Abstract:
A method of progressing a melt front ( 55 ) around a curved progression path ( 20 ) via a pattern (LP) of transverse laser scan lines (S 1 -S 8 ) of differing lengths. Multiple area bands (B 1 -B 8 ) conceptually divide a width of the curved path. The multiple transverse scan lines distribute the laser power among the bands with a predetermined uniformity that provides relatively consistent power density across the melt front. The scan lines may extend from a less curved side ( 24 ) of the curved path, through a band (B 4  or B 8 ) of largest area, toward a more curved side ( 22 ) of the path. At least one of the scan lines (S 1 , S 8 ) may cross all bands. Other scan lines are shorter and extend by varying distances into the inner bands (B 1 -B 3  or B 1 -B 7 ), normalizing the power density across the bands.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates generally to the field of materials technology, and more particularly to scanning an energy beam over a material surface to produce a progressing melt front, and more particularly to rastering a laser beam on a curved progression path with scan lines that differ in length transverse to the progression path to provide uniform surface heating. 
       BACKGROUND OF THE INVENTION 
       [0002]    Laser beam optical scanning over a surface has been used to melt or sinter materials for additive manufacturing and for purposes such as hardfacing, corrosion overlay, refurbishment or cladding. It is often valuable to provide a uniform power (and/or energy or heat) distribution to the surface to ensure minimal and consistent melting of the substrate, and thus low and uniform dilution. Low dilution is important for application of hardfacing, corrosion overlay or for refurbishment cladding—especially with materials that are difficult to weld because they are prone to cracking. Uniform power distribution is also important for uniform transformation hardening of surfaces. For cladding of broad areas, buildups in grooves, and hardening of complex surfaces such as gear teeth, customized optics such as mirrors rocked by specifically contoured cams were developed by the present inventor in the late 1980&#39;s. Such motor driven laser mirrors have been superseded by advanced galvanometer driven optics capable of moving the beam in three dimensions. 
         [0003]    In conjunction with robotics, such optics are used for spot welding in automotive parts manufacture. Rastered laser cladding has also been performed with such optics. There are two common modes of rastering with such optics—“wobble” and “normal”. In wobble rastering, the beam follows a path similar to the projection of a helical spring as viewed from the side. The beam spends more time at the ends of the scan lines, resulting in a power distribution to the substrate that applies more power at such locations and less at the center. This can result in over-melting at the sides producing inconsistent surface properties. 
         [0004]    Normal rastering resolves the issue of over-melting associated with wobble rastering. With normal rastering, scanning is performed from left to right with periodic incrementing forward. The forward incrementing is nearly instantaneous compared to the left to right motion, so power distribution is nearly uniform over the area being exposed. 
         [0005]    While normal scanning solves the issue of over-melting at both sides of the scan, an additional problem persists. That is, normal scanning provides uniform power for straight linear paths, but where the path curves or, for example, turns a corner, forward incrementing at the outer edge of the corner must be relatively large compared to forward incrementing at the inner edge of the corner in order to cover the larger circumferential distance at the larger radius. This results in a power density that is not uniform over the area being processed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The invention is explained in the following description in view of the drawings that show: 
           [0007]      FIG. 1  is a top view of a curved surface path to be laser heated. 
           [0008]      FIG. 2  is a top view of a pattern of laser scan lines progressing along the beginning portion of the curved path of  FIG. 1 , which is straightened for clarity. 
           [0009]      FIG. 3  shows how the pattern of  FIG. 2  may progress around a curved surface path. 
           [0010]      FIG. 4  shows the pattern of  FIG. 3  using overlapping scans. 
           [0011]      FIG. 5  shows repetitions of the pattern of  FIG. 4  around the curved path. 
           [0012]      FIG. 6  shows an adjustment of the scan line lengths for improved uniformity. 
           [0013]      FIG. 7  shows another embodiment with higher band resolution. 
           [0014]      FIG. 8  shows an embodiment with the same power density ratios as in  FIG. 6  using half the number of scan lines. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0015]    The inventor recognized that certain patterns of transverse scan lines of varying lengths can provide a more uniform power density along a progressing melt front across the width of a curved path of scan progression. This maintains the melt front in an optimum temperature range for consistent surface properties. 
         [0016]    One skilled in the art will understand that the concepts of power (e.g. joules/second or watts) and energy (e.g. watt-seconds or joules) are both important when controlling a melt front, since the material being heated will naturally disperse energy via conduction, convection and radiation. For example, for a constant power, as more energy per unit area is delivered to a given depth of powder and substrate, more melting of the substrate (dilution) will occur. Also, for a constant energy per unit area of a moving energy beam, as more power is delivered to a given depth of powder and substrate, more melting of the substrate (dilution) will occur. The reason for this is that a greater rate of energy delivery (power) per unit area enhances more melting because it overwhelms the ability of the substrate to conduct heat away. (This may be considered to be analogous to a single large wave of the ocean penetrating farther onto shore than many smaller waves of equivalent total energy.) For simplicity of discussion, the word “power” is used herein to include both the concepts of power and/or energy to the extent that they affect the control of the melt front. 
         [0017]      FIG. 1  shows a surface area of a material to be heated by an energy beam such as a laser. It includes a curved path  20 , having an inner first side  22  with a smaller radius of curvature (sharper curvature) than an outer second side  24 . The two sides may be inner  22  and outer  24  concentric arcs about a center point  26 , or they may be other curves, such as elliptical or irregular curves. The curved path  20  begins  30  and ends  32  at lines normal to one or both sides  22 ,  24 . In this example, lines  30  and  32  are aligned with radii from the center  26 . The width of the curved path  20  may be conceptually subdivided into area bands B 1 -B 4 , which may have equal width. In this example, the bands are concentric with the sides  22 ,  24 . The bands have respectively different areas as a percentage of the total area of the progression path  20 . For example, with relative radii of R 1 =2, R 2 =4, R 3 =6, R 4 =8, and R 5 =10, the band area percentages of the total area around the curved path are approximately: B 1 =12%, B 2 =21%, B 3 =29%, and B 4 =38%. 
         [0018]    Herein, “greater curvature” or “more curved” means more sharply curved, or having a relatively smaller radius of curvature. “Less curvature” means less sharply curved or having a greater radius of curvature. Herein “transverse scan line” means a scan line that is less than 20 degrees from normal to one or both sides  22 ,  24  of the scan progression path  20 . 
         [0019]    A linear sequence of transverse scan lines  40 ,  42 ,  44 ,  46  is shown progressing by forward increments  41 ,  43 ,  45 ,  47  along a straight path  50 . This is simplified for clarity, since the scans may overlap as later shown. The laser beam may have a predetermined beam width represented by the diameter of the circles, and a predetermined power. Although illustrated as discrete circles, it will be appreciated that the laser may be pulsed to transfer energy in such discrete circles, or it may be continuous to transfer a given amount of energy per area per time, represented by each circle, at a particular scan speed, such as ¼ m/s. The laser may move faster along the increment lines, such as 3 m/s. If the linear progression of scan lines  40 - 46  were continued and wrapped around the curved path  20  by aligning each successive scan line with radii R 5 , the power density would be uneven. This is exemplified by a radial scan line  52 . It has two circles out of eight in each band B 1 -B 4 , so 25% of the power is applied to each band. But band B 1  has only 12% of the total area of the curved path  20 , while B 4  has 38% of the total area, so the power density by band is uneven. If this type of scan line is repeated around the curved path  20 , the power density (W/m 2 ) differs by a factor of 3 between the inner and outer bands ((25/12)/(25/38)=3.1) as shown in the table below. This wide variation in power densities across the width of the curved path produces a large temperature range along a melt front, resulting in inconsistent surface properties, as shown in Table 1. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 Band 
                 Power % 
                 Area % 
                 Power %/Band Area % 
               
               
                   
                   
               
             
             
               
                   
                 B1 
                 25 
                 12 
                 2.08 
               
               
                   
                 B2 
                 25 
                 21 
                 1.19 
               
               
                   
                 B3 
                 25 
                 29 
                 0.86 
               
               
                   
                 B4 
                 25 
                 38 
                 0.66 
               
               
                   
                   
               
             
          
         
       
     
         [0020]      FIG. 2  represents a beginning portion of the curved path  20  of  FIG. 1  as if it were straightened for clarity. The beginning line  30  is shown. A sequence of transverse scan lines S 1 -S 8  of different lengths forms a pattern that provides power more closely proportional to the respective areas of the bands B 1 -B 4 . The total power applied in this pattern (or sub-pattern of an entire curved path  20 ) along its length LP is represented by 40 circles. Four of the circles or 10% of the power is in band B 1 . Eight of the circles or 20% of the power is in band B 2 . Twelve of the circles or 30% of the power is in band B 3 . Sixteen of the circles or 40% of the power is in band B 4 . This power distribution more closely matches the respective areas of the bands B 1 -B 4 , as shown in Table 2. 
         [0000]                                        TABLE 2                       Band   Power %   Area %   Power %/Band Area %                           B1   10   12   0.83           B2   20   21   0.95           B3   30   29   1.03           B4   40   38   1.05                        
The worst case in this example is a difference of less than 27% between B 1  and B 4  (1.05/0.83=1.265).
 
         [0021]      FIG. 3  shows how the pattern of scan lines S 1 -S 8  of  FIG. 2  may be wrapped around the curved path  20 , and repeated multiple times from the beginning  30  to the end  32  of the path. In other embodiments where the direction of the melt front progresses around a smaller angle, for example around only a 45 degree bend, then the sub-pattern S 1 -S 8  need not be repeated, or may be repeated fewer times. This is simplified for clarity, since the beam circle diameters normally overlap at the scan lines as later shown. Each scan line S 1 -S 8  in this example is aligned with a radius from the center  26  of scan progression path  20 . In non-circular curved paths, the scan lines may be aligned with lines normal to the inner curved side  22  and/or the outer curved side  24 . Perfect alignment of any or all of the scan lines along such radii or normals of the curve is not essential. Some of the scan lines may be aligned to within less than 20 degrees of such a radius or normal line, but especially less than 10 or 5 degrees thereof. Transverse alignment maintains a wet melt pool across the width of the curved path  20  as the scans progress. 
         [0022]      FIG. 4  shows the pattern of scan lines S 1 -S 8  of  FIG. 3  with an overlap in scan width (beam spot circle diameter; e.g. focus) at the radially outer ends of the scans. The overlap may be ⅔ in scan width or at least 1/10 in scan width. The overlap of adjacent scans increases toward the inner band B 1  as the radial or normal lines of scan alignment converge.  FIG. 5  shows repetition of the pattern of scan lines S 1 -S 8  of  FIG. 4  around the curved path  20 , which maintains a consistent transverse melt front  55 . 
         [0023]      FIG. 6  shows how power density differences among the bands can be further reduced or eliminated by finer adjustment of the scan line lengths. Here, the scan line lengths are not limited to multiples of the laser spot diameter, so scan lines S 6  and S 7  have been lengthened to overlap into band B 1 . Scan lines S 4  and S 5  have been lengthened to overlap into band B 2 . This enables the different lengths of the scan lines to fully normalize the power applied per band area among the different bands B 1 -B 4 . Perfect uniformity in power density among bands, is illustrated in Table 3 below. However, differences of less than 35% but especially less than 10% or 5% in power density between each combination of two bands can be acceptable. 
         [0000]    
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
               
                   
                   
               
               
                   
                 Band 
                 Power % 
                 Area % 
                 Power %/Band Area % 
               
               
                   
                   
               
             
             
               
                   
                 B1 
                 10 
                 10 
                 1.00 
               
               
                   
                 B2 
                 20 
                 20 
                 1.00 
               
               
                   
                 B3 
                 30 
                 30 
                 1.00 
               
               
                   
                 B4 
                 40 
                 40 
                 1.00 
               
               
                   
                   
               
             
          
         
       
     
         [0024]      FIG. 7  shows how power density variation over the total area of the scan path  20  can be reduced by finer banding B 1 -B 8 . Here, the increment lines  54  need not be normal to the scan lines, so successive scan lines can have individually different lengths as shown. The laser beam may be powered on or off during the increments  54 ,  56 . If powered on, the increment lines and times are included in the power density calculations. By adjusting the different lengths of the scan lines S 1 -S 8 , the power density can be highly normalized across the width of the curved path  20 . 
         [0025]      FIG. 8  shows an embodiment with the same power density proportions as in  FIG. 6  (assuming the increments are powered off in both cases), but with only half the number of scan lines S 1 -S 4 . This pattern returns the beam to the inner band sooner than in  FIG. 6 , and thus refreshes the inner end of the melt front sooner. 
         [0026]    The scan patterns herein may be designed using computer-aided design and applied using computer-aided manufacturing technology. When cladding near an edge, it is useful to apply lower power density along that edge to prevent over-melting near such limited heat sink. This may be accomplished with fewer scan increments near such edge and/or alternating the spacing of the scan lines from that edge. Conversely, when cladding near a wall, additional short scans may be provided on that side of the path to improve melting against such a greater heat sink. 
         [0027]    While various embodiments of the present invention have been shown and described herein, it will be obvious that such embodiments are provided by way of example only. Numerous variations, changes and substitutions may be made without departing from the invention herein. Accordingly, it is intended that the invention be limited only by the spirit and scope of the appended claims.