How do steel and aluminum bond over time?

[My Username]

I recently replaced a wheel bearing assembly on my car. The bearings and race were steel, and they were seated in the aluminum steering knuckle. Removing the bearing assembly required a sledge hammer and a lot of time and effort. Why does that happen? I'm asking here because after an hour of searching on the internet I could only find things about it that were practical, such as putting a graphite paste between the two metals next time. But I couldn't find an explanation of what happens on the molecular level to cause the bond.

Edited January 3, 2017 by My Username

[Acme]

I recently replaced a wheel bearing assembly on my car. The bearings and race were steel, and they were seated in the aluminum steering knuckle. Removing the bearing assembly required a sledge hammer and a lot of time and effort. Why does that happen? I'm asking here because after an hour of searching on the internet I could only find things about it that were practical, such as putting a graphite paste between the two metals next time. But I couldn't find an explanation of what happens on the molecular level to cause the bond.

Galvanic corrosion

 

Overview

 

Dissimilar metals and alloys have different electrode potentials, and when two or more come into contact in an electrolyte, one metal acts as anode and the other as cathode. The electropotential difference between the dissimilar metals is the driving force for an accelerated attack on the anode member of the galvanic couple. The anode metal dissolves into the electrolyte, and deposit collects on the cathodic metal.[1]

 

The electrolyte provides a means for ion migration whereby metallic ions move from the anode to the cathode within the electrolyte. This leads to the metal at the anode corroding more quickly than it otherwise would and corrosion at the cathode being inhibited. The presence of an electrolyte and an electrical conducting path between the metals is essential for galvanic corrosion to occur. ...

In your case the electrolyte is likely dirty water.

[arc]

To add to Acme's good analysis;

 

The two metals also have quite different coefficients of thermal expansion between them. And when exposed to cycles of heating and cooling, such as through friction from braking during deceleration, and forces from dynamic loading during cornering for example, these changes can allow moisture from the weather and condensation from thermal cycling to penetrate more easily between the dissimilar materials. This can accelerate the galvanic corrosion, or on their own, bring enough microscopic foreign material into the part's contact surface area to make them difficult to remove years later.

 

The greater the difference in the coefficient the greater the space for water and foreign material to occupy. So, if your part's contact areas are still smooth and do not have the visible corrosion from electrolysis it is probably the thermal cycling alone and/or foreign material.

 

Some manufactures actually preheat some parts before the assemblies are combined and use negative thermal expansion to hold the parts together. This, of course would require you to heat the proper part to disassemble the components.

 

https://en.wikipedia.org/wiki/Thermal_expansion

 

"The coefficient of thermal expansion describes how the size of an object changes with a change in temperature. Specifically, it measures the fractional change in size per degree change in temperature at a constant pressure. Several types of coefficients have been developed: volumetric, area, and linear. Which is used depends on the particular application and which dimensions are considered important. For solids, one might only be concerned with the change along a length, or over some area."

 

 

[RiceAWay]

To add to Acme's good analysis;

 

The two metals also have quite different coefficients of thermal expansion between them. And when exposed to cycles of heating and cooling, such as through friction from braking during deceleration, and forces from dynamic loading during cornering for example, these changes can allow moisture from the weather and condensation from thermal cycling to penetrate more easily between the dissimilar materials. This can accelerate the galvanic corrosion, or on their own, bring enough microscopic foreign material into the part's contact surface area to make them difficult to remove years later.

 

The greater the difference in the coefficient the greater the space for water and foreign material to occupy. So, if your part's contact areas are still smooth and do not have the visible corrosion from electrolysis it is probably the thermal cycling alone and/or foreign material.

 

Some manufactures actually preheat some parts before the assemblies are combined and use negative thermal expansion to hold the parts together. This, of course would require you to heat the proper part to disassemble the components.

 

https://en.wikipedia.org/wiki/Thermal_expansion

 

"The coefficient of thermal expansion describes how the size of an object changes with a change in temperature. Specifically, it measures the fractional change in size per degree change in temperature at a constant pressure. Several types of coefficients have been developed: volumetric, area, and linear. Which is used depends on the particular application and which dimensions are considered important. For solids, one might only be concerned with the change along a length, or over some area."

 

Thermal expansion Wikipedia.png

 

It is important that you should realize that these bearings were installed at or near the same temperature. Some have the aluminum casing heated a bit to ease installation. They are generally press fit tight enough to prevent rotation of the outer bearing race. But as you can see from your listing that the actual difference in expansion between the two metals on a 3" bearing is not worth discussing.

 

Acme is no doubt correct.

Edited January 17, 2017 by RiceAWay

[Bender]

It is important that you should realize that these bearings were installed at or near the same temperature. Some have the aluminum casing heated a bit to ease installation. They are generally press fit tight enough to prevent rotation of the outer bearing race. But as you can see from your listing that the actual difference in expansion between the two metals on a 3" bearing is not worth discussing.

 

Acme is no doubt correct.

Difference in thermal expansion on 75 mm for a moderate 100°C increase in themperature is 75x10^-5*100=75 µm, which is plenty to go from a loose fit (eg H7/h6) to a tight fit (eg H7/s6) (or vice versa). The first is mountable by hand or with a little force, the second is very difficult to assemble/disassemble without temperature change.

Edited January 18, 2017 by Bender

[arc]

Plus 1 bender.

 

The original post probably gives the greatest clue as to the reason for the difficult disassembly.

 

I recently replaced a wheel bearing assembly on my car. The bearings and race were steel, and they were seated in the aluminum steering knuckle. Removing the bearing assembly required a sledge hammer and a lot of time and effort. Why does that happen? I'm asking here because after an hour of searching on the internet I could only find things about it that were practical, such as putting a graphite paste between the two metals next time. But I couldn't find an explanation of what happens on the molecular level to cause the bond.

 

Bold mine. Aluminum is a much softer metal than the high carbon steel of the bearing raceway, and assemblies consisting of steel races pressed into a steel or aluminum housing require extremely accurate alignment during the insertion and removal processes. This process is properly done on a press designed to provide gradual and more important, aligned force, to remove and install press fit components together.

 

Aluminum is very easily misaligned with the bearing race when it is subjected to blunt forces at angles beyond dead center to the parts axis. In other words do not use a hammer! This causes galling of the aluminum's machined surfaces which lead to the need of even more increase of forces to dislodge the raceway and many times to the permanent damage of the part through fracturing.

 

Many smaller procedures involving aluminum require the use of an arbor press which allow for a very subtle and direct control of the ram where the operator can feel through the direct mechanical linkage the movement and forces that result. Whereby a hydraulic press could easily overload the aluminum part before the operator can respond in time to prevent damage.

 

I have replaced many bearings over the last 40 some years. I do my own automotive repair in my own very well equipped shop which includes a 20 ton hydraulic press that I would not use to disassemble an aluminum assembly that shows signs of the corrosion or contamination mentioned without first preheating it. Any degree of the corrosion that Acme mentioned or the contamination as I suggested will only compound the difficulties in doing this process correctly with even the proper equipment let alone a hammer.

[foxy john]

It's also possible that the sides of the bearing are tapered. These would fit very tightly indeed. I remember years ago, undoing a tapered steering joint using severe hammering, getting it free, then accidentally letting the two parts join again without any pressure. Yet more hammering!