RSW Trouble! - Machine Qualification per AMS-W-6858A

[CoffeeMan]

Hello,

I have been charged with qualifying a Unitek/Miyachi High Frequency DC Inverter resistance spot welding unit for use with Nickel/Cobalt superalloys (URL:

http://www.miyachi1.com/Product_Item_View.asp?ID=13).

I am experimenting with settings for .059 thick Hastelloy X, and have been plagued with failures.


The problems stem from the shear strength requirements of the spec in Table #2. Strengths are grouped according to the Ultimate Strength of the material. There are four ranges for UTS: under 90 ksi, 90-149 ksi, 150 -185 ksi, 185+ ksi. It so happens that the Hast X has a strength ~110 ksi putting it in the second category. Experimentation and head scratching have produced welds with strengths of 2100 lbs or so - the absolute minimum strength for the category and thickness.

Obviously, to get more strength from my welds I should increase time or current, right? Well, I did increase these parameters ever so conservatively. To my chagrin, these welds flashed or developed pores (I found upon macroetching the cross-sections).

Gee, now that I've got a porous nugget what do I do? I can't increase my welding force because I am maxed out at 480 lbs (2.5 in dia. pneumatic cylinder @ 100 psi shop air).

If anyone can offer me any good advice or tips...
that would be great! I am at my wits' end.


[flush]


BTW - I am using .480 OD flat tip electrodes (to maximize nugget size).

 

[Goahead]

Hi,
looking up on Metals Handbook vol. 6 (8th ed.) for standard schedules for spot welding of stainless steel,
I find two main differences.
Electrodes for thickness of 0.050 to 0.062" are given as follows:
Body diam. (min) 0.5 in , Face diam. (max) 0.25 in.
Net electrode force, lb 1200 to 1500 for same thicknesses.
Maybe you should try to overcome the limits of your setup.
Of course the major differences between nickel alloys and stainless steel are lower thermal expansion, lower thermal conductivity, and higher electrical resistivity,
Good luck!

http://www.welding-advisers.com/

 

[griffengm]

CoffeeMan,
Recommend you get a copy of Resistance Welding Manual and work from there.
P. 246, third ed. "The thermal and electrical conductivities of this alloy (Hastalloy) are also quite similar to those of Inconel."
P. 250, Table IV, Recommended Schedule...Inconel Sheet,
.063"T, Elec Dia. 5/16", F(lb) 2100#, T (cycles)12, Amp 13800, Weld Dia..310".
The biggest advantage to the book is that it allows you to troubleshoot problems such as yours. There are newer editions that may cover your application better than mine.

Griffy

 

[GRoberts]

From the other posts, it looks like you may need to increase you electrode force. Can you put an pneumatic pressure booster in line with your welder to increase the electrode force? Of course the air-cylinder would have to be rated for it.