Metallurgical bonding during consolidation of titanium particulates

2017-03-03T00:06:14Z (GMT) by Ravichandran, Thyagarajan
Titanium and titanium alloys, due to their higher cost are seldom used for varied applications, unlike aluminium, magnesium and steel, which are widely used. Powder metallurgical route has given a new dimension to produce low-cost, near-net shaped parts using titanium powders. The conventional method of producing titanium alloys is through either blended elemental process of pre-alloyed process. The limitation of the aforementioned processes are that, not all elements can be alloyed, owing to their higher costs. As an alternative to the conventional blending process, Electroless plating is introduced to alloy titanium powders in the present work. Two types of commercially pure grade-2 titanium powders, which differ in their particle sizedistribution and morphology are used and are named as grade-A and grade-B. The grade Aand grade B powders were coated with nickel-phosphorus (Ni-P) and nickel-boron (Ni-B) separately. The Ni-P coating on the powders was performed in acidic condition and Ni-B coating, in alkaline condition. The uncoated, Ni-P coated and Ni-B coated powders were characterized for their particle size distribution using Malvern Mastersizer instrument, particle shape through Richardson plot and chemical composition using Inductive Coupled Plasma - Atomic Emission Spectroscopy (ICP-AES) and LECO analyser. It was found that on coating the powders with Ni-P and Ni-B separately, a change in the apparent density and tap density values were observed. The Hausner ratio, which signifies the rearrangability of the powders, was found to be highestfor the powders in uncoated condition and lowest, when coated with Ni-B. The morphology of the particles characterized using Richardson plot revealed that on coating, the shape of the particle changed from irregular (in uncoated condition) to being more regular (after coating with Ni-P and Ni-B). Also, the particle roughness was found to have decreased upon coating. A novel method, combining the aspects of the Kawakita and Alderborn approaches, has been employed in order to understand the compaction behaviour of the uncoated and coated powders. The overall consolidaton is described by the Kawakita-Ludde relationship, and the transition pressures which demarcate the limits of ”rearrangement only” and ”plastic deformation only” are identified using a modification of the Alderborn relationship. A significant increase in the green strength is achieved with the Ni-B coated powders, and this is attributed to a modification of friction conditions and the number of contacts, which lead to an increased contribution from plastic deformation. The Mohr-Coulomb and Ohyane equations are used to describe the effect. The compaction behaviour was further analysed using the Drucker-Prager-Cap model in the combined ”rearrangement + plastic deformation” and ”plastic deformation only” regions. The aforementioned mechanisms are explained in detail by constructing the cap surfaces at different stress ratio’s (ratio between hydrostatic stress and deviatoric stress). The early stage sintering behaviour of the uncoated and coated powders were studied using the sintering data (relative sinter density vs. temperature), dilatometer and differential thermalanalyser (DTA) curves, X-ray diffraction peaks at different sintering temperatures, optical and scanning electron micrographs. The sintered uncoated powders showed greater densification in the beta-titanium phase field than in the alpha-titanium phase field. Also, from the dilatometer and DTA curves, it was found that the transformation of alpha-titanium to beta-titanium phase field is not a sudden process, rather a gradual process. The formation of secondary phases like Ti2Ni, Ti3P in the Ni-P coated powders were found to influence the densification. The relative sinter density values of the Ni-P coated powders were found to be less than that of the uncoated powders. The sintering activation energy values for uncoated and Ni-P coated powders were calculated using Wang and Raj method and Maqueda method. Plastic flow was found to be the dominant sintering mechanism in uncoated grade A and grade B powders in the temperature range between 900oC and 1300oC. The Ni-P coated powders showed grain boundary diffusion as the dominant sintering mechanism between 623oC and 827oC and plastic flow between 900oC and 1300oC. The Ni-B coated grade A and grade B powders showed poor sintering behaviour when compared to the uncoated and Ni-P coated powder. The swelling of compacts were observed after sintering, which was attributed to the presence of elements like sodium and chlorine, which were co-deposited during the electroless plating process. The uncoated, Ni-P coated and Ni-B coated powders were powder rolled and their mechanical properties were determined. It was found that the mechanical properties of Ni-P coated powder rolled strip were found to be superior to the uncoated and Ni-B coated powders. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of the Indian Institute of Technology Bombay, India and Monash University, Australia.