 2013; revised October 18, 2013; accepted October 30, 2013
Copyright © 2013 Chukwuka Nwoye et al. This is an open access article distributed under the Creative Commons Attribution Li-
cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
The predictability of hardness of the heat affected zone (HAZ) in aluminum weldments cooled in palm oil, based on
hardness of similarly cooled mild steel and cast iron weldments has been ascertained. The general model:
1.2769











indicates that HAZ hardness of aluminium weldment is dependant on the ratio of product to sum
of HAZ hardness of mild steel and cast iron weldments cooled in palm oil under the same conditions. The maximum
deviations of the model-predicted HAZ hardness values α, μ and β from the corresponding experimental values αexp, μexp
and βexp were less than 0.04% indicating the reliability and validity of the model.
Keywords: Model; Hardness; Heat Affected Zone; Aluminum Weldments; Mild Steel; Cast Iron
1. Introduction
In recent times, great importance has been placed on the
weldability of some alloys as a way of repairing struc-
tural components damaged in service. The application of
fusion welding and other conventional welding processes
in the repair and fabrication of nickel based superalloy
such as RR1000 has been seriously restricted. This is
because these alloys especially those containing signifi-
cant amount of Al and Ti (>3 wt%), have been consid-
ered highly susceptible to heat affected zone (HAZ)
cracking during welding and post weld heat treatment
strain age cracking [1].
Welding cracks of superalloys have been reportedly
attributed to large shrinkage stress occurring as a result
of rapid precipitation of particles during cooling from the
welding temperature [2].
Heat affected zone microfissuring susceptibility has
been traced to depend on the composition and micro-
structure of a material [2]. It has been discovered that
liquation which could occur by different mechanisms, is
the primary cause of low HAZ crack resistance in most
austenitic alloys including precipitation hardened Ni base
superalloys [3]. Based on the foregoing, the synergetic
effect of thermally induced welding strain and very low
ductility in the alloy due to localized melting at the grain
boundaries results in HAZ liquation cracking.
Result of previous study [4] shows that pre weld heat
treatment operation has resulted in considerably mini-
mized HAZ cracking in some superalloy weldments.
Reports [5] have shown that weldment cracking is one of
the reasons for low mechanical properties such as hard-
ness and impact strength in welded parts. HAZ is the area
adjacent to the immediate welded area or fusion zone.
The formation of hard and brittle martensite in all the sub
zones of the HAZ or increase in the martensite region in
size is relative to the other regions results from too rapid
cooling. The presence of martensite in the HAZ results in
a very high hardness value for the heat affected zone.
Slow cooling favours a better microstructure needed for
engineering applications. Also, increased rapidity of the
*Corresponding author.

C. NWOYE ET AL. 359
quenching rate, results to greater HAZ hardness.
Quadratic and linear models have been derived [6] for
predicting the heat-affected zone (HAZ) hardness of wa-
ter cooled cast iron weldment in relation to the combined
and respective values of the heat-affected zone hardness
of aluminum and mild steel welded and cooled under the
same conditions. The model:



2
3.074923.0749 2

 

 




(1)
was found to be the solution to a quadratic equation:
23.0749

   (2)
where: γ = Model-predicted hardness of HAZ in alumi-
num weldment cooled in water (VPN); β = Model-pre-
dicted hardness of HAZ in mild steel weldment cooled in
water (VPN); θ = Model-predicted hardness of HAZ in
cast iron weldment cooled in water (VPN).
It was found that the validity of the model is rooted on
the fractional expression:
3.07493.07493.0749 1

  .
The respective deviations of the model-predicted heat-
affected zone hardness values of aluminum, cast iron and
mild steel from the corresponding experimental values
were less than 0.01% which is quite insignificant, indi-
cating reliability of the model.
Successful attempt has also been made [7] to derive
quadratic and linear models for predicting the HAZ
hardness of air cooled cast iron weldment in relation to
the combined and respective values of HAZ hardness of
aluminum and mild steel welded and cooled under the
same conditions. It was discovered that the general
model:



2
2.977422.9774 2

 

 




(3)
predicts the HAZ hardness of cast iron weldment cooled
in air as a function of the HAZ hardness of both alumi-
num and mild steel welded and cooled under the same
conditions. The linear models; θ = 2.2391γ and θ =
1.7495β on the other hand predict the HAZ hardness of
cast iron weldment cooled in air as a function of the HAZ
hardness of aluminum or mild steel welded and cooled
under the same conditions. Re-arrangement of the gen-
eral model also resulted to the evaluation of the corre-
sponding HAZ hardness in aluminum and mild steel
weldments. It was found that the validity of the model is
rooted on the fractional expression:
2.97742.97742.9774 1

  
since the actual computational analysis of the expression
was also equal to 1, apart from the fact that the expres-
sion comprised the three metallic materials. The respec-
tive deviations of the model-predicted HAZ hardness
values θ, γ, and β from the corresponding experimental
values θexp, γexp, and βexp was less than 0.003% indicating
the validity and reliability of the model.
The hardness of the heat affected zone in aluminium
weldments has been evaluated using computational mod-
els [8]. The general model:


1.2714









 (4)
was found to predict the HAZ hardness of aluminum
weldment cooled in water as a function of the HAZ
hardness of both mild steel and cast iron welded and
cooled under the same conditions. The maximum devia-
tions of the model-predicted HAZ hardness values γ, α
and β from the corresponding experimental values γexp,
αexp and βexp were less than 0.02% respectively.
The hardness of HAZ in aluminum, cast iron and mild
steel cooled in kerosine was found to be exactly the same
as the hardness value of the same materials cooled in
groundnut oil [9]. This implies that
GK
H
H (5)
Where: HG = Hardness of HAZ cooled in groundnut
oil; HK = Hardness of HAZ cooled kerosene.
Nwoye [9] reported that 8% - 10% less hardness than
that from water occurs when kerosine or groundnut oil is
used as quenchant for HAZ. It was discovered that
quenching the HAZ with kerosine or groundnut oil gives
approximately 8% - 10.7% more hardness than that from
quenching with air. The researcher found that palm oil
gave the lowest hardness and cooling rate on the HAZ.
Recent research [10] focused on the hardening charac-
teristics of medium carbon steel and ductile cast iron in
SAE engine oil, neem oil and water as quenching media.
The samples were quenched to room temperature in
Neem oil, water and SAE engine oil for a clear compari-
son of the associated microstructures and mechanical
properties of the quenched samples. The result shows
that hardness value of the medium carbon steel increased
from 18.30 HVN in the as-cast condition to 21.60, 20.30
and 20.70 HVN while that of ductile cast iron samples
increased from 18.90 HVN in the as-cast condition to
22.65, 20.30 and 21.30 HVN for water, neem oil and
SAE40 engine oil respectively. The as-received steel
sample gave the highest impact strength value and water
quenched sample gave the least impact strength. The
impact strength of the medium carbon steel samples are
50.84, 41.35, 30.50 and 45.15 Joule and that of ductile
iron are 2.71, 1.02, 0.68 and 1.70 Joule for as-cast condi-
tion, neem oil, water and SAE 40 engine oil quenched
respectively. The microstructure of the samples quenched
in the Neem oil revealed the formation of martensite.
Hence, neem oil can be used where cooling severity less
than that of water but greater than SAE 40 engine oil is
required for hardening of plain carbon steels and ductile
Open Access JMMCE

C. NWOYE ET AL. 360
cast iron.
The present study aims at ascertaining the predictabil-
ity of the hardness of the heat affected zone (HAZ) in
aluminum weldment cooled in palm oil, as a function of
the respective and combined values of HAZ hardness of
mild steel and cast iron welded and cooled under the
same conditions.
2. Materials and Methods
Aluminum, mild steel and cast iron were cut and welded
using the shielded metal arc welding technique and the
hardness of the HAZ (cooled in palm oil maintained at
room temperature) tested. The hardness of the HAZ is as
presented in Tabl e 1. The full details of the experimental
procedures and equipment used are presented in the pre-
vious report [9]. Table 2 shows the welding current and
voltage used.
Where: Ctype = Current type; WCt = Welding current
(A); WVe = Welding voltage (V); DC = Direct current
(A); AC = Alternating current (A).
2.1. Model Formulation
Experimental data obtained from research work [9] car-
ried out at Metallurgical and Materials Engineering De-
partment of Federal University of Technology, Owerri
was used for this work. Results of the experiment as pre-
sented in the report [9] and used for the model formula-
tion are as shown in Table 1. Computational analysis of
the experimental data [9] is shown in Table 1 resulted in
Table 3.
Where: HHR (Symbol) = Ratio of HAZ Hardness
(Symbol); HHR (Values) = Ratio of HAZ Hardness (Val-
ues).
Table 1. Hardness of HAZ in weldments [9].
Materials HAZ Hardness (VHN)
Aluminum
Cast Iron
Mild Steel
407
870
503
Table 2. Variation of materials with welding current and
voltage [9].
Materials CType WCt WVe
Aluminum
Cast Iron
Mild Steel
DC
AC
AC
120
180
180
280
180
180
Table 3. HAZ Hardness ratio betwee n aluminum, mild steel,
and cast iron weldments cooled in palm oil.
HHR (Symbol) HHR (Values) Results
α/μ
α/β
μ/β
407/870
407/503
870/503
0.4678
0.8091
1.7296
Table 3 shows that the hardness of HAZ in aluminum
weldment cooled in palm oil is a function of the hardness
of HAZ in cast iron and mild steel weldment also cooled
in palm oil. Therefore,
0.4678



(6)
0.8091



(7)
1.7296



(8)
Adding Equations (6) and (7) as arranged in Table 1:
0.4678 0.8091



 

 (9)
1.2769
 

 

 (10)
1.2769

 

 (11)


1.2769

   (12)
1.2769











(13)
The derived model (general model) is Equation (13).
Where: α = Model-predicted hardness of HAZ in alu-
minum; weldment cooled in palm oil (VPN); β = Model-
predicted hardness of HAZ in mild steel; weldment
cooled in palm oil (VPN); μ = Model-predicted hard-
ness of HAZ in cast iron weldment cooled in palm oil
(VPN).
2.2. Boundary and Initial Conditions
The welding operation was carried out under atmospheric
condition. After welding, weldments were also main-
tained under atmospheric condition. Welding current and
voltage used are 180 A and 220 V respectively. SiO2-
coated electrodes were used to avoid oxidation of weld
spots. The coolants used were maintained at 25˚C (room
temperature). Volume of coolants used; 1000 cm3. No
pressure was applied to the HAZ during or after the
welding process. No force due to compression or tension
was applied in any way to the HAZ during or after the
welding process. The sides and shapes of the samples are
symmetries.
3. Results and Discussions
The derived model is Equation (13). The computational
analysis of Table 1 gave rise to Table 3.
A comparison of the HAZ hardness values from ex-
periment and those of the model show model values very
much within the range of the experimental values. The
HAZ hardness of aluminium weldment was found to be
dependant on the ratio of product to sum of HAZ hard-
ness of mild steel and cast iron weldments. Results of
Open Access JMMCE

C. NWOYE ET AL. 361
this comparison are presented in Tables 4 and 5. Model
values of α evaluated from Equations (6) and (7) and
tabulated in Table 4 show that all the equations are valid
since all of them gave almost the same corresponding
experimental values αexp. The value of μ in Equation (8)
was evaluated to establish the validity of the model. It
was found that the model-predicted μ value was also al-
most the same as the corresponding experimental value
μexp. This is a clear indication that the HAZ hardness of
any of aluminum, mild steel and cast iron weldments
cooled in palm oil can be predicted as a function of the
HAZ hardness of any of the other two materials, provid-
ing each pair was cooled in palm oil. Table 5 also indi-
cates that the model-predicted value of α is approxima-
tely the same as the corresponding experimental value
αexp.
3.1. Model Validation
The derived model was validated by evaluating the
model-predicted values of HAZ hardness in aluminum
weldment cooled in palm oil α and comparing them with
the corresponding values obtained from the experiment
αexp [9]. Following re-arrangement of the model Equation
(13), the values of μ and β were also evaluated as:
1
1.2769 1








(14)
1
1.2769 1








(15)
and compared with their respective corresponding ex-
perimental values μexp and βexp to further establish the
validity of the model. The model-predicted values of α, μ
and β are shown in Table 5.
Table 4. Comparison of the hardness of HAZ in aluminum,
mild steel and cast iron weldments cooled in palm oil as
obtained from experiment [9] and as predicted by derived
model (each material as a function of 1-material).
N Models derived PH E
H Dn (%) Cf (%)
1
1
1
α = 0.4678μ
α = 0.8091β
μ = 1.7296β
406.9860
406.9773
869.9888
407.00
407.00
870.00
−0.0030
−0.0056
−0.0013
+0.0030
+0.0056
+0.0013
Table 5. Comparison of the hardness of HAZ in aluminum,
mild steel and cast iron weldments cooled in palm oil as
obtained from experiment [9] and as predicted by derived
model (each material as a function of 2-materials).
N Models derived PH E
H Dn (%)Cf (%)
2
2
2
α = 1.2769 [(μβ/μ + β)]
μ = [(1.2769/α – 1/β)]–1
β = [(1.2769/α – 1/μ)]–1
406.9805
870.3220
503.0181
407.00
870.00
503.00
−0.0048
−0.0370
−0.0036
+0.0048
+0.0370
+0.0036
3.2. Deviational Analysis
Analysis and comparison between the model-predicted
values α, μ, β and the respective corresponding experi-
mental values αexp, μexp and βexp reveal deviations of
model data from the experimental data. This is attributed
to the non-consideration of the chemical properties of the
coolant and the physiochemical interactions between the
materials (aluminum, mild steel and cast iron) and the
coolant which is believed to have played vital roles in
modifying the microstructure of the HAZ during the
cooling process. These deviations necessitated the intro-
duction of correction factor to bring the model-predicted
values to exactly that of the corresponding experimental
values.
Deviation (Dn) of model-predicted HAZ hardness
from that of the experiment [9] is given by
– 100
HH
H
PE
Dn E




 (16)
Correction factor (Cf) is the negative of the deviation
i.e.
Cf Dn

 (17)
therefore
– 100
HH
H
PE
Cf E

 


(18)
EH. where: PH = Model-predicted HAZ hardness (VHN);
EH = HAZ hardness from experiment (VHN); Cf = Cor-
rection factor (%); Dn = Deviation (%).
Where: N = No. of materials constituting the corre-
sponding model as independent variable.
Introduction of the value of Cf from Equation (18) into
the models give exactly the corresponding experimental
values αexp, μexp and βexp [9].
It can also be seen from Table 5 that the model-pre-
dicted values of μ and β are also almost the same as the
corresponding experimental values of μexp and βexp re-
spectively. Tables 4 and 5 indicate that the respective
deviations of the model-predicted HAZ hardness values
α, μ and β from those of the corresponding experimental
values αexp, μexp and βexp are all less than 0.04% which is
quite negligible and within the acceptable model devia-
tion range from experimental results. Furthermore, the
values of μ and β (from Equations (14) and (15) respec-
tively) evaluated to be approximately equal to the respec-
tive corresponding experimental values μexp and βexp con-
firm the validity of the model. This also implies that the
general model; Equation (13) can predict the HAZ hard-
ness of any of aluminum, mild steel and cast iron weld-
ments cooled in palm oil as a function of the HAZ hard-
ness of the other two materials, providing the three mate-
rials constituting the model (aluminum, mild steel and
Open Access JMMCE

C. NWOYE ET AL.
Open Access JMMCE
362
cast iron) were cooled in palm oil. Equation (13) is re-
garded as the general model equation because it com-
prises of the HAZ hardness of all the materials consid-
ered for the model formulation. Based on the foregoing,
the models in Equations (6), (7) and (13) are valid and
very useful for predicting HAZ hardness of aluminum,
mild steel and cast iron weldments cooled in palm oil
depending on the material of interest and the given HAZ
hardness values for the other materials.
4. Conclusion
The derived models: α = 0.4678μ, α = 0.8091β, and μ =
1.7296β, can predict the HAZ hardness of aluminum
weldment cooled in palm oil as a function of the HAZ
hardness of mild steel or cast iron welded and cooled
under the same conditions. Similarly, the general model:

1.2769



can predict the HAZ hardness of aluminum weldment
cooled in palm oil as a function of the HAZ hardness of
both mild steel and cast iron welded and cooled under the
same conditions. Furthermore, re-arrangement of these
models could be done to evaluate the HAZ hardness of
mild steel or cast iron respectively as in the case of alu-
minum. The respective deviations of the model-predicted
HAZ hardness values α, μ and β from the corresponding
experimental values αexp, μexp and βexp were less than
0.04% indicating the reliability and validity of the model.
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