Thread Failure in Aluminum Casting

[Pedee68]

What the common failure modes for an aluminum casting where threads fail during assembly of bolt. The failure is stripped threads causing the bolt to not engage during assembly. Casting is A304 alloy with threads for M5 bolt. The torque value is 5.5 Nm.

Looking for areas to investigate either the casting, bolt or torque process.

 

[GregLocock]

Inadequate thread engagement

Galling

Clumsy operators

Poor tapping technique

 

[MintJulep]

Off hand, that seems like a pretty high torque for tapped threads in a relatively weak grade aluminum casting.

Look at the stress in the female threads. Consider all the variability associated with torque-controlled tightening.

 

[mfgenggear]

OP
Threads in aluminum is not good practice.
Inserts are a better option.
If the threads seize it will strip out easily.
Over torque strip out.
Cross thread strip out
And more.

 

[JohnRBaker]

I agree, a threaded insert would seem to be the best way to go.

 

[waross]

https://www.stanleyengineeredfastening.com/en/brands/Optia/heli-coil

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The Heli-Coil® portfolio of wire threaded inserts includes offerings of a variety of configurations whether it be plating, material, locking, or non-locking variants that make it fully adaptable and configurable for industries spanning automotive, industrial, aerospace, and defense. There is a strong industry trend toward the lightweighting of materials, yet achieving structural integrity of bolted or fastened joints. Heli-Coil® by nature of its design is an ideal solution to be incorporated as part of these assemblies.

https://www.boellhoff.com/ca-en/products/special-fasteners/helicoil-thread-inserts-for-metals/

 

[waross]

AI Overview
Learn more…

The German company Böllhoff Grouphas owned the Heli-Coil trademark since October 1, 2021. Böllhoff acquired the trademark from Stanley Engineered Fastening.



The Heli-Coil thread insert was originally patented in the United States in 1938. Josef Böllhoff licensed the production and sale of Heli-Coil thread inserts in 1954.

 

[ctopher]

I would also try the insert.
Also, make sure the tapped threads are clean. I have seen too many threads with metal bits in them from the tapping process that will cause the bolt to strip or get stuck.

 

[ghostdigital]

Direct threading into Aluminum is rarely a good idea. Baseline stainless steel threaded inserts.

 

[Brian Malone]

Direct threading into aluminum is not a problem and is commonly done. There are issues only when multiple cycles of assembly/disassembly occur or extremely high torque is applied or the fastener is undersized to the loading. Inserts are used when a product or application supports the cost and/or the use profile requires maximum strength and highly reliable assembly/disassembly. Go take apart your car or motorcycle and count the number of inserts used in aluminum parts with threads. Certainly, this is probably different for an airplane or aerospace application or similar but the use case is different. Verify the threads were formed properly, the torque applied is correct for the material strength and the environment is not promoting corrosion.

The items noted by GregLocock are items I would check first, especially length of engagement. 5.5 Nm is a reasonable torque for an M5 but at the high end for most aluminum alloys but verify the specific properties of the A304 material you are using.

 

[GregLocock]

Our front suspension was aluminium for the most part. For production the aluminium was threaded and passed durability. All our design engineers and most of the rest had done the Bolt Science workshop so most of our bolted joints were properly designed (famous last words). For prototype work our SOP was to helicoil the suspension to avoid problems with repeated reassembly.

 

[mfgenggear]

Any one who cheapskate out and especially on automotive and not install an insert . Is well not good.
Example, old Buick heads had spark plugs blowing out. Ford v10 engine had spark plugs blow out. Most head mfg don't give a Torque value. Because of failures. And the list goes on.
On tool cast fixtures, I fix to many to count stripped out threads.
Heat and corrosion is not your friend. Aluminum will corrode and seize. Heat is the enemy. As well as contaminated.

 

[ghostdigital]

Direct threading into aluminum is not a problem and is commonly done.

Not a practice in my world or industry ever .

In what industry or product is this "commonly done"?

 

[desertfox]

What the common failure modes for an aluminum casting where threads fail during assembly of bolt. The failure is stripped threads causing the bolt to not engage during assembly. Casting is A304 alloy with threads for M5 bolt. The torque value is 5.5 Nm.

Looking for areas to investigate either the casting, bolt or torque process.

Hi Pedee68
I would check the shear stress on the internal thread created by the bolt torque and compare it with the allowable shear stress for the casting. I think you will find that the failure is most likely to be the shear stress on the internal thread is too high for the material

 

[Brian Malone]

Not a practice in my world or industry ever .

In what industry or product is this "commonly done"?

Have you looked at any car or motorcycle? I have worked on many cars and dirt bikes and there are very few inserts in the multitude of threaded holes in the aluminum parts - occasionally I'll see one in an alternator body. I am missing what the deal with threads in aluminum is coming from. If done properly and correctly loaded they are appropriate and highly reliable. I have worked in various industries ranging from medical/laboratory analyzers and processing / industrial equipment and thread inserts in aluminum parts are not used as standard practice - only in locations that will be disassembled with regularity or if the thread loading of the tapped side indicated additional strength is required. I have used more inserts to repair threads that were damaged due to cross threading or insufficient grip or over torquing - all user error.

 

[WKTaylor]

A few related questions... 'seez the guy late to the discussion'...

I am unfamiliar with casting alloy A304... what is the procurement specification [ASTM, etc] for your part? What temper are you heat treating the A304 to... none/as cast, -O, -T3, -T4, -T6? Have you validated the alloy/temper, 'in-house'?

Are You thread-tapping for a coarse or fine thread bolt? What is the thread you are cutting [Diameter and threads-per X]. What is the starting hole diameter? What is the depth of the threading? 'Blind' or 'thru' threading? What is the depth[length] of bolt-thread engagement within the tapped hole, IE: 1.0xDia, 1.5xDia, 2.0xDia, etc?

Are you installing a straight bolt... or a shoulder-bolt [or equivalent]?

Are you torque-turning to 5.5-Nm... or are you also adding-in 'free-running' torque [to a slightly higher torque value due to 'thread-drag']. Are you torque-turning the bolt 'dry' or 'wet'**? Are you using a calibrated/precise-range torque wrench... clicking or indicating... for this size bolt and the required torque?

**Are you torque-turning the bolt 'dry' or 'wet' with sealant or with thread-locking compound, or with primer, or with oil, or with...???

Is the threaded installation supposed to be permanent, IE: 1-time install; or multi-times remove/install?

Was this install ever prototyped/validated by certified techs? Was the install 'strenth/durability tested'... tensile/bending... max-torque... etc.

/NOTEs/
I have an old aircraft spec on this subject... NAS141... for a stud into a threaded aluminum casting [aircraft alloys at HT temper -T6, I think]. It only permits coarse threads.
For a full discussion/understanding of torque-turning fasteners... read...
SAE J1701M TORQUE-TENSION TIGHTENING FOR METRIC SERIES FASTENERS
SAE J1701 TORQUE-TENSION TIGHTENING FOR INCH SERIES FASTENERS
SAE AS1310 FASTENER TORQUE FOR THREADED APPLICATIONS, DEFINITIONS OF

 

[stevenal]

If threads in aluminum were a bad idea in general, I wouldn't think using an insert that was itself threaded into the aluminum would be any better.
I can see an advantage where disassembly/reassembly is frequent to lessen chances things go wrong then.

 

[ghostdigital]

Have you looked at any car or motorcycle? I have worked on many cars and dirt bikes and there are very few inserts in the multitude of threaded holes in the aluminum parts - occasionally I'll see one in an alternator body. I am missing what the deal with threads in aluminum is coming from. If done properly and correctly loaded they are appropriate and highly reliable. I have worked in various industries ranging from medical/laboratory analyzers and processing / industrial equipment and thread inserts in aluminum parts are not used as standard practice - only in locations that will be disassembled with regularity or if the thread loading of the tapped side indicated additional strength is required. I have used more inserts to repair threads that were damaged due to cross threading or insufficient grip or over torquing - all user error.

I've worked design engineering in big aerospace, spacecraft and weapons systems - 38 years experience...

We use threaded inserts in aluminum, always tangless and mil spec.. Helicoils are discouraged because they are challenging to remove and replace if damaged. To manage dissimilar metal corrosion there is usually a coating barrier between the insert and the aluminum.

 

[GregLocock]

In what industry or product is this "commonly done"?

Automotive. Every motorbike I've ever worked on has bolts into aluminium holding the cover plates to the crankcase. Our vehicle front suspensions were often aluminium and for production were bolts into aluminium. For prototype work we helicoiled them as they were often disassembled. All our design engineers and most others did the Bolt Science workshop and so know how to design bolted joints, and if they couldn't figure it out we had a few specialists who would. It's an engineering problem, engineers sort it out.

 

[waross]

Automotive. Every motorbike I've ever worked on has bolts into aluminium holding the cover plates to the crankcase. Our vehicle front suspensions were often aluminium and for production were bolts into aluminium.

Greg, are you able to comment on different grades, alloys and casting methods and how threadability is affected thereby?
Thanks.