JPS63224890A - Laser build-up welding method - Google Patents

Abstract

PURPOSE:To absorb the shrinking stress of a build-up metal with the compressing stress of a hardening part and to prevent a crack by locally hardening the surface layer equiv. to the build-up bead width both edges of a cast iron base metal and subjecting a dissimilar metal to laser padding. CONSTITUTION:The surface layer at both edge parts 3 in the build-up bead 5 width direction of the boat formed recessed part 2 to be subjected to build-up welding of a cast iron base metal 1 is made in hardened state by heating with laser beam irradiation locally. A dissimilar metal powder 4 is put into the boat formed recessed part 2, a laser beam 29 is oscillated in the width direction by an oscillating mirror 17 and the base metal 4 is continuously moved in the length direction to effect a build-up bead welding. The shrinking stress due to the cooling of the build-up bead 5 is absorbed by off-setting with the compressing stress of the hardening part 3.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention improves locally the surface properties such as wear resistance and corrosion resistance in cast iron members such as automobile parts such as cylinders of automobile engines and other various mechanical parts. The present invention relates to a method of overlay welding a corrosion-resistant and wear-resistant alloy onto a cast iron base material using a laser as a heat source, and particularly to a method of forming a wide overlay bead with a width of about 5# or more.

Conventional technology Recently, in order to improve the properties of parts made of cast iron, such as wear resistance and corrosion resistance, a laser, which is a high-density energy source, is used as a heat source to produce metals according to the required properties on a cast iron base material. The method of overlaying (alloy) welding is becoming popular. For example, as a laser overlay welding method for improving sliding wear characteristics, as shown in JP-A No. 61-116056, a laser beam is focused by a lens at the overlay processing width or slightly larger than it. However, it is usual to irradiate a corrosion-resistant and wear-resistant alloy as a build-up material on a cast iron base material with the laser light to melt and solidify it. As another method, as described in JP-A-50-101254, Gaussian distribution or ring mode laser light oscillated from a laser oscillator is once shaped by a beam oscillation optical system, and then this is overlaid. It is known how to irradiate the material.

On the other hand, as a method for overlaying a particularly wide area,
As shown in U.S. Pat. No. 3,952,180, it is also known to build up a wide area by laser overlapping a number of narrow beats.

Problems to be solved by the invention
- When forming a wide overlay bead by a conventional general laser overlay welding method as shown in No. 101254 etc., especially when forming an overlay bead with a width of 5# or more,
When the overlay material melted by laser irradiation solidifies on the base material, the shrinkage stress increases significantly, which causes damage to the cast iron base material below the overlay material, especially in the parts corresponding to both widthwise edges of the overlay bead. Because a significantly large tensile stress is applied to the base material,
If the base material is a material with low tensile strength or toughness, such as cast iron,
Due to the tensile stress caused by the solidification and shrinkage, cracks often occur in the base material at positions corresponding to both edges in the width direction of the overlay bead, for example, as shown in FIG. 5, which will be explained later.

If such a rack occurs on the base material, even if the overlay part and its surroundings are finished, fine cracks and ranks will remain on the finished surface, resulting in fatigue in applications where repeated mechanical stress is applied. It is necessary to avoid the occurrence of the rack as described above, since it has an adverse effect on the characteristics and also has an adverse effect on the wear resistance.

On the other hand, as described in the above-mentioned U.S. Pat. As shown in FIG. 6, when a subsequent build-up bead is formed with an overlapping portion over a previously formed build-up bead, a problem arises in that cracks are likely to occur in the previously formed build-up bead. The reason why cracks occur in the previously formed overlay bead is considered to be as follows.

In other words, since overlay using a laser, which is necessary with a high-density energy source, is rapid heating and rapid cooling using the minimum necessary thermal energy, it is possible to By the time the next overlay bead is formed, the previously formed overlay bead has already cooled to a temperature similar to that of the base material. A stress of σ = σS + σt, which is the sum of the stress σS generated during melting and solidification of the overlay material and the contraction stress σ℃ corresponding to the thermal expansion coefficient during the cooling process to the same temperature as the material after solidification, has already been applied. ing. If new laser energy is applied to form the next overlay bead at that point, it is thought that the stress balance will be disrupted and cracks will occur in the previously formed overlay bead. If a crack occurs in the overlay bead in this way, it will have a significant negative effect on fatigue and wear characteristics, just like cracks in the base material, and therefore this method is not suitable for forming a wide overlay bead. I could not say it.

This invention was made against the background of the above circumstances, and effectively prevents the formation of cranks in the base material around the bead without causing other problems when forming a wide overlay bead by overlay welding. The object of the present invention is to provide a laser overlay welding method.

Means for Solving the Problems The method of the present invention is to overlay dissimilar metals over a certain width onto the surface of a base material made of cast iron using a lorry as a heat source. As a preliminary treatment, the surface layer of the cast iron base material in portions corresponding to both widthwise edges of the overlay bead is locally hardened in advance.

Here, it is desirable that the width T of the portion subjected to the hardening treatment as a preliminary treatment is a width that satisfies T2O,2t with respect to the overlay bead width t.

Function: Prior to laser overlaying, if the surface layer of the cast iron base material is locally quenched as a preliminary treatment at the portion corresponding to both edges in the width direction of the overlay bead to be finally formed, that area can be hardened. The surface layer of the cast iron base material is given compressive residual stress, and
Strengthened by quench hardening.

As already mentioned, in laser welding, tensile stress acts on the cast iron base material due to shrinkage during melting and solidification of the welding material, and especially tensile stress is applied to the base material in the parts corresponding to both widthwise edges of the welding bead. is concentrated. In particular, when forming a wide overlay bead with a width of about 5 mm or more, the tensile stress becomes significantly large. However, in the case of the method of the present invention, compressive residual stress is applied in advance to the base material in the portions corresponding to both widthwise edges of the overlay bead through quenching treatment, so that the stress due to solidification shrinkage during overlay is applied to the base material in advance by quenching. The tensile stress is canceled out by the compressive residual stress mentioned above, and the tensile stress in that area becomes smaller, or conversely it becomes a compressive stress, and as a result, it is possible that cranking occurs in the cast iron base material around the overlay bead. Prevented. Furthermore, the fact that the cast iron base structure in that part is strengthened by the quenching treatment performed in advance also greatly contributes to preventing cracks from forming in the base material due to tensile stress caused by solidification shrinkage during overlaying.

Here, it is desirable that the width T of the portion subjected to the pre-hardening treatment is a width that satisfies T2O,2t with respect to the width t of the overlay bead. If the hardening width T is smaller than 0.2t, cracks may occur in the base material at the edges of the hardening layer when laser overlaying is performed after hardening, and the hardening width should be set to 0.2t or more. By doing so, it is possible to more reliably prevent the occurrence of cracks in the base material.

Figure 3 shows the stress distribution on the surface of the cast iron base material, especially the stress distribution on the outside of the welding bead in the width direction, the stress caused by laser overlaying (if no hardening treatment is performed before overlaying), A, The results of a simulation of the stress B generated by quenching before overlay and the stress C remaining after pre-hardening and laser overlay are shown. Note that the simulation results were conducted using the same tJ material, hardening conditions, and laser build-up conditions as in the examples shown later, and the build-up width was 13 m.

As shown in FIG. 3, it is clear that the residual stress after laser build-up is reduced by performing the hardening process prior to laser build-up.

In addition, the crack generation stress limit in the cast iron base material used here is thought to be approximately +30 kgf/- or less, but the residual tensile stress was reduced to 30 kgf/- by quenching as a preliminary treatment.
7 or less is at a position outside the position 2.0 mm from the outer edge boundary of the built-up layer. Therefore, the hardening width should be adjusted so that the hardened layer covers the area from the outer edge boundary of the built-up layer to a position 2.0 mm or more. If the hardened layer is formed so that T is 0.2 times or more the main overlay width, cracks can be more reliably prevented from occurring.

Example [Example 1] Qr1 was applied to a base material made of JIS Fe23 gray cast iron.
5-20wt%, Fe3-8wt%, 3i1-5wt%
, BO, 7 wt %, C 2, 0 wt %, and the balance Ni was used for laser overlay welding.

That is, first, as shown in FIG. 1 (A>), the depth D2.
A boat-shaped recess 2 with a width W13# is formed in the base material 1 by machining, and self-cooling hardening is performed on both sides of the boat-shaped recess 2 using a laser, as shown in FIG. 1(B). A hardened layer 3 was formed.

The laser hardening conditions at this time are as follows.

Laser power: 3 kW Beam scanning speed: 300 to 700 s/min Beam mode: Defocus mode beam diameter: 3 to 10 degrees in diameter The surface of the area to be laser hardened should be preheated to 5 to 10 degrees in order to improve laser energy absorption efficiency. A graphite coating was applied to a film thickness of 20 strands.

After laser hardening, the self-shaped recess 2 is formed as shown in Figure 1 (C).
The above-mentioned N1-based alloy powder 4 was placed on the substrate, and laser irradiation using an oscillation beam was performed under the following conditions to form a build-up bead 5 as shown in FIG. 1(D).

Laser output 3kW Beam spot diameter: Diameter 1.5-2.0mm Beam scanning speed: 150-200mm/man Beam oscillation frequency: 2001-1z Beam oscillation-1 Width: 14
FIG. 2 shows the optical system used for laser overlay in this example, and this optical system will be explained below.

A laser beam 12 emitted by a laser oscillator 11 is reflected by a bend mirror 13, and is further reflected by a 7nSe lens 1.
4, the diameter of the beam is adjusted to be appropriate on the cast iron base material 1 to be overlaid. The laser beam 12 that has passed through the lens 14 is oscillated to a constant frequency and constant amplitude by a beam oscillating mirror 17 that is vibrated by an electromagnetic exciter 16.

Note that this oscillation direction is determined to be perpendicular to the moving direction A of the base material 1 on the base material 1, in other words, to be in the width direction of the metal powder 40 to be overlaid.

Here, the beam mode of the laser beam 12 immediately after passing through the lens 14 is a Calashan mode as shown by the reference numeral 19, but the oscillated beam shaped by the beam oscillation mirror 17 is as shown by the reference numeral 20. It has an apparent shape with a peak on the upper cylinder side and a flat central part, and the peak parts on both sides correspond to both sides in the width direction of the metal powder 4 to be overlaid. The oscillated beam 20 thus shaped is roughly reflected by a mirror 21 and irradiated onto the metal powder 4 to be overlaid on the cast iron base material 1 moving at a constant speed in the six directions of arrows in the figure.

By irradiating the overlay metal powder 4 with the oscillating beam beam 20 as described above, the overlay metal powder 4 melts and
It solidifies to form a built-up bead 15 (first
Figure (C)).

Note that in order to make the oscillation beam high in frequency and widen the oscillation width, the focal length f of the lens 14 should be adjusted to f > 500, taking into consideration the capability limit of the electromagnetic exciter 16 for driving the beam oscillation mirror. It is necessary to do so. However, as the f value increases, there is a problem that it becomes difficult to narrow down the beam spot diameter, and in order to set the beam spot diameter to an appropriate value of 3M or less, f<780g is required, so the lens 14 should be 500m<780g.
A lens with a focal length within the range of f<780 m was used.

Further, the laser beam narrowed down by the lens 14 is irradiated onto the oscillating mirror 17 or the oscillating mirror 1
There is a limit to the energy that can be irradiated to the oscillating mirror 17 because the mirror 7 is made lightweight in order to oscillate at high speed.

Therefore, if the irradiation laser output P, the focal length f of the lens, and the radius of the laser beam emitted from the laser oscillator 11 are r, then the distance between the lens 4 and the oscillating mirror 17! was set to satisfy the following equation.

Note that when a 3i substrate mirror is used as the oscillating mirror 17, it is desirable to set it so that the following equation is satisfied.

FIG. 4 shows the situation near the edge in the width direction of the overlay bead which has been overlaid by laser according to the above embodiment. In this case, the build-up bead width t was 13 mm, the hardened layer width T was 4.0 mm, and T≧0.21 was satisfied.

As is clear from FIG. 4, no cracks were generated in the base material at the portions corresponding to both widthwise edges of the overlay bead.

[Comparative Example 1] Laser overlay welding was performed without hardening as a preliminary treatment, as has been conventionally practiced. The base material and overlay metal powder used were the same as in Example 1, and the laser overlay conditions and optical system were also the same as in Example 1.

FIG. 5 shows a cross-sectional view of the area near the edge in the width direction of the built-up bead obtained in this way. As is clear from Figure 5,
In the case of Comparative Example 1 in which quenching was not performed as a preliminary treatment, it can be seen that cranks were generated in the base material around the overlay bead due to solidification shrinkage of the overlay material.

[Comparative Example 2] A method of overlapping beats according to the conventional method shown in U.S. Pat. After cooling to a certain temperature, laser overlay welding was performed by forming the next bead with an overlapping portion over the previous bead. Note that the same base material and overlay metal powder as in Example 1 were used. In addition, the laser processing conditions are irradiation energy per unit area (Example: 1)
In order to make it the same as in the case of , the following conditions were set.

Laser output 3kW Beam spot diameter: Diameter 1.5-2.0 Simple beam Scanning speed: 300-600#/Sonobeam Oscillation frequency:
200Hz Oscillator Width Near M Figure 6 shows the cross-sectional state of a build-up bead obtained by laser build-up welding after superimposing the two leads under these conditions. As is clear from Fig. 6, the central part of the bead,
In other words, cracks were generated at the bead overlapping portion.

[Other Examples] In Example 1 shown in FIG.
Since the boat-shaped recess 2 is built up after having been formed, it goes without saying that the method of the present invention can also be applied to the case where the boat-shaped depression is not particularly formed.

Further, in Example 1, laser hardening was used as the local hardening method in the preliminary treatment, but other local hardening methods such as induction hardening can also be applied.

Further, the hardening as a preliminary treatment may be performed as a completely separate process from the laser build-up, or may be performed simultaneously as a parallel process. In other words, even if the laser build-up is performed after completing the hardening process on the part of the workpiece to be hardened, or even if the laser build-up is performed, the position may precede the irradiation position of the laser gun for domestic use to some extent. Hardening such as laser hardening may be performed in the process.

Effects of the Invention According to the laser welding method of the present invention, a wide welding bead,
In particular, when forming a build-up bead with a width of 5 m or more, local hardening prior to laser build-up applies compressive residual stress to the parts of the cast iron base material corresponding to both edges of the build-up bead, and strengthens the base structure. Therefore, it is possible to effectively prevent cracks from occurring in the cast iron base material due to the tensile stress caused by the solidification shrinkage of the overlay material and the shrinkage during the subsequent cooling process.Therefore, the cracks can impair fatigue resistance, sliding wear properties, etc. It is possible to form a wide overlay bead with a high yield of non-defective products without deteriorating the material.

[Brief explanation of the drawing]

FIG. 1 (A> to (D>) are schematic sectional views showing step-by-step Embodiment 1 of the method of this invention, and FIG. 2 is a schematic sectional view showing the optical system used in Embodiment 1 of this invention. FIG. 3 is a diagram showing the stress distribution in the surface layer of the base material, and FIG. 4 is a photograph of the metal cross-sectional structure near the widthwise edge of the overlay iron formed in Example 1 (5x magnification). ), Fig. 5 is a photograph (10x magnification) of the metal cross-sectional structure near the widthwise edge of the build-up bead formed in Comparative Example 1, and Fig. 6 is a metal cross-section of the build-up bead formed in Comparative Example 2. This is a photograph of the structure (5x magnification). 1... Base material, 3... Hardened layer, 4... Metal powder for overlay, 5... Overlay bead. = 17- Fig. 1

Claims (1)

[Claims] When overlaying dissimilar metals to a certain width on the surface of a base material made of cast iron using a laser as a heat source, both edges in the width direction of the overlay bead are pre-treated as a preliminary treatment prior to laser overlay welding. A laser overlay welding method characterized by locally hardening the surface layer of a cast iron base material in a portion corresponding to the above.