| Abstract |
Friction drilling is used to process special holes with a bushing and a boss
in thin sheet metals without producing chips via a non-traditional tool-drill.
Friction drilling parameters involve the feed rate, rotational speed and profile
dimensions of the drilling tool, which directly affect the induced bushings
dimensions, as well as, the microstructure of the produced hole.
In the present study, friction drilling parameters were manipulated during
the performance of friction drilling of 6082 and 7075 Al-alloys, the length,
width, and thickness dimensions of the sheets are approximately 100, 50, and 4
mm, respectively. It was drilled by friction using tool cone angles with values of
40, 45 and 50⁰ under different feed rates (100, 200 and 315 mm/min) and
rotational speeds (1000, 1250 and 1600 rpm). Moreover, the temperature
variation in the tool-work-piece interface was recorded during the drilling
processes via an infrared camera and four thermocouples located at different
positions near the drilling zone. The characteristics of the thermally induced
bushes (shape, dimensions, and surface roughness) were inspected.
Furthermore, in the present study, the microstructure evolution and hardness
distribution in the thermally-formed bush and in the heat-affected zone around
the bushing with and without heat treatment were premeditated. Corrosion tests
were carried out to evaluate the corrosion resistance of friction drilled hole for
different conditions.
During the formation of the bushing and the boss in the investigated
aluminum sheet metals, the minimum measured temperature was 220 °C and the
maximum measured temperature was 380 °C. It was found that the temperature
in the tool-work-piece interface increased with the reduction of the feed rates
and the increase of both of the rotating speeds and the tool cone angles. It was
found that higher temperature resulted in greater bushing heights and smaller
II
bushing thickness. No systematic effect of process parameters was observed on
resulting hole diameter and boss height. Furthermore, the surface roughness of
the drilled holes were found to be increased with the increase of the rotational
speeds and lower feed rate.
It was found that the hardness of the bush was slightly increased with
increasing of the tool cone angle and reduction of the tool rotational speed.
However, the hardness of the thermally-induced bush showed values lower than
the parent metal. The hardness values increase with moving away from the edge
of the induced bush. The hardness near the drilling surface was approximately
65±10 HV, while it recorded hardness values of 75±10 HV at 5 mm away from
the drilling surface. In addition, the microstructure of the friction drilled
specimens showed a very fine structure in the drilling zone due to crushing of
the original structure during the friction drilling process. The thermal cycle of
friction drilling resulted in a reduction of micro-hardness of a hole. The
softening phenomenon of 6082 and 7075 AL- alloys is due to the dissolution or
coarsening of the strengthening precipitates. The hardness of aluminum is
improved by artificial ageing, the hardness increases to 145±15 HV after
artificial ageing. Consequently, a performance of a heat-treatment of the friction
drilled 6082&7075 Al-alloys are required to ensure a homogenization of the
produced structure.
The result is obvious that there is an improvement in pitting corrosion of
Al- alloys (BM 6082&BM7075) after drilling process. The obtained results, as
shown by SEM, reveal the dangerous growth of localized attack of the BM
alloys surfaces in 3.5% NaCl solution. Ageing treatment improved corrosion
resistance of friction hole drilled for 6082 Al-alloy. After heat treatment, Mg2Si
precipitates are more concentrated in the α-Al matrix, resulting in increasing
corrosion resistance.
III
The optimum drilling process parameters, which affect the surface
roughness values and bushing dimensions, were predicted separately by
applying the main effects plot and analysis of variance. The best settings of
parameters for the lowest surface roughness value were discovered to be
rotational speed of 1250 rpm, feed rate of 200 mm/min, and cone angle of 45°.
However, the rotational speed at level 0 (1250 rpm), the feed rate at level 1 (315
mm/min), and the cone angle at level 0 (45°) have been determined as the
optimal input parameter levels for bushing thickness. The rotational speed at
level 1 (1600 rpm), the feed rate at level 0 (200 mm/min), and the cone angle at
level 0 (45°) are the multi-optimum input parameter levels identified of the
output responses of grey relational grade (surface roughness (Ra), Δd, height of
bushing (ha), height of boss (hb), and thickness of bushing (t) for 7075 Al-alloy.
The rotational speed at level 0 (1250 rpm), the feed rate at level -1 (100
mm/min), and the cone angle at level 0 (45°) have been established as the ideal
input parameter levels for the grey relational grade of 6082 Al-alloy.
|
