Tool-travel speed

On the other hand, using high machining voltages (more than 32 V) at low tool travel speeds results in a non-smooth channel surface with significant depth variation along the channel. As a general rule, the quality of the microchannels deteriorates as the tool travel speed is decreased. This can be attributed to the poor material removal rate and the accumulation of melted material inside the microchannel immediately behind the tool. 

Well-defined linear channel edges and smooth channel surface This type of contour is characteristic of low voltages with appropriate tool travel speeds. Typical values are 28 V with the tool travel speed ranging from 5 to 10 pm/s and 30 V with the tool travel speed varying between 15 and 30 pm/s (Fig.6.12(a)). 

Jagged outline contours with smooth channel surface This contour is observed for low machining voltages (less than 32 V) with tool travel speeds lower than in the previous contour type. 

Heat affected edges with non-smooth channel surface and thermal cracks When the tool travel speed is low and the machining voltage is high (typically 32 V with speed less than 30 pm/s and 35 V with speed less than 40 lm/s) the edges are unclear and heat affected and the surface is not flat and smooth. Thermal cracks are observed (Fig. 6.12(d)). 

Deteriorated microchannels As the tool travel speed is increased the channels are non longer continuous and the inner surface becomes very rough. For example, at 28 V increasing the tool travel speed above 40 lm/s results in deteriorated microchannels (Fig. 6.12(e))... 

Figure 6.13 summarises the type of microchannel obtained for different combinations of the machining voltage and the tool travel speed. Machining voltages less than 32 V result in acceptable microchannel quality. In this voltage range, an appropriate selection of the tool speed will result in the best... 

Figure 6.13 Characterisation diagram for microchannels machined using SACE technology as a function of the machining voltage and the tool travel speed (for tool-electrode distances less than 15 J.m). Machined using a 0.5 mm stainless steel cathode in 20 wt% NaOH. Reprinted from [23] with the permission of the Journal of Micromechanics and Microengineering.

If the trajectory of a channel can be directly controlled by the motion of the tool-electrode, the depth of the channel cannot be monitored directly. Depending on the tool travel speed and the machining voltage, different depths are achieved. Another important issue is the chemical etching of the substrate. Due to this effect, the depth of the channels will not remain constant over machining time, but on the contrary increases slightly. A typical example is shown in Fig. 6.16, which illustrates the depth of a channel machined at 28 V... 

Figure 6.18 shows the profile of a microchannel machined at 30 V and with a tool travel speed of 30 pm/s at different tool distances from the glass surface. The average depth of the microchannels decreases with higher tool distance. The quality of the machined microchannels does not change significantly for tool distances up to 15 pm. Above 25 pm no acceptable results can be achieved. 

Whorl Pin. The next evolution in pin design is the Whorl pin developed by TWI (Ref 88, 89). The Whorl pin reduces the displaced volume of a cylindrical pin of the same diameter by 60%. Reducing the displaced volume also decreases the traverse loads, which enables faster tool travel speeds. The key difference between the truncated cone pin and the Whorl... 

Wayne Thomas at TWI has recently focused on FSW tool designs that increase the tool travel speed, increase the volume of material swept by pin-to-pin volume ratio, and/or increase the weld symmetry (Ref 106, 107). Many of these tool designs have focused on tool motion and... 

Ref 112, 125) and lasers (Ref 126). Also, a finite element model has shown that a current passing between the tool and anvil can reduce the normal forces during tool plunge and at least double the tool travel speed, when compared to conventional FSW (Ref 127). 

Tool travel speed influences total heat input... 

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