Tool rotation

Statistical methods are used to measure the density variation as the tool rotates, the stand-off can be estimated and the density corrected. A density caliper can be computed that works for cavings of 2 in. (5 cm) or less when the tool rotates at a speed ranging from 6 to 150 rpm. 

Subsurface obstructions and dense soil or waste layers may cause tool rotation to halt. Shallow obstructions (<20 ft deep) should be excavated prior to applying the MecTool system. 

Figure 7.7 Comparison between a channel machined (a) without and (b) with tool rotation. The sharpness of the edges is improved. Reprinted from [128] with permission from Elsevier.

In 2D-S ACE machining, tool rotation also contributes to increased performances (Fig. 7.7). In particular, it sharpens the edges of the machined structures and smoothes the machined surface [123,128]. 

One way to overcome this difficulty was proposed by Jain et al. [62]. They used abrasive cutting tools. The feed of the tool-electrode was chosen in such a manner so as to always keep contact between the tool and the workpiece. Thus, the machining gap is of the same order of magnitude as the size of the abrasive particles on the tool-electrode (a few micrometres). The researchers showed that the material removal rate achieved with the abrasive tools is higher than that obtained with the conventional tools. This effect may be attributed to the increased gap, but, at the same time, as tool rotation (20 rpm)... 

By mounting the tool-electrode holder on a motor-controlled stage, drilling can be done at a constant speed. Depending on the motor controller used, more or less complex velocity profiles can be used. By synchronising the vertical motion with tool rotation, it is possible to drill threads in glass as shown by Lee et al. [79]. 

The tool serves three primary functions, that is, heating of the workpiece, movement of material to produce the joint, and containment of the hot metal beneath the tool shoulder. Heating is created within the workpiece both by friction between the rotating tool pin and shoulder and by severe plastic deformation of the workpiece. The localized heating softens material around the pin and, combined with the tool rotation and translation, leads to movement of material from the front to the back of the pin, thus Ailing the hole in Ihe tool wake as the tool moves forward. The tool shoulder restricts metal flow to a level equivalent to the shoulder position, that is, approximately to the iniAal workpiece top surface. 

Threads are used to transport material from the shoulder down to the bottom of the pin for example, a clockwise tool rotation requires left-handed threads. A round or domed end to the pin tool reduces the tool wear upon plunging and improves the quahty of the weld root directly underneath the bottom of the pin. The best dome radius was specified as 75% of the pin diameter. It was claimed that as the dome radius decreased (up to a flat-bottom tool), a higher probability of poor-quality weld was encountered, especially directly below the pin (Ref 85). The versatility of the cylindrical pin design is that the pin length and diameter can readily be altered to suit the user s needs. Also, machining a radius at the bottom of the threads will increase tool life by eliminating stress concentrations at the root of the threads. 

Rorvik (Ref 31) demonstrated that a zirconia-coated steel anvil retained more heat, with the workpiece allowing the tool to travel three times faster to obtain the same heat-affected zone width produced with a steel anvil. A statistical analysis of nine input parameters determined that cooling the anvil had a minimal impact on the friction stir weld in fact, tool rotation rate, travel speed, and tool depth were more important (Ref 24). Weld quality and performance is affected by differences in heat transfers observed when comparing the friction stir welding of flat plate versus extrusions (Ref 97). Extrusions typically have complicated cross sections, with features that quickly draw heat away from the friction stir weld. This dissipation of heat through the extrusion increases the tool heat input necessary to create a quality friction stir weld. 

Tool rotation rate influences total heat input... 

Mechanical Properties. As noted previously, caution must be exercised when interpreting strain within weldments, due to the potential for strain localization in transverse tensile tests. However, yield and tensile strength results require no special consideration. Yield strength is often related to hardness, and based on the hardness curves typically obtained for 2024 Al, yield strength as well as fracture location should correlate with the lowest-hardness location in the W -hardness curve. Mechanical properties for 2024 Al have been reported in numerous publications as a function of FSW variables, including tool rotation rate, travel speed, and sheet thickness (Ref 2,4—8,11, 15). 

Friction shr welds were produced using tools machined from a W-25%Re alloy. A tool with a shoulder diameter of 14 mm ( /i6 in.) and a pin length of /.9 mm (0.075 in.) was used to produce all of the welds discussed here. No threads or other profiles were used on the pin. Tool plunging was completed under displacement control. The tool was maintained at a forward tilt angle of 1° for welding, and the welds were run under load control of the axial (z) force. The welds were produced with a tool rotation rate of 200 rpm and a travel rate of 100 mm/min (4.0 in./min), using an axial load of either 9.8 or 10.7... 

Slipping, with the rotational speed of the material being an arbitrary constant fraction of the tool rotation speed... 

Fig. 12.25 (a) Failure loads for friction stir spot welded 5052. (a) 1 mm (0.04 in.) single overlap, (b) 1 mm (0.04 in.) double overlap, (c) 1.6 mm (0.06 in.) singleoverlap configuration. Mechanical properties are not significantly influenced by plunge depth with increasing tool rotation rate. The thicker sheet exhibits lower weld strength. Source Ref 21... 

Traverse speed and tool rotation rate influence abnormal grain growth. 

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