05-04-2012, 10:28 AM
Ultrasonic machining of titanium and its alloys
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Introduction
Ultrasonic machining (USM) is of particular interest for the
machining of non-conductive, brittle work piece materials such
as engineering ceramics. Because the process is non-chemical
and non-thermal, materials are not altered either chemically or
metallurgically [1]. The process is able to effectively machine
all materials harder than HRC 40, whether or not the material is
an electrical conductor or an insulator [2–8]. Holes as small as
76min diameter can be machined, however, the depth to diameter
ratio is limited to about 3:1 [4,8]. The history of USM began
with a paper by R.W. Wood and A.L. Loomis in 1927 [9,10]
and the first patent was granted to American engineer Lewis
Balamuth in 1945 [3,11,12].
Ultrasonic machining of titanium alloys
Titanium has been recognized as an element (Symbol Ti;
atomic number 22; and atomic weight 47.9) for at least 200 years.
However, commercial production of titanium did not begin until
the 1950s. At that time, titanium was recognized for its strategic
importance as a unique lightweight, high strength alloyed structurally
efficient metal for critical, high-performance aircraft,
such as jet engine and airframe components [46]. Significant
unused worldwide sponge, melting and processing capacity for
titanium can accommodate continued growth in to new, highvolume
applications. These alloys are branded as difficult to
machine materials but have high utility in manufacturing sector
[47]. Poor thermal conductivity of titanium alloys retard the
dissipation of heat generated, creating,
Basic elements of an ultrasonic machine tool
The machines for USM range from small, tabletop-sized
units to large-capacity machine tools. In addition to the partsize
capacity of a USM machine, suitability for a particular
application is also determined by the power rating [1]. Fig. 4
shows compact 500W USM machine for small, light-weight
work piece [58].
Ultrasonic transducer
In the case of USM transducer, electrical energy is converted
in to mechanical motion [1,13,56,57]. With a conventional
generator system, the tool and horn are set up and mechanically
tuned by adjusting their dimensions to achieve resonance
[13]. Recently however, resonance following generators has
become available which automatically adjust the output high
frequency to match the exact resonance of the horn/tool assembly
[2]. They can also accommodate any small error in set
up and tool wear, giving minimum acoustic energy loss and
very small heat generation [20].
Conclusions
1. USM is a non-thermal process, which does not rely on a
conductivework piece and is preferable for machiningwork
pieces with low ductility and hardness above 40 HRC.
2. It is possible to ultrasonically drill holes in titanium without
causing excessive surface integrity damage; specifically
cracking using ultrasonic assisted drilling. Higher surface
finish is attained when machining on titanium alloy is undertaken
by USM and it is not always necessary that if work
piece with higher toughness value is machined, it will have
less MRR rather it is combination effect of material composition
(hardness of work piece) relative to the tool and work
piece. In otherwords selection of operating parameter levels
is critical in order to achieve acceptable productivity.