Tom Shelley describes a technique for enhancing
fatigue strength using
ultrasound that has a surprising number of other uses
Ultrasonic finishing is being used to greatly enhance the fatigue
strength of steel and other metal alloy components in a fraction
of the time required by alternative techniques and at a much
lower cost.
It can be incorporated into a machining process as a separate
step, without needing to remove the workpiece, and is already in
commercial use in Russia where experience of the process and its
capabilities is beginning to build.
Ultrasonic finishing was primarily developed for treating
crankshafts and spherical and conical surfaces. However,
ultrasonics also appears to work well as a means of eliminating
lime scale build-up in hot water and steam systems running on
hard water. It also has a host of other uses.
The technology is the result of a lifetime's research work by
Professor Yuri Holopov. This has now spun off from St Petersburg
State Technical University into the North West Centre of
Ultrasonic Technology or INLAB.
The underlying principle is the same as that used in shot peening
and vibratory finishing (Eureka, March 2002) and super precession
polishing (Eureka, April 2002), which is to put the surface
layers of the material into compression to produce a smoothed,
polished finish. Both effects are known to be effective, because
they remove defects which may act as fatigue crack initiation
sites, and close up any that are open.
The underlying technique is to put a tool with an ultrasonically
driven hard steel tip into the tool post of a lathe. It does its
work as the last stage of a set of machining operations - without
requiring removal of the workpiece from the machine.
Ultrasonic finishing can be undertaken in similar timescales as
finish machining, Professor Holopov mentioning feed rates of 0.05
to 0.2mm per revolution for crankshaft journal polishing at
machining speeds of 500 to 600rpm. Residual tensile stresses of
300 to 400N/mm2 are reversed and turned into compressive stresses
of 240 to 700N/mm2. The depth effect is 50 to 500 microns, which
is slightly higher than for shot peening. Holopov claims general
fatigue strength increases of around 50%, depending on
circumstance and fine machined finishes of 1.6 to 3.2µm are
turned into mirror finishes of 0.2 to 0.4µm. The fatigue
strength enhancement is less than that claimed for shot peening
under NC control plus vibratory finishing under best
circumstances, but seems realistic.
If engine crankshafts could be made thinner and lighter, even by
a small amount, it has an immediate, fairly dramatic effect on
engine acceleration and fuel consumption. Research by most motor
manufacturers on improved crankshafts seems to be mainly focussed
on using exotic materials, such as metal matrix composites. Shot
peening and vibratory finishing has been shown to bring such
dramatic benefits to conventional steel gears and gearboxes, but
only at motorsport-type prices. Ultrasonic finishing looks to be
a lot cheaper than either technique.
The tools are driven using magnetostrictive technology and
deliver power outputs of 400 to 600W. They are made in two parts:
an acoustic head in contact with the workpiece, which weighs 4 to
6kg; and a separate electronic converter, which turns the normal
220V 50Hz mains supply into 22kHz. The latter weighs 7.5kg.
About 100 sets are currently in service in Russian factories with
more on order and a large number of serious enquiries under
discussion. The technique has been found to give benefits to
mild, construction, tool, cast, alloy and bearing steels; gray
cast and wrought iron; and copper, brasses, bronzes and aluminium
alloys. It has been successfully applied to shafts, faces,
conical and spherical surfaces (external and internal), grooved
channels, fillets and the insides of blind holes
Selling price for a complete set is currently $8,000, down from
$15,000 a short while ago. The company generally seems to be
cutting prices as it gets its production under way and the
equipment comes with a two year guarantee.
Sound softened water
As well as increasing the fatigue strength of metals, Holopov
finds that ultrasound is also a good way of preventing the
build-up of scale on the insides of hot water pipes and boiler
tubes.
Scale build-up is a hugely expensive problem. A 1mm scale
layer, for example, on the inside of a typical heat exchanger can
result in an increase in fuel consumption of 2 to 2.5% and a 5mm
scale layer requires a fuel increase of 8 to 10%. Overheating and
corrosion under the scale layer can also substantially reduce
tube life.
The most common way of softening water is to use chemicals to
precipitate out lime and other contaminants; ion exchange is
another. Both procedures incur significant cost. There have,
however, been persistent reports that various other techniques
can cause the precipitation of calcium salts as a fine suspension
of apatite crystals, instead of calcite coatings - eliminating
harmful effects at much lower cost. We are already aware of the
use of permanent magnets and radio frequency magnetic fields but
Holopov and his colleagues recommend low levels of ultrasound.
Research into this method apparently began in the USSR in the
1930s, and started to see service in the 1950s and 60s. However,
it is only now that the cost of the equipment has dropped to a
point where the method is economically attractive.
As well as initiating precipitation within the bulk of the water,
ultrasound apparently offers the additional benefit of being able
to initiate fractures in - and eventual break-up of - existing
scale coatings. Power consumptions in INLAB's commercially
available equipment is 30, 70, or 380W, depending on application.
A plethora of potential
Other processes mentioned by the company include: reinforcing of
plastic with metal; hole broaching and engraving of ceramic,
glass and stone; reduction of residual stresses in welds;
dispersion and de-aeration of fluids; metal finish turning,
drilling, countersinking, hole reaming and threading; removal of
cast ceramic from a casting hollow; and 'impregnation of friction
pairs with geomodificators'.
This last is apparently a new process for incorporating naturally
occurring mineral ceramic materials into the surfaces of metals,
thereby reducing friction and wear. We are told that, in internal
combustion engines, it can increase intervals between repairs and
servicing by four to six times and reduce fuel consumption by 8
to 15%. For further details, watch this space! (For more
information see www.utinlab.ru
and contact Tom
Shelley in England and Yuri
Neshitov in Russia )
Design Pointers
Ultrasonic vibration may be applied to a smoothing tool in a
final finishing operation to induce compressive stresses in
surface layers, greatly enhancing fatigue life
Ultrasonic vibrations may also be used to prevent and remove
scale in pipes carrying hot water and in a host of other
applications