CN-115943011-B - Laser welding method

Abstract

A method for laser keyhole welding is disclosed for welding together two pieces made of a metal alloy. The method independently adjusts the power of the focused central beam and the power of the concentric focused annular beam. At the end of the weld, the power of the annular beam (PA) decreases, the movement of the focused beam stops, the power of the central beam (Pc) increases, and the power of the two beams initially ramps down rapidly and then slowly. Increasing the power of the center beam equalizes the temperature of the two blocks before solidifying and cooling at the weld termination. Additional power pulses may be applied to prevent the formation of defects or to eliminate any defects.

Inventors

• R. Briscoe

Assignees

• 相干公司

Dates

Publication Date

20260512

Application Date

20210506

Priority Date

20200522

Claims (20)

1. 1. A method for laser welding a workpiece along a weld line, comprising the steps of: Delivering a focused beam of laser radiation to the workpiece, the focused beam of laser radiation having a focused central beam and a concentric focused annular beam, the focused central beam having a diameter less than a diameter of the focused annular beam on a surface of the workpiece exposed to the focused beam of laser radiation; Moving the focused laser radiation beam laterally relative to the workpiece along the weld line, the central beam having a central processing power and the annular beam having an annular processing power; reducing the power of the annular beam from the annular processing power; stopping lateral movement of the focused laser radiation beam relative to the workpiece when the focused laser radiation beam reaches an end position on the weld line; increasing the power of the central beam from a central processing power; Ramping down the power of the annular beam at a first annular ramp rate while ramping down the power of the central beam at a first central ramp rate, and The power of the annular beam is ramped down at a second annular ramp rate while the power of the central beam is ramped down at a second central ramp rate, the second annular ramp rate being less than the first annular ramp rate and the second central ramp rate being less than the first central ramp rate.
2. 2. The method for laser welding a workpiece along a weld line of claim 1, wherein the laser radiation is delivered from a laser source through an optical fiber to a focusing lens, the focusing lens forming the focused beam of laser radiation.
3. 3. The method for laser welding a workpiece along a weld line of claim 2, wherein the optical fiber includes a central core for guiding the central beam and an annular core for guiding the annular beam.
4. 4. The method for laser welding workpieces along a weld line of claim 1, wherein the workpieces comprise two pieces to be lap welded together.
5. 5. The method for laser welding workpieces along a weld line of claim 4 wherein the two pieces are separated by a gap.
6. 6. The method for laser welding workpieces along a weld line of claim 4 wherein the ratio of said center process power to annular process power is selected to form a weld of uniform width across said two blocks.
7. 7. The method for laser welding a workpiece along a weld line of claim 1, wherein the ratio of center process power to annular process power is less than 1:3.
8. 8. The method for laser welding a workpiece along a weld line of claim 1, wherein focusing is at a depth of focus relative to an exposed surface of the workpiece that is in a range between 1 millimeter above the exposed surface and 2 millimeters below the exposed surface.
9. 9. The method for laser welding a workpiece along a weld line of claim 1, wherein ramping down the power of the annular beam at a second annular rate and ramping down the power of the central beam at a second central rate ramps down the energy of the annular beam and the central beam to off power.
10. 10. The method for laser welding a workpiece along a weld line of claim 9, wherein the off power is 0 watts.
11. 11. The method for laser welding a workpiece along a weld line of claim 9, wherein the shut-off power is less than a power to melt the workpiece surface.
12. 12. The method for laser welding a workpiece along a weld line of claim 1, wherein the step of reducing the power of the annular beam is from the annular processing power to a lower power and the step of increasing the power of the central beam is from the central processing power to a higher power, then maintaining the lower power and the higher power for a dwell time before the step of ramping down the power of the annular beam and ramping down the power of the central beam.
13. 13. The method for laser welding workpieces along a weld line of claim 12 wherein the workpieces comprise two pieces to be welded together and the lower power, the higher power, and the dwell time are selected to equalize the temperatures of the two pieces at end locations.
14. 14. The method for laser welding a workpiece along a weld line of claim 12, wherein the lower power, the higher power, and the dwell time are selected to equalize a temperature on an exposed surface and a temperature at which the keyhole penetrates the workpiece deepest.
15. 15. The method for laser welding workpieces along a weld line of claim 12 wherein said lower power, said higher power, and said dwell time are selected to form a cross-sectionally symmetrical weld at an end position.
16. 16. The method for laser welding a workpiece along a weld line of claim 12, wherein the power of the ring beam is reduced from the ring processing power to between 10% and 20% of the lower power.
17. 17. The method for laser welding a workpiece along a weld line of claim 1, further comprising the step of applying a laser power pulse after the step of ramping down the power of the annular beam and ramping down the power of the central beam.
18. 18. The method for laser welding a workpiece along a weld line of claim 17, wherein the laser power provided by the pulses is sufficient to slow solidification of molten material at the end position.
19. 19. The method for laser welding a workpiece along a weld line of claim 17, wherein the laser power provided by the pulses is sufficient to remelt the exposed surface.
20. 20. The method for laser welding a workpiece along a weld line of claim 1, wherein the workpiece is made of a metal alloy selected from the group consisting of Gen3 steel, XGen steel, DP600 steel, and DP980 steel.

Description

Laser welding method Priority The present application claims priority from U.S. patent application Ser. No.16/881886, filed 5/22/2020, the disclosure of which is incorporated herein by reference. Technical Field The present invention relates generally to welding using a focused beam of laser radiation. The invention relates in particular to welding metal alloys using a focused central beam and a focused annular beam. Background Laser beams are increasingly used to cut, drill, weld, mark and scribe workpieces made of a variety of materials, including metals and metal alloys. Conventional machining can create undesirable defects such as microcracks that can propagate when the work piece is stressed, thereby degrading and weakening the work piece. Laser machining minimizes such undesirable defects, is generally cleaner, and results in a smaller heat affected zone. Laser machining uses a focused laser beam to produce precise cuts and holes with high quality edges and walls while minimizing the formation of unwanted defects. In laser welding, a focused laser beam precisely locates each weld spot or line while minimizing incidental heating. It is useful to distinguish between the two main laser welding conditions. Conductive welding is performed at a lower laser power and lower power density. The absorbed laser power heats the irradiated material, melts the material in each part to be joined, and the materials flow, mix, and then solidify. Keyhole welding is performed at higher laser powers and higher power densities, which are sufficient to vaporize some of the irradiated material. The pressure of the vaporized material against the surrounding molten material opens a channel through the molten material that has a typically narrow and deep profile that allows for deep penetration of the laser beam. The finished keyhole weld is typically narrower, deeper, and more robust than the conductive weld. However, maintaining a stable keyhole in a hot, dynamic pool of molten material is difficult. One problem when laser welding certain metals and metal alloys is the formation of defects, particularly cracks, at the ends of the weld line. Some of the defects are caused by stresses that occur when the workpiece cools. These initial defects can weaken the welded workpiece and may propagate further if thermal or mechanical stresses are applied when the finished workpiece is put into service. Unreliable welds in the structure may lead to catastrophic failure. One known solution to mitigate the defect is to rapidly reduce the laser power at the end of the weld, rather than digitally shutting down the power. Another known solution is to rapidly raise the focused beam at the end of the weld, so as to irradiate progressively larger areas on the workpiece with progressively lower intensity beams. While these solutions have been successful on many materials, they have proven inadequate for modern high strength metal alloys having relatively high thermal conductivity. These materials are still very prone to cracking at the beginning of laser welding, particularly at the end of laser welding. U.S. patent application Ser. No. 16/786623, filed on even 10/2/2020, which is assigned to the assignee of the present invention, the entire disclosure of which is incorporated herein by reference, describes a method of welding such materials. The method uses a composite laser beam having a central beam and a larger annular beam focused on the workpiece being welded. As the focused laser beam approaches the end of the wire weld, the power of the ring beam ramps down, while the power of the center beam ramps up. Then, the power of the central beam is ramped down. While this approach works well to prevent defects, during these power variations, the focused laser beam moves across the workpiece surface, creating a gradual taper across the weld width. Such tapers are typically a few millimeters long and are unacceptable in certain applications where a uniform cross-section along the entire length of the weld is desired. The ends of the weld may also taper in depth. There is a need for a simple and reliable method to produce welds with uniform cross-sections in metals and metal alloys that are prone to cracking when the weld is terminated. In particular, a weld is produced that maintains a minimum width along its entire length. Preferably, the process does not compromise any advantages of laser welding, such as welding speed, accuracy, welding quality, and cost per weld. Summary of The Invention A method according to the invention for laser welding a workpiece along a weld line includes delivering a focused beam of laser radiation to the workpiece. The focused beam has a focused central beam and a concentrically focused annular beam. The focused central beam is smaller than the focused annular beam exposed on the workpiece surface of the focused beam. The focused beam moves laterally relative to the workpiece along the weld line. The central beam has a