Could my motor case survive this weld?

[Frightening Man]

I'm trying to attach my aluminium fins to my also aluminium motor case. I know welding can severely weaken the motor case, so I won't be welding the part that will be under pressure. My plan would be to weld the bottom half of the fins, at the part of the rocket where the concrete nozzle is located. I would put some sort of foam / glue around the non-welded part for aerodynamic reasons. My questions are:
1. Would I weaken the motor case this way?
2. Would the fins be attached strongly enough? (At least for going mach 1)


Thank you for your time.

 

[FredA]

concrete nozzle

Sounds Frightening.

The whole case is under pressure. You'll destroy the temper and likely ruin the surface on the inside so things won't slide in and out.

 

[jderimig]

If you are using 6061-T6 you can retemper to T4 at home in your kitchen.

However more frightening is your "concrete" nozzle. Do you know what concrete does when exposed to high heat? I do.

 

[FredA]

However more frightening is your "concrete" nozzle.

Yes - I should have been more specific.

 

[curtisheisey]

Ditto, to all the above words of heed.

Furthermore, welding on a thin tube can also cause dimpling on the inside. If so, you may not be able to slide your motor grains in.

Consider a non-welding fin attachment method. See the many TRF threads on "tip-to-tip" lamination for attaching the fins on the surface using a layup of carbon fiber. You would also want to use a high temperature resin.

 

[Frightening Man]

Thank you for your replies!
I haven't been all set about using a concrete nozzle, but if you think it could form a problem I will definitely look into it. I thought only APCP was too hot for concrete nozzles, and I'm using sugar.
The case is 1.5 mm thick (0.059 inch) which I believed to be thick enough to avoid these kind of problems.
The reason I'm avoiding resins is because of how expensive the stuff is.

Would the case suffer any strength, besides the deformation of the aluminium?

 

[GalantVR41062]

There are a few issues with welding thin aluminum.

I have welding experience in MIG, TIG, oxygen acetylene. Welding mild steel, aluminum, stainless steel of various thickness, 22ga or less and 1" plus. I have learned while working on making cars go fast, welded at CAT road paving devision for a while and a few other places.

If the parts to be welded are used, dirty, anodized, painted, un known aluminum alloy, acheiving a good weldment that will withstand the stress and pressure of being a hobby or high power experimental motor could be difficult and without previous working experience and testing could be un safe.

Welding process: braze, TIG, MIG spool gun. I have not braze welded aluminum but other metals, a spool gun will burn to hot for this kind of weld.

TIG, not just any TIG, but one with pure Argon shielding gas, 1/16" pure (green) tungsten, the ability to weld in AC, high frequency and ball the end of the tungsten.

What thickness is the fin? The case at .059" is thin, I have welded car radiators, the tubes in the core of the radiator are thin, it's not fun but doable.

The other issue I see is only having the back edge of the fin welded could cause instability and the epoxy or what ever glues the front part of the fin may snap off, or degrade with the heat of the burning motor.

The biggest issue I see is distorting the aluminum, if welded properly the reduction of the temper of the aluminum from welding could be offset by original material thickness and the additional filler rod creating the weld joint, if its 6061 T6 aluminum I would select either 4043 or 5356 alloy 1/16" filler rod. The 4043 will be easier, cleaner to weld, but be softer overall. The 5356 is a structural welding rod and will have a different look but be a lot stronger. The internal bore will distort during welding, the more welding rod you add the more will migrate to the ID of the tube, the oxidation layer on aluminum melts at 3700° F, where as the aluminum itself melts at 1221° F. There are many ways to remove this oxide layer and is critical to the welding process.

When I try this later this year I will have a elaborate fin jig to hold the fins during the welding process, I will be over sizing the tube I am welding the fins onto, then boring the ID to what I need to attach to the airframe or slide over a motor case, I do not plan to weld onto the motor case directly.

~John

 

[TimothyG]

I love that his chosen persona is frightening man and his first post is frightening.

 

[Frightening Man]

There are a few issues with welding thin aluminum.

I have welding experience in MIG, TIG, oxygen acetylene. Welding mild steel, aluminum, stainless steel of various thickness, 22ga or less and 1" plus. I have learned while working on making cars go fast, welded at CAT road paving devision for a while and a few other places.

If the parts to be welded are used, dirty, anodized, painted, un known aluminum alloy, acheiving a good weldment that will withstand the stress and pressure of being a hobby or high power experimental motor could be difficult and without previous working experience and testing could be un safe.

Welding process: braze, TIG, MIG spool gun. I have not braze welded aluminum but other metals, a spool gun will burn to hot for this kind of weld.

TIG, not just any TIG, but one with pure Argon shielding gas, 1/16" pure (green) tungsten, the ability to weld in AC, high frequency and ball the end of the tungsten.

What thickness is the fin? The case at .059" is thin, I have welded car radiators, the tubes in the core of the radiator are thin, it's not fun but doable.

The other issue I see is only having the back edge of the fin welded could cause instability and the epoxy or what ever glues the front part of the fin may snap off, or degrade with the heat of the burning motor.

The biggest issue I see is distorting the aluminum, if welded properly the reduction of the temper of the aluminum from welding could be offset by original material thickness and the additional filler rod creating the weld joint, if its 6061 T6 aluminum I would select either 4043 or 5356 alloy 1/16" filler rod. The 4043 will be easier, cleaner to weld, but be softer overall. The 5356 is a structural welding rod and will have a different look but be a lot stronger. The internal bore will distort during welding, the more welding rod you add the more will migrate to the ID of the tube, the oxidation layer on aluminum melts at 3700° F, where as the aluminum itself melts at 1221° F. There are many ways to remove this oxide layer and is critical to the welding process.

When I try this later this year I will have a elaborate fin jig to hold the fins during the welding process, I will be over sizing the tube I am welding the fins onto, then boring the ID to what I need to attach to the airframe or slide over a motor case, I do not plan to weld onto the motor case directly.

~John

Thank you for your reply!
The case and fins are both .059" indeed.
I'll be careful to use a heat resistant glue / foam.
I am not too worried about deformation of ID, as I can insert all components from the above end.
Sorry for the late reply.

 

[OverTheTop]

I know of an aluminium fin can that was made by brazing and successfully flown with an N5800. The fins were not brazed to the case though. Would the lower temperature of brazing destroy the temper? I am not a metallurgist but I suspect it would.

 

[tomsteve]

https://binderdesign.com/store/page13.html

 

[Joekeyo]

Sugar rocket? Isn't factory manufactured (I didn't say assembled) engines and the related NAR safety record the basis for the excellent safety record of the hobby? If you want to blow up yourself that is your own business. I discourage you from using these motors for the safety of others. I think this is a troll.

 

[dhbarr]

Sugar rocket? Isn't factory manufactured (I didn't say assembled) engines and the related NAR safety record the basis for the excellent safety record of the hobby? If you want to blow up yourself that is your own business. I discourage you from using these motors for the safety of others. I think this is a troll.

It may surprise you to find there is a Tripoli Research Safety Code, and an associated restricted Experimental section of this forum.

 

[jderimig]

There are a few issues with welding thin aluminum.



TIG, not just any TIG, but one with pure Argon shielding gas, 1/16" pure (green) tungsten, the ability to weld in AC, high frequency and ball the end of the tungsten.

What thickness is the fin? The case at .059" is thin, I have welded car radiators, the tubes in the core of the radiator are thin, it's not fun but doable.



The biggest issue I see is distorting the aluminum, if welded properly the reduction of the temper of the aluminum from welding could be offset by original material thickness and the additional filler rod creating the weld joint,

When I try this later this year I will have a elaborate fin jig to hold the fins during the welding process, I will be over sizing the tube I am welding the fins onto, then boring the ID to what I need to attach to the airframe or slide over a motor case, I do not plan to weld onto the motor case directly.

~John

John, great post. I am doing something similar and would like your comments, not a thread hijack because its on the same topic.

1. Electrode, I am going to use a lanthenated tungsten, I heard its somewhat better for Aluminum?

2. I was thinking a 5000 series aluminum might be a better choice for fin material because of its strength without needing a heat treat. But available tubing is predominately 6061 (or 6063). Any thoughts on welding a 5000 aluminum to a 6061 tube?

 

[GalantVR41062]

https://www.thefabricator.com/thewe.../aluminum-workshop-choosing-an-aluminum-alloy

With the exception of 5052 aluminum 5356 rod needs to be used, and you can weld 5000 to 6061. Any good weldment with aluminum will crack at the edge of the HAZ, not down the weld. A crator in the weld will cause a crack down the weld joint.

Aluminum does not have a infinite stress barrier. At some point the part may fail.

I have not used Lanthenated tungsten.

https://images.app.goo.gl/46xZHSk64WRzuufi8

I see it can be used for AC, yes you can and I have welded aluminum with 2% thoriated, pure tungsten (green) or even the "new" E3 purple tungsten works well for aluminum.

I also use and prefer my "old school" Airco 250 water cooled torch TIG welder. It is full of copper and has an adjustable air gap for the high frequency. Do not use if radio signals, VHS tapes or electronic devices such as cell phones, pace makers, ballast florescent lights etc are in the area of this welding process. NAVY ships used high frequency for communications back in the day.

Last words of warning when welding with a TIG welder, using argon gas. The heat of the plasma arc, argon shield gas (normal welding situation), and the presence of chlorine (not normal) will make mustard gas, that can kill you.

I have only heard of people using brake cleaner that has chlorine in it to clean parts to weld.

Always use acetone and or a clean SS brush to prep your surface, you can also rub off the oxide layer off your filler rod with sandpaper (Emery cloth) and watch the humidity, electricity is lazy and takes the path of least resistance, ground your work.

If it's on a welding bench or someone is holding it with welding gloves or a pliers, its HOT, don't touch it. Use proper PPE, shade 12 lense in a quality hood.

~John

 

[Omniasa]

A lot of useful information and advice right here, thanks for this.

 

[rfjustin]

I love that his chosen persona is frightening man and his first post is frightening.

#nailedit

 

[boatgeek]

On the engineering side of things, unwelded 6061-T6 has a yield strength of 35 ksi. My marine references have an as-welded yield strength of 15 ksi. If you can get that back to T4 temper, you only get back to 16 ksi. In the 5000 series, you see similar results. While the heat affected zone isn't going to be huge at these thicknesses, it's still going to be big enough to worry about.

As far as distortion goes, thickness doesn't really help you. You can easily locate a bulkhead on the far side of an aluminum plate by the distortion the welds cause in the near side, even up to 3/8" thickness.

A final option you have is to design your case for T0 (ie as-welded) yield strength and just accept the higher wall thickness. In that case, you might as well weld the entire fin, keeping the distortions in mind in your design process. At that point, you'd probably be better off to take the weight hit of a fin can or body tube around the motor casing.

 

[Zeus-cat]

Every time I read one of these posts I just think that the video of the launch would get reviewed by Big Jim McBob and Billy Sol Hurok on Farm Film Report.

He Blowed up Real Good

 

[cbrarick]

and let's just pause and ask, "if you can't afford R45, how can you afford the rest of the rocket?

not trying to be a dork, but a realist. go for a smaller rocket you can actually afford to fly - and have fun with. I still fly the heck out of my 38 mm rockets, despite flying multiple O motors.

-or ignore me for I know not what I speak and then-

This should be good, real good.
I need video of the flight
and prospectively nominate it for "Cool and really Cool 6"

 

[Omniasa]

The construction won't be stable in my opinion.

 

[AHansom]

I also use and prefer my "old school" Airco 250 water cooled torch TIG welder. It is full of copper and has an adjustable air gap for the high frequency.
~John

Older school than this? Ive kind of settled in on the 2% Lanthanated for everything. Works excellent on Aluminum, Steel and Stainless. Best of all I don't have to remember to change the electrode.

I appreciate people experimenting with there rocket design. Not sure I believe in this approach though. Doesn't really matter, he's been off the forum making his own way.