Hi All,
I have a medical application where I have to join two parts made of 420ss. Our welder of choice is a Lasag YAG laser. The parts are small about .250 diameter and .5 long. Due to their size there is no filler involved the two parts just butt up against each other and are fusion welded. As expected the joints tends to crack on about 30% of the parts we weld. As much as I'd like to see a different grade of stainless it's just not going to happen. Short of pre and post heating of the parts is there anything I can try to help the situation. Tomorrow I'll be running a test matrix with varying wave forms and surface speeds. Oh and to add fun to the party, the parts are coated with a proprietary coating of ME92 which acts like chrome.
Bill
Just sorta brain stromin here without more info but 420 SS is a very hard high carbon alloy with a martensitic microstructure with hardnesses in the neighborhood of ~50 HRC. Even more I would guess with a rapid quench as you would get with laser welding. I'm thinkin the weld and HAZ are like granite.
Are you preheating? If you already are is there any strain due to fixturing that prevents the part from accomodating the shrinkage stress from cooling?
Soemthiong that small I am assukming you have to hold in place. does it prevent accomodation of cooling stresses?
Even with these small of parts it seems to me you would need to reduce stresses wihich given the high thermal gradients of laser welding would need to be minimized.
Where do the cracks seem to be initiating from?
Have you done hardnesses on the weld metal and/or HAZ?
And you might do chems on your weld metal. Your coating may be the very problem.
Good thoughts from js55.
Pulse Forming should help (are you using a KLS246 C60?), but is not always straight forward path to success.
Fiber beam delivery or hard optics?
How big is the laser spot size and required penetration?
Something that can be helpful is to enclose part in a bag or other enclosure and purge work zone atmoshpere with assist gas.
I've had similar issues with 316L SS. Not so much with other 300 series materials.
I would be interested in the results you achieve in manipulating your pulse form.
Good luck!
-Wayne
Can you also use the laser to temper the part immediately after welding by turning the power down and keeping the part rotating?
Also, you may consider compressing the part as welding takes places in order to minimize the residual tensile stresses.
Check toe chrome layer as well for contaminants. Some surface coatings are high in contaminants such as sulfer, which as we all know is a severe welding problem.
By bsinc
Date 11-19-2007 13:06
Thanks for all your input so far.
We ran some tests on Friday centering on 3 factors. Wave form, intensity, and surface speed. The two forms we tried were a square wave and a multi-wave with two peaks. The two levels of intensity were the current setting and plus 20%. The speed settings were our standard setting and half speed. So far it appears that cutting the speed in half shows some promise. I believe it may be attributed to the haz holding a little more heat and slowing down the ability to cool too quick and cold crack.
A little more background on the shape of the parts. They are used for canular tips for arthroscopic medical devices. They are two semi circle halves that when joined together form a round parts .25 in diameter and about .5 long. The welds are done along the tangent seams.
As far as the coating goes because it's proprietary we don't really know the chemistry involved other than that it's standard for this type of application in the medical industry and is FDA approved.
Anyway the parts are in our QC department for testing and I probably won't get any results for a few days. After that we'll probably run more tests.
