Investigating into advanced coatings for bandsaw blades

Khan, Fahd (2011) Investigating into advanced coatings for bandsaw blades. Doctoral thesis, Northumbria University.

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Abstract

Bandsawing is an important metal cutting operation carried out in a variety of industries in order to remove raw material for secondary operations. Due to its continuous cutting action, bandsawing has over taken other cutting processes such as power hack sawing and circular sawing. Bandsawing operation offers numerous advantages such as high cutting rate, low kerf loss, longer tool life and high automation possibilities, due to its efficient and continuous cutting action.

It is costly and time-consuming to test the wear of the full bandsaw products on a full-scale bandsaw machine. In order to overcome this, a single tooth test rig has been developed at Northumbria University, which utilizes a single bandsaw tooth instead of the complete bandsaw loop. Previous research has utilized this test rig for evaluating bi-metal saws while machining steels.

Development of new, wear resistant and difficult-to-cut materials such as titanium alloys (e.g. Ti-17) imposes greater demands on bandsawing operations. Traditionally, high speed steels and cemented carbides have been employed to cut/machine these materials. The main disadvantage of high speed steel cutting tools is that it undergoes severe plastic deformation when cutting at temperatures above 600ºC. Tungsten carbide cutting tools have proven their supremacy in almost all the machining processes and interrupted cutting of these difficult-to-cut titanium alloys.

One of the challenges in design of cemented tungsten carbide tools is the optimization of toughness and wear resistance. This has led to the development of coated carbide tools, which accounts for the major portion of all commercial metal cutting inserts sold worldwide.

This current research has furthered the use of single tooth test rig, by using un-coated and coated tungsten carbide tipped bandsaw blades while machining high performance titanium alloys (Ti-17). The purpose is to evaluate and assess the performance of un-coated and coated carbide bandsaw teeth and ascertain wear mechanisms and modes of single bandsaw tooth, in a way that is representative of full product testing.

Two different coatings (AlTiN and TiAlSiN) were chosen to be deposited using arc evaporation PVD technique. These coatings were selected due to their properties in terms of wear resistance and structure: TiAlSiN is nano-structured, while AlTiN is conventional in terms of its grain size. These coatings were characterized using various techniques, such as electron microscopy and nano-indentation.

Cutting tests were carried out using un-coated and coated carbide bandsaw teeth. Adhesive wear and diffusion wear were identified as the wear mechanisms, while flank wear and chipping were confirmed as the principal wear modes for the un-coated carbide bandsaw teeth. Cutting forces were found to be less while machining Ti-17 alloy using coated teeth as compared to the forces obtained while machining with un-coated teeth. Less material was found to be adhering to the coated teeth as compared to un-coated teeth.

Finite element analyses (FEA) were carried out on interaction of the cutting tool and the workpiece to determine the stress concentration during the cutting process. It was found that the increase in the honing lengths on the carbide teeth reduced the stresses and moved the maximum stress from the edge of the rake face to the honed edge.

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