Although there are many micro drilling techniques covered in this paper, we will only focus on the non-conventional techniques which include laser micro drilling, EDM micro drilling, electro chemical micro drilling, electron beam micro drilling, spark assister chemical engraving and ultrasonic vibration micro drilling. The first topic is laser micro drilling, this is a technique that uses a high-density laser beam that melts and vaporizes the to be drilled material. The diameter of laser drilled holes ranges from 1 to a few 100 microns. There are many operating parameters to consider when the use of laser drilling is needed. There are three main groups of parameters which include the characteristics of the laser, the setup of the machining process and the material specifications e.
g. laser frequency, gas types and pressure, material composition and thickness. There are two phases during laser micro drilling. The first one is the photo-chemical phase, in this phase the photon energy from the laser breaks down the bonds and ejects the material from the specimen. In the photo-thermal phase, second phase, the material absorbs the laser energy and this causes the temperature climbs above the melting or evaporation temperature. The material then boils or evaporates away. The advantages of laser micro drilling are that it is a technique well suited for more complex parts and hard to machine materials. Also, this technique has an high aspect ratio, high production rate and a local treatment of the material.
some disadvantages are the poor hole quality and the residual stresses. The stresses come from the differences in cooling rates around the heat affected zone. These differences result in varying shrinkage between the external and internal layers in the material. All this will cause the poor hole quality and sub-surface cracking. Another big drawback of laser drilling is spatter. spatter is the incomplete ejection of material. This spatter will set itself upon the surface around the drilled hole.
The removal of spatter requires additional machining, which causes a higher cost and production time. Decreasing the amount of generated splatter can be done with the optimization of the laser parameters. Ultrafast lasers also known as femtosecond (fs) lasers overcome many of the drawbacks of traditional lasers. As shown in figure 3 below, due to the shorter pulse duration of the laser, the material will heat up more rapidly directly to the vapor temperature so that the material will vaporize away from the surface and thus no splatter is formed. The second non-conventional method is EDM or electro discharge machining micro drilling.
Here material is removed due to the thermal energy from sparks generated between the two electrodes. One electrode will be the workpiece and the other is a micro wire which passes through the material to make a mirror copy of itself. The ablated material solidifies and is removed from the gap between the two electrodes with the help of a pressurized dielectric liquid.This technique can be used on complex shapes and hard to machine materials as long as they’re electrically conductive. Some more advantages are that there is no cutting force and the easy manufacturing of the micro electrodes. The quality of the drilled holes, this means roundness and surface roughness, comes close to that of conventional micro drilling but much better than the quality obtained by laser drilling. The main disadvantages are a high tool electrode wear rate and production rate which add up to the cost of this technique.
The unstable nature of this technique is also a problem, also the material is removed from the specimen through melting and vaporizing which brings thermal stresses and poor surface quality. There are many factors that have a direct influence on the operational performance and the accuracy and quality of the drilled holes. These factors can be linked with either the process and system parameters or thermal properties the workpiece examples are voltage, peak currant, pulse duration, electrode gap, dielectric fluid, tool properties… As seen in figure 4, EDM gives a much better roundness and surface quality in comparison with ECM- drilling and laser drilling techniques.
However, there is still some recast formation on the sidewalls of the holes. Third is electro chemical micro drilling or ECMD, this technique is comparable witch electroplating in an anodic solution. The anode in this process is the to be drilled workpiece, and the tool is the cathode. Between the two is an electrolyte solution. According to Faraday’s law, when there is a pules voltage applied on the electrolyte and the tool is held at a certain distance from the workpiece, the anode will start to dissolve locally. This way there will form a negative image of the tool in the workpiece. ECDM is a promising technique because of all the advantages such as the inexpensive setup, high removal rate, no subsequent stresses witch results in no micro cracks, good surface quality and environmentally acceptable.
There are two disadvantages with ECMD, the first one is the failure of tool insulation. This happens when the salt electrolytes obstruct the holes. The second one is stray removal, this has a huge effect on the reliability of ECDM. ECDM also does not offer the same precision as conventional micro drilling. Next in line is electron beam drilling or short EBMD, this technique is also a thermal drilling process but this time in vacuum conditions to stop the electron beam (EB) from reacting with air molecules and thus losing its energy.
The EB is produced by a cathode on high voltage, the electrons are then accelerated by an electric field that is located between the cathode and anode. After the acceleration, the EB is focused on the specimen by an electromagnetic lens. When the EB comes in contact with the specimen it instantly melts and vaporizes the material. At the backside of the specimen a backing material is placed. When the EB cuts through the specimen, the backing material reacts with the EB and produces gas. As this gas is produced, it pushes the molten material out of the hole.
There are two operation types namely single pulse and multi pulse mode. The single pulse mode also called perforation is more used due to its higher productivity rate for high aspect ratio holes compared to laser, EDM or ECM. The multi pulse mode is mainly used to expand the hole shape.
In the future Electron beam drilling has the potential in applications where more than 10 000 holes are needed on complex shapes and with hard to machine materials. A promising feature of this technique is that it can be combined with CNC controlled machines. Spark assisted chemical engraving micro drilling or SACE is micro drilling based on electro chemical discharge and thermal assisted etching. Again there are two electrodes, one the tool and one the workpiece plus an electrolyte solution between the two.
First a potential difference is applied between the two electrodes. Due to the electrochemical discharge, near the tool, the temperature rises up to 500-600°C and this helps the etching process. This technique can be used to drill micro holes in non-conductive materials such as ceramics and quartz.
Brittleness and non-conductiveness are two material properties that make micro drilling a very difficult job, especially when a high aspect ratio is needed. Traditional techniques have a relative long process time and poor surface quality on these materials. Thermal processes for example laser micro drilling speeds up the process time but has the negative effect of micro cracks. Chemical procedures take too long and can’t produce a high aspect ratio. SACE on the other hand is based on thermal assisted etching and can provide a solution for micro drilling in glass-like materials. There are two main types of SACE micro drilling namely gravity and constant velocity feed micro drilling. In gravity feed micro drilling, a constant force is applied to the tool-electrode.
This gives combined with tool-rotation and a pulsed voltage a micro hole with minimal surface cracks, see figure 5. This type is also the most used in research. The second type has only had a little amount of research.At last there is micro drilling by virtue of ultrasonic vibration or ultrasonically assisted machining. There is still many research going on in this field, this type of micro drilling uses an ultrasonic vibration on either the workpiece or the tool, see figure 6.
Usually this frequency is higher than 20kHz. The main benefit of this technique is that it can produce a micro hole on its own. But it can also be added to other conventional or non-conventional types of micro drilling as shown in figure 4. Ultrasonic assisted machining improves the hole surface quality, reduces cutting force, makes it easier to remove chips and to obtain higher aspect ratios. The only drawback for this technique is the slow production rate.
Studies have shown that ultrasonic assisted micro drilling improves the oversize, roundness and center displacement. With micro-EDM researchers were able to reduce the EDM gap, taper, machining time significant and this with a higher surface quality. This shows that this technique can be used to enhance other fundamental techniques. For example, figure 5 shows that the roughness of ultrasonic assisted drilled holes is much lower than that of normal drilled micro holes.In conclusion, today laser micro drilling is one of the most popular micro drilling techniques. But although it is the most popular, it still had some drawbacks for example: poor surface quality of the drilled hole, splatter, residual stresses which cause micro cracks and a higher production cost.
A lot of these drawbacks can be solved with the right implementation of the process and laser parameters. Also out of figure 6 we can conclude that laser beam drilling is the best cost effective technique that can be used when drilling micro holes with a large aspect ratio. In the future however it may be possible to improve laser micro drilling via ultrasonic vibrations. In this paper there was no sign of any research done on this topic but when you see the improvements ultrasonic vibrations have on micro-EDM and micro drilling techniques it may be worth a shot. Maybe ultrasonic assisted micro laser drilling will be the future.