52
J. Karloopia et al.
(a) Initially due to cracking of the reinforcing TiB 2 particulates.
(b) The occurrence of decohesion at the matrix (Al-Si)—reinforcement (TiB 2 )
interphase results in the nucleation of fine microscopic voids, and
(c) As a direct result of the progressive growth of the fine microscopic voids and
their eventual coalescence to form macroscopic voids and fine microscopic
cracks.
The complications that can be associated with processing of the in situ metal
matrix composites can also contribute to failure of the test specimen. The slag traps
that occur during the reaction for in situ formation of the TiB 2 particulate phase can
also create defects, which provides conditions that are conducive for the initiation
of failure by fracture. Most of the fracture that occurred was due to mixed mode
of fracture, which is attributed to the presence of facets and voids for the sample at
the low values of the tool rotation speed and tool welding speed. Typically, a mixed
fracture mode results in an overall decrease in elongation when compared to failure
by ductile fracture. Thus, growth and coalescence of the dimples are suppressed
when they are challenged or confronted by the hard, brittle and essentially elastically
deforming reinforcing TiB 2 particulates. It can also be pointed out that both ductility
and degree of plastic deformation depend on the degree of growth of both the dimples
and the fine microscopic voids through the microstructure of the composite. The
typical size of dimples ranged between 8 and 5 µm as shown in Fig. 5b and was
calculated using the conventional line intercept technique.
Mechanical Properties of TiB 2 /Al-Si Weld Joint
The experimental results for friction stir welding of the in situ 3%TiB 2 /Al-12Si
composites using different combinations of the process parameters are summarized
in Table 3. It is concluded that the ultimate tensile strength revealed the following:
(i) Maximum at a tool rotational speed of 708 rpm and a tool welding speed of
20 mm/min and with a peak value of 141.153 MPa.
(ii) Tool Variant # 2 having 20 mm diameter gives the lowest value for the ultimate
tensile strength to be 125.021 MPa for the combination of 931 rpm for tool
rotational speed and 40 mm/min as the tool welding speed.
(iii) The base material of in situ 3%TiB 2 /Al-12Si had an ultimate tensile strength
(UTS) of 158.942 MPa.
The maximum value achieved for the friction stir welded in situ 3%TiB 2 /Al12Si composite was 141.153 MPa. This validates an overall joint efficiency of the
friction stir welded plate to be 88.8%. It can be used as a safety factor, which can be
introduced in numerous API and ASME codes. The numerical value is calculated as
a percentage by taking a ratio of strength of welded joint of the composite material
to strength of the base composite material. A high strength of the weld suggests a
J. Karloopia et al.
(a) Initially due to cracking of the reinforcing TiB 2 particulates.
(b) The occurrence of decohesion at the matrix (Al-Si)—reinforcement (TiB 2 )
interphase results in the nucleation of fine microscopic voids, and
(c) As a direct result of the progressive growth of the fine microscopic voids and
their eventual coalescence to form macroscopic voids and fine microscopic
cracks.
The complications that can be associated with processing of the in situ metal
matrix composites can also contribute to failure of the test specimen. The slag traps
that occur during the reaction for in situ formation of the TiB 2 particulate phase can
also create defects, which provides conditions that are conducive for the initiation
of failure by fracture. Most of the fracture that occurred was due to mixed mode
of fracture, which is attributed to the presence of facets and voids for the sample at
the low values of the tool rotation speed and tool welding speed. Typically, a mixed
fracture mode results in an overall decrease in elongation when compared to failure
by ductile fracture. Thus, growth and coalescence of the dimples are suppressed
when they are challenged or confronted by the hard, brittle and essentially elastically
deforming reinforcing TiB 2 particulates. It can also be pointed out that both ductility
and degree of plastic deformation depend on the degree of growth of both the dimples
and the fine microscopic voids through the microstructure of the composite. The
typical size of dimples ranged between 8 and 5 µm as shown in Fig. 5b and was
calculated using the conventional line intercept technique.
Mechanical Properties of TiB 2 /Al-Si Weld Joint
The experimental results for friction stir welding of the in situ 3%TiB 2 /Al-12Si
composites using different combinations of the process parameters are summarized
in Table 3. It is concluded that the ultimate tensile strength revealed the following:
(i) Maximum at a tool rotational speed of 708 rpm and a tool welding speed of
20 mm/min and with a peak value of 141.153 MPa.
(ii) Tool Variant # 2 having 20 mm diameter gives the lowest value for the ultimate
tensile strength to be 125.021 MPa for the combination of 931 rpm for tool
rotational speed and 40 mm/min as the tool welding speed.
(iii) The base material of in situ 3%TiB 2 /Al-12Si had an ultimate tensile strength
(UTS) of 158.942 MPa.
The maximum value achieved for the friction stir welded in situ 3%TiB 2 /Al12Si composite was 141.153 MPa. This validates an overall joint efficiency of the
friction stir welded plate to be 88.8%. It can be used as a safety factor, which can be
introduced in numerous API and ASME codes. The numerical value is calculated as
a percentage by taking a ratio of strength of welded joint of the composite material
to strength of the base composite material. A high strength of the weld suggests a