Abstract
Abstract : This project investigated the drilling of Titanium (Ti) and Ti6Al4V powder compacts made using uni‐axial powder compaction. The aim was to determine whether green machining can improve the machinability of Ti‐based alloys. The objective of the study was therefore to understand the interaction between the properties of the compacted powders (green compacts) and the drilling parameters with a view of providing some understanding of the green machinability. Green machining needs powder compacts with sufficient strength to withstand the clamping and machining forces. Two approaches were used to enhance the (green) strength of the powder compacts in this study: changing the binder used during powder compaction and using different pressures (430 and 600 MPa) to compact the powders. An unconventional polymeric binder based on poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA), was investigated as a green strength enhancer. The investigation involved understanding the thermal behavior of the blended polymers using thermogravimetric analysis (TGA) and swelling tests, molecular bonding using Fourier Transform Infrared (FTIR) spectroscopy, and finally, optimizing the quantity of binder by determining the green strength of powder compacts using the Brazilian Disk test. Green compacts for drilling experiments were produced using the optimized polymeric binder. For comparison purposes, compacts made using conventional Acrawax (at 0.7wt. %) as a binder were also produced. The green compacts were characterized for density, using mass and volume, and strength using the Brazilian Disk test. Some of the compacts with polymeric binder were subjected to a curing treatment in order to increase their strength. The compacts were then subjected to drilling experiments. Two types of twist drills, the cheaper uncoated high speed steel (HSS‐Co), which cannot machine wrought Ti‐based components, and the more expensive TiAlN‐coated solid tungsten carbide‐cobalt (WC‐Co), which is the main machining tool for Ti‐based components, were used. The drilling speed and feed rates were varied. Machinability was characterized using the size of breakouts, the iv surface roughness of the drilled holes, and for one set of drilling parameters, and the drilling/cutting force. Analysis of the polymeric binder indicated it possessed the ability to form a network structure of higher strength when cured (heated to an optimized temperature of 200 ⁰C and soaked for an optimized duration of 1 hour). When used as a binder, the polymeric binder considerably increased the green strength of Ti powder compacts compared to when Acrawax C was used. This was attributed to the curing that was observed to have occurred in the polymeric binder. For this reason, its use was extended to the Ti6Al4V powder compacts. The drilling tests showed that all the powder compacts investigated could be clamped and machined, regardless of their green strength and type of tool used. This was an important observation, because using the cheaper HSS‐Co tools has potential to lower the cost for machining Ti‐based alloys. However, except for the observation that Acrawax imparted better machinability than the polymeric binder on Ti compacts did, the results did also indicate that generalizations could not be made regarding the machining responses of the powder compacts because the behaviour was a function of the type of powder, type of binder, and tool used. For example, using Acrawax as a binder for Ti powders, increasing feed rate and cutting speed reduced machinability (increased breakout sizes) for samples machined using HSS‐Co but increasing speed enhanced machinability (decreased breakout sizes) for machining with the carbide twist drills. On the other hand, regardless of the tool used, the breakout size in Ti powder compacts pressed with polymeric binder increased with speed and feed rate. In the case of Ti6Al4V, compacts obtained using the polymeric binder, breakout size increased with increasing cutting speed and reducing feed rate (i.e. a lower feed rate increased the size of the breakout). In the same vein, this research found that green strength was not a good indicator of machinability. For Ti compacts, the machinability of Acrawax and uncured polymeric binder compacts was improved by a higher green strength. However, for the cured polymeric binder, a lower green strength conferred better machinability. For Ti6Al4V powder v compacts, different green strengths gave similar breakout sizes and/or higher green strengths caused larger breakouts. Also, while the cutting force increased with green strength (Acrawax<uncured<cured for Ti and uncured<cured for Ti6Al4V), for both the Ti and Ti6Al4V compacts, the cutting forces for a specific category of samples, e.g. cured, compacted obtained at the two different compaction loads (430 and 600 MPa) and therefore possessing different green strengths, were similar. For example, Ti‐Acrawax samples pressed at 430 and 600 MPa had similar cutting forces even though they had different green strengths. A comparison of the machinability of Ti compacts and the sintered compacts found that while HSS‐Co twist drills could drill through Ti green compacts, they failed catastrophic (broke) while trying to drill through the sintered parts. This was a confirmation of what is known in literature, but highlighted the generally improved machinability offered by green machining.
M.Phil. (Engineering Management)