Combined laser and plasma-arc processing torch and method

Details Combined laser and plasma-arc processing torch and method Combined laser and plasma-arc processing torch and method

1999-07-15

2002-05-14

Elve, M. Alexandra (Department: 1725)

Electric heating

Metal heating

By arc

C219S121450, C219S121460, C219S121630, C219S121640, C219S121840

Reexamination Certificate

active

06388227

ABSTRACT:

FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to processing torch devices used for welding, cutting, coating, cladding, and more specifically, to a combined laser and plasma-arc welding torch, and a corresponding method for using the combined laser and plasma-arc welding torch of the present invention, which combines features of laser and plasma-arc welding technologies for producing higher energy density and coupling efficiency for welding workpieces than is achievable by using current configurations of laser and plasma-arc welding devices.
Welding is a vital manufacturing technology in many industries. Welding processes are energy intensive as they require the production of high energy densities in order to create and move a pool of liquid material. In most common welding methods, the energy coupling efficiency between the welding tool and a workpiece is twenty to thirty per cent at best, depending upon the material and welding technology used. Thus, significant economic benefits can be obtained if the coupling efficiency can be increased. Other aspects of the welding process, such as weld quality and productivity are also of interest and can impact the economics of the process. Because all of these factors are in some way dependent upon the energy density, which is incident on the workpiece, much effort has been made to increase this quantity by developing additional technologies and welding tools.
Hereinafter, the term “workpiece” refers to a material, typically, metal, and subjected to a welding process involving the use of a welding torch. Hereinafter, the term “high energy density spot” refers to a very is localized region, or portion, on a workpiece, of highly concentrated energy originating from a welding torch.
One of these technologies, plasma welding, is a process in which a constricted arc is used as an energy source to melt and then fuse two metal pieces together. Plasma welding is routinely used in heavy industry because it can be used to weld thick plates quickly with a single pass, while producing a high quality weld. This technology is based on producing a high temperature partially ionized gas stream by forcing an inert gas through an electric arc. The arc heats the gas to a temperature s where it becomes ionized and conducts electricity.
If an electric field is set up between an electrode and the workpiece, the plasma-arc formed by the ionized gas will impinge on the workpiece and melt the material. In plasma-arc welding, appropriate choices of plasma gas flow rate, arc current, and weld travel speed will create conditions in which the high energy and momentum of the plasma-arc produces a dynamic pressure which causes the arc to penetrate the molten pool of material, forming a small hole which penetrates completely through the base metal. The hole is termed a “keyhole” and the welding technique in which such a feature is formed is termed is “keyhole welding”. In the keyhole technique, molten metal is displaced to the top surface of the bead of material by the plasma vapor as the vapor penetrates the material and forms the keyhole. As the plasma-arc torch is moved along a weld joint, metal melted at the front of the keyhole flows around the plasma-arc to the rear to form a weld pool. The principal advantage of this form of welding is the ability to perform relatively fast welding of materials with a single pass, with minimal preparation of joints. In addition, a general benefit of plasma welding is that it reduces stress or deformation in the workpiece because the plasma-arc is concentrated inside the keyhole.
FIG. 1
shows the components of a typical prior art plasma-arc welding torch
10
. Torch
10
is composed of an electrode
12
, which is recessed inside of, and surrounded by, a constricting nozzle
14
having an exit orifice
15
. The space formed in-between electrode
12
and nozzle
14
is referred to as the plenum chamber
16
. Nozzle
14
is partially surrounded by an outer or shielding gas nozzle
17
.
In the operation of torch
10
, an electric current is set up between electrode
12
and workpiece
18
or between electrode
12
and nozzle
14
. An orifice gas is forced into plenum chamber
16
, thereby surrounding electrode
12
. The orifice gas becomes ionized in the electric arc, thereby forming plasma. The plasma issues from orifice
15
as a plasma-jet
20
and impinges on workpiece
18
. Because electrode
12
is recessed inside plenum chamber
16
, plasma-jet
20
is collimated and focused by constricting nozzle
14
(and the electric field set up between electrode
12
and workpiece
18
if such is the case) onto a small region of workpiece
18
. This serves to increase the energy density on workpiece
18
. An auxiliary shielding gas is commonly forced through outer nozzle
17
and is used to blanket the region on workpiece
18
at which the plasma-jet
5
impinges in order to reduce atmospheric contamination of the melted material pool formed by the jet.
Even though plasma-jet welding has many important advantages as a welding method, there are several serious limitations to plasma welding technology. The depth of keyhole penetration and therefore weldable material thickness, as well as the achievable welding speed, are limited by the energy density of the plasma-are. In addition, the keyhole may collapse under some operating conditions, thereby creating an obstacle to finishing the weld joint. Another limitation is that plasma instabilities and plasma width restrict the use of the technique to certain types of materials.
In plasma welding, the energy density at the location of the workpiece is the most important parameter in establishing the keyhole. The keyhole forms under a range of welding currents from 10 to 250 amps, depending on the material and velocity of the workpiece with respect to the welding torch. In addition, the available energy density in the plasma-arc and therefore into the heated spot on the workpiece depends on the mechanisms of heat transfer within the plasma-arc.
In this regard, there are three modes of heat transfer loss from the plasma-are to the environment: convection, conduction, and radiation. These modes of heat transfer reduce the temperature of the plasma-arc, and consequently the energy density at the workpiece. The conduction mechanism is usually negligible under most operating conditions. When the plasma-arc operates at relatively low temperature, convective heat losses to the environment are dominant. However, as the temperature of the arc increases, radiative heat losses, which are proportional to the fourth power of temperature, become dominant. An equilibrium condition exists in which any increase in plasma-arc energy due to dissipative electrical current flow and temperature is offset by the radiative losses. This condition limits the maximum power density of the plasma welding process, thereby limiting the ability to weld thicker plates or increase the welding speed, and therefore the productivity of this welding process.
During normal plasma-arc welding, radiative heat transfer becomes dominant for currents of about 200-250 amps, and plasma power densities of about 3-3.5 kilowatts. It is physically impossible with existing technologies to achieve higher power densities with plasma welding. Any attempt to increase power density by increasing power consumption from the welding torch leads to a reduction in welding efficiency. If higher speed welding is attempted, the plasma-arc becomes unstable and poor quality welding results. High-speed plasma welding is difficult to achieve because the heating spot on the workpiece quickly falls behind the welding torch axis. Such spatial instability is a reason for poor weld quality.
Another type of welding process, which can achieve high energy densities at a weld point on a workpiece, is laser beam welding. This welding process also relies on forming a keyhole in the material to be welded and has found many applications in industry. In terms of the power density applied to the workpiece, laser beam welding can be compared with electro

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