CA1244523A - Method for producing parallel-sided melt zone with high energy beam - Google Patents

Abstract

ABSTRACT
A method is disclosed for controlling a high energy radiation beam to produce a melt zone with parallel sides in a workpiece having first and second opposed surfaces between which the zone extends. The method comprises (a) directing the beam at a slight angle to the normal through the first surface of the workpiece with the focus of the beam being at or adjacent the second surface; (b) oscillating the beam and workpiece relative to each other about a point located on the beam axis at or adjacent the first surface and in a plane perpendicular to the lateral line of advancement of the directed beam so that opposite sides of the melt zone in the plane will each be sequentially rotated to assume an orientation substantially perpendicular to the first surface; and (c) laterally advancing the directed beam along a on the first surface of the workpiece while carrying out the oscillations of step (b) at a selected frequency above a minimum.

Description

5~3 METHOD FOR PRODUCING PARALLEL-SIDED
MELT ZONE WITH HIGH ENERGY BEAM
This invention relates to the use of high energy beams for melting workpieces and, more particularly, to the technology of using electron or laser beams for creating sound welds in metallic workpiece~.
In ~he following description, reference is made to the accompanying drawings, wherein:

Figure l is a cros~-sectional schematic illustration of a typical workpiece subjected to a particle beam creating a melt zone in accordance with prior art;
Figure 2 is a view similar to Figure l, illustrating the weldment after solidification;
Figure 3 is a somewhat enlarged view similar to that of Figures l and 2, illustrating the scope of oscillation of the particle beam in accordance with the method of this invention to theeeby create a parallel-sided melt zone;
Figure 4 is a composite of two sequential views similar to Figures 1-3, illustrating a eirst alternative mode for effecting the oscillation of said particle beam relative to the workpiece, the workpLece is manipulated relative to a ~itationary particle beam;
Figure 5 is a view similar to Figure 3, illustrating a second alternative mode ~or carrying out the oscillation of the particle bea~ optlc~ are ~ed to shift the particle beaIn axi.~I;
Figure 6 i!l 'Itl I I another alternatlve rnode for eEfecting osci11ation oE the particle beam, varying differential magnetic fields are used to ~hift the beam axis; Flgure 6a illustrateY side elev~tional view Oe magnets; and ~'igure 7 is sti11 another alternative embodiment, illustrating the use of a magnetic lens to shift the particle beam axis.

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5~3 Autogeneous fusion welds made by a high energy beam, such a~ an electron beam or laser beam, usually exhibit a triangular melt zone A (in cross-section), such as shown in Figure 1, for most process speeds Qelected.
The upper head B of the weld is usually wider ~width C) than the width (D) of the welding beam (E) itself; this is due to heat conduction from the super-heated melt which is pcoduced in the actual beam impact area with the workpiece. A~ the beam burns its way through the material~ it creates the triangular heat affected zone a~
a function of time. As a consequence of the triangular cross-section of the melt zone A, the shrinkage during solidification of the weld is not uniform. If two unrestrained plates (10-11) are joined in this way, the weldment 13 causes warpage away from plane 14, as shown in Figure 2. If the plates are restrained as in a box structure, warpage cannot take place, but high internal stresses will develop. Such stresses will be very high at the broad side of the triangular fusion line weld and can lead to weld failure.
It would be advantageous if the high energy radiant beams can be controlled during welding ~o as to produce a parallel-sided fusion weld zone, thus eliminating or reducing warpage and peak stres~e~ in the weldments.
We are unaw~re of any publication which teaches the control of high energy radiant beams to obtain a parallel-sided heat affected or fusion weld zone, Particle beam control in the prior art ha~ included optical focu~ing oE la~er~ eOr rectLILnear pattern control (.~ee U.S. patent 3,965,327) and has included the use oE masks over the workplece to eliminate need for precise optics (see U.S. patent 3,742,182). These patentY fail to disclose a means o~ oscillating the beam in a manner to obtain a parallel-sided melt zone. In Japanese patents 54-101596 and 54-116356, there is utilized an oscillating beam control to obtain a sinu~oidal or spiral pattern for the print of th~ heat S'~3 affected zone, but they do not teach how to ob~ain parallel-sided weld or melt zones. All of the melt or heat affected zones of these patents contain the typical and conventional V-shaped cross-sectio~ because the teachings do not provide anything that would compensate for such shape.
In accordance with one aspect of the present invention, there is provided a method of providing parallel-sided melt zones in a metal workpieee subjected to a focussed high energy radiation beam, comprising con-trolling the relationship between the workpiece and beam so that the cross-seetion of the melt zone in the workpiece is oscillated at a selected minimum frequency along a lateral path of movement of the beam so that opposite sides of the melt zone assume a substantially perpendicular position relative to the surface through which the beam enters the workpiece. By using this method, a high energy radiation beam produces a melt zone with parallel sLdes in a workpiece having first and second opposed surfaces between which the melt zone extends.
In another aspect, the method comprises the steps of (a) directing the particle beam at a small angle to the normal through the first surface of the workpiece, with the focus of the beam being at or adjaeent the second surface to preferably generate a conlcally-shaped melt zone in the workpiece with the apex oE the cone being substantially at or adjacent the second surface; (b) oscillating the beam and workpiece relative to eac11 other about a point l.ocatccl on the bea~ lx:L~.7 at or ~1c~ cent the :E:Lrst ~urEnco ~1nd :lu ~1 p~1no perponc1:1c~ 1r 1:o thc l.nteral L1.ne Or nc1va~ o111c:~11t oE t11l~ c11rc~ctoc1 ber11n so that 0p1)0s:ltr! s.Lc1c!s of the me:1.t zone .Ln the p:1.ane w:l..LI. eacll be sequentially rotatec1 to assume an orientatLon ;ubst/1nt:La1.1y perpen(1-lculQr to the fIrsL surEace; ar1d (c) lateral:ly uc1vunc:Lng the c1Lrectec1 be,1n1 along a path on the first surfa(:e of tho workpioce wll:L:1.e carryl.ng out the osclL-lations of step (b) at a selected frequency above a 5Z~

minimum. Preferably, the frequency should be high when compared wi~h the travel speed of the beam.
Preferably, the workpiece is a pair of metal parts to be welded at a seam, the seam constituting the path for laterally advancing the directed beam, and the melt zone enveloping such seam. Advantageously, both the fiest and ~econd surfaces are parallel, and the sideQ of the resultant melt zone are sub~tantially perpendlcular to both such surfaces.
The oscillations may be carried out by optically controlling the source of the beam to shift the beam axis about the point of oscillation or uqing magnetic coil~ to deflect the beam axis about the point of oscillation.
Alternatively, the workpiece itself may be tilted eelative to the fixed beam causing the beam to assume a different angular orientation with respect to the melt zone first established.
To provide parallel-sided melt zones in a metal workpiece, the invention herein contemplates controlling the relationship between the high energy particle or laser beam and the workpiece so that the cross-section of the melt zone is oscillated at a high frequency along the seam oc weld line and in the plane of the cross-section so that opposite sides of the melt zone assume a substantially perpendicular position relative to the surface through which the beam enter~ the workpiece.
Therefore, as the beam axis is advanced along the seam or weld line~, the extremities of the oscillations create a flat-sided melt zone whlch overcome~3 the problem~ Oe warpage in an unre~trained structure or heat ~tress In a restralned structure.
High energy radiation beams with which thi~
invention is concerned are primarily of the electron or la~er beam type, but may include, within the definition, 3'j ~2'~ 3 proton or helium ion beams from a duo-plasmatron. These beams will melt and subsequently weld all metals if focused to a high enough power density. The beams are characteriæed by a high radiance, typically 106 to 108 Watt cm 2sr 1 for a laser beam and 108 to 1011 Watt cm 2sr 1 Eor an electron bearn. If a beam with a radiance of W*=108 W cm 2sr 1 is focused to an angular cone of 2 ~ ~2, alpha being the half aperture angle of the cone, then the focal spot has a power density of N*=2~ ~ 2W*. If ~=.01 rad (or .57 degree), we get N*=6.3x104 W cm 2. If we increase the aperture of the focusing cone to ~ =20, the power density becomes 10M~ cm 2. The power in the beam is another, independently controllable quantity and can range from lOOW to lOOKW. Such beams are used routinely today for welding. The impact alignment of these beams, according to the present state of the art, is normal to the workpiece surface and this results in the triangular melt zone as shown in Figure 1.
2Q Particle beams (electron) of this nature have particle energy chan~ing with the square of the velocity and have thermal energy cau~sing an elementary beam cone to be emitted from each single point of the source. At larger distances from a finite sized source, any particle beam of this type exhibits a therrnal beam spread and always has a finite anyuLar aperture. In ~act~ a ~parallel~ beam could not carry arly energy7 all deductions base(l or~ e a~-~slllnptlorl of ~lamln~r~ elow Oe the electrons in a bealrl ar:e ttlearefore simply lncorrect.
BecaUse of this beam ~pread, there must be some means for focusing the elementary rays of the pacticle beam and this is usually carrried out ~y the use c)f lenses which can include electrostatic or rnagnetic mean~.
Focusing creates an energy concentration effective to melt the material and creates the melt zone previously described. As shown in FigUre 3, such focused 4~;23 beams have a conical configuration which draws together the rays at a focus point 15 along its beam axis 16. It is this focus point 15 or cross-over of the elementary ray cones which should be located at or adjacent the lower surface 17 of a flat metal workpiece 18. Thus, a autogeneous fusion weld made by a laser electron beam E
will exhibit usually a triangular melt æone A or a V-shaped configuration, as shown in Figure 1. The upper head B of the melt zone is wider than the bottom because heat conduction from the super-heated melt, in the direct path of the beam, is produced in the actual beam impact area. As the beam burns its way through the material, it creates the triangular melt zone as a function of time.
A~ a consequence of the triangular or V-shaped cros~-section of the melt zone, shrinkage during solidification i5 not uniform and the two separated parts of the assembly to be welded will warp in a fashion as shown in Figure 2.
Unique oscillation of the beam relative to the workpiece overcomes this problem. The sides 21 and 22 of the V-shaped melt zone are reoriented so that such sides are swung to a new position ~22a and 21a) substantially perpendicular to the upper surface 20 (and preferably also to the lower surface 17) of the workpiece. The extremities 21a and 22a of these oscillations, when joined together a~ the bealn l~ advanced alorlg a lat~ral path relative to tl-le workpLece, ~(~rm~ ~t~lLght E)lanar side~. If the~ o~qcLll;ltiorls arel ca~ried out. ~o that the beam is first swung to a positLon ~-1 and then to a 3a position E-2, each side of the weld æone V zone iq brought to a perpendicular positlon relative to the upper ~urface ~0. The re~ult is a parallel-31ded zone as shown in Figure 3~ The oscillations should be at a selected frequency above a minimum. At least one oscillation should take place while the beam's peak spot has moved a distance along the weld path equal to its diameter. For example, if the spo~ size is 1 mm and the traveling speed (welding speed) is 100 mm/sec, then one oscillation should take place every 1~100 of a second~ This minimum corresponds to an oscillating frequency of 100 cycles/sec. At such frequency and any frequency higher than that, the derived melt zone will have parallel sides.
Two criteria are important to the function of such relative oscillation at each station along th~
lateral path of the beam. First, the focus 15 of the 1~ beam E ~hould be at or ad~acent the lower or second surface 17 of the workpiece it is this beam focus 15 which is moved during oscillation to traverse a distance substantially commensurate to the width C of the upper interface of the melt zone aligned with the first surface). The second criteria is that the pOil3t 25 on the beam axis 16, about which the beam is shifted or oscillated relative to the workpiece, should be at or adjacent the upper or fir.st surface 2U of the workpiece.
It is preferable that such point of rotation or oscillation be slightly above the first surface so that the sides of the weldment are as close to a straight line in cross-section as possible. similarly, it i9 advantageous Lf the focus is sllghtly below the second surface to more easily achieve the same result.
The method particularly comprises controlling a high energy radlant beam ~ to produce a meLt zon~ A wLth paeallel sides (2la-22rl) in a workpiece LN h-lvlng Elr~t and second oppo~a-l sllr~.lceCl (2ll-l7) between whLch the melt zone exterlds, by the steps o~: (a) dLrecting the particle beam 1` along an axLs 16 through the eirst æurface of the workpLece wLtl3 the Eocus 15 Oe the beam E
being at or adjacent the æecond surface 17 and the beam generating a conical melt zone A in the workpiece 18 with the apex 26 of the cone being substantially at or adjacent the second surface 17; (b) oscillating the beam and workpiece (as shown in FigUreS 3 and 4) relative to :~2~SZ3 each other about a point 25 on the axis 16 at or adjacent the first surface 20 and in a plane 27 perpendicular to the lateeal line of advancement 28 of the directed beam E
so that opposite sides 21-22 of the conical melt zone in the plane 28 will each be rotated to assume an orientation 21a-22a substantially perpendicular to the first surface 20; and (c) laterally advancing the directed beam along a series of points 29 to define a path 28 relative to the workpiece while carrying out the oscillations of step (b) while traveling along path 28.
As shown in Figure 4, the workpiece may be periodically tilted with an appropriate frequency (the latter, of course, being higher for faster welding speeds and lower for slower welding speeds). Such tilting should be arranged so that as the beam axis 16 remains stationary, the melt zone A will be shifted so that it~s side in any cros~3-sectional plane will assume a perpendicular relationship with re~pect to the upper and lower surfaces of the workpiece.
As shown in FigUre 5, the beam alternatively may be oscillated by deflecting the beam E through the use of a rotating or oscillating mirror 30, the resultant reflected beam E-l or E-2 then being focused by a curved mirror 31. The beam reflecting from the curved mirror will hit the point of oscillation 25 from variouC3 directions as 80 desired. rrhe ~ocal l~ngkh 3~ Oe th~
curved mirror 31 ~should be approxLInately twice lt~
curvature if the be.;lln l~ nearly a p,-lrallel heam~
In Figure 6 theee is ~hown ~till another alternative mechanism For bringing about the osclllation of the beam relative to the workpiece. Magnets 40-Al are used to create two maglletlc fieldq which can be varied in strength and direction. The fir~3t field is directed opposite to the second magnetic field. If the current and windings are judiciously chosen, one and the same magnetizing current can drive both coils 42 and ~3; the ~2~4523 g current is, of course, an alternating current. The intensity ratio of the two magnetic fields must be changed i~ the workpiece distance 45 changes. The difference in magnetic fields creates a deflection effect upon the beam depending upon the strength and polarity of the two fields.
Yet ~till another method for deflecting or refocusing the beam is shown in Figure 7. A first deflection by the angle 52 is produced by the magnet 40 (as before in Figure 6). The angle 52 varies with the strength of the magnetic field produced by 40. ~ second deflector (counter-deflection) is then produced by the magnetic lens 50. It images the center of the field 40 onto the workpiece at point 25. The lens field has constant strength, i.e., the current through lens 50 is constant, which i~ a generally simpler situation than before, where the Eield current ~oe 41 (Figure 6) had to be varied in unlson with the field current of 40. In Figure 7, only the current ~hrouqh magnet 40 has to be 2Q varied to osciLlate the beam, the len~ action of 50 will always bring it: back to point 25, regardless of how it goes through the lens 50, whether by path El or E2. The constant field strength of the lens determines the po~ition of the focal point 15. This focal point can be adju~ted independently Oe the variattons in the magnetlc lens by chan~tng the excitation (len~l cll~rerl~) o~ '5n.
This permit~ an e~ y, elnpirlcal adJllfltmerlt: o~ the ~ocu~.
The magrletlc len~ 3hould be o~ a type which does not rotate the image as a ~imple solenoid type oE lens would do. Tandum lenses which hold the Einal image rotation to zero are well known in the art and can be used here.

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of providing parallel-sided melt zones in a metal workpiece subjected to a focused high energy radiation beam, comprising controlling the relationship between the workpiece and beam so that the cross-section of the melt zone in the workpiece is oscillated at a selected minimum frequency along a lateral path of movement of the beam so that opposite sides of the melt zone assume a substantially perpendicular position relative to the surface through which the beam enters the workpiece.

2. The method as in claim 1, in which the focus of said radiation beam is at or adjacent the surface from which the beam exits from the workpiece.

3. A method for controlling a high energy beam to produce a melt zone with parallel sides in a workpiece having first and second opposed surfaces between which the melt zone extends, the method comprises the steps of:
(a) directing said beam substantially parallel to the normal through said first surface of said workpiece with the focus of said beam being at or adjacent said second surface;
(b) oscillating said beam and workpiece relative to each other about a point located on said axis at or adjacent said first surface and in a plane perpendicular to the lateral line of advancement of the directed beam so that opposite sides of said melt zone in said plane will each be sequentially rotated to assume an orientation substantially perpendicular to the first surface; and (c) laterally advancing said directed beam to define a path along the first surface of the workpiece while carrying out said oscillations of step (b) at a preselected frequency above a minimum.

4. The method as in claim 1, in which the workpiece is a pair of metal parts to be welded at a seam, the melt zone enveloping said seam.

5. The method as in claim 1, in which said beam is a laser beam and in which step (b) is carried out by optically controlling said source of said beam to shift the beam axis about said point.

6. The method as in claim 1, using a charged particle beam and in which in step (b) said oscillation is carried out by use of magnetic fields effective to selectively shift the beam axis about said point.

7. The method as in claim 1, using a charged particle beam and in which in step (b) said oscillation is carried out by use of differential magnetic fields commonly supplied by the same current and which in combination are effective to shift the beam axis about said point.

8. A method of controlling particle beams to make a substantially parallel-sided melt zone in a workpiece having opposed first and second surfaces, comprising:
(a) directing said particle beam at said workpiece, said beam entering said workpiece through said first surface and controlling said beam to be focused at a point at or adjacent said second surface, said beam defining a conical melt zone between said first and second surfaces; and (b) while advancing said beam along a predetermined linear path, oscillating said beam about a point on the axis adjacent said first surface to cause the interface of said beam with said second surface to substantially traverse a width substantially commensurate with the width of the interface of the melt zone at said first surface.

9. The method as in claim 8, in which during oscillation a side of said melt zone is moved to assume an angle with said surface of substantially 90°.