In recent years, the transportation industries have increasingly turned to aluminum as they look for “lightweight” products. As a result, metal fabricators need new tools to work with this challenging material.
Aluminum alloys offer an improved strength-to-weight ratio compared to traditional steel alloys. Lightening trends in the transportation sector lead to the need for fast and efficient aluminum grinding tools. Typical angle grinder wheels designed for steel are not for use on aluminum because the surface of the wheel can quickly become clogged with metal filings that adhere to the abrasive.
By investigating the mechanisms by which metal chips can adhere (load) to the abrasive wheel surface, strategies to avoid metal loading can be applied to abrasive design. This then leads to new aluminum grinding products with significantly improved grinding speeds and longer lasting performance, without the need for waxes or lubricants.
The use of aluminum is increasing
Aluminum used in industry is usually not a pure element, but rather one of a number of families of aluminum alloys, depending on the end use. Although the properties of specific aluminum alloys can vary widely, it is safe to make the following generalizations:
- Aluminum alloys are much lighter, only about one-third the density of steel alloys (2.7 g/cc vs. 7.85 g/cc).
- Although they are not as mechanically strong as steel, their strength-to-weight ratio is higher.
The production and use of aluminum is increasing. Although total steel production is higher than aluminum production at present, the growth rate of aluminum production is about 60% higher than that of steel. From 2008 to 2018, global aluminum production grew at a compound annual growth rate (CAGR) of 4.8%, while global steel production grew at a CAGR of 3.0%.
Aluminum usage growth is driven primarily by the automotive and transportation industries, followed by the aerospace and defense and marine industries. The automotive and transportation industry accounts for about 80% of the global high strength aluminum alloy usage value at a forecast CAGR of 7.7% from 2018 to 2023.
The strong need for aluminum in the automotive and transportation industry is driven by trends in lightweighting. Car manufacturers are under constant pressure to increase the fuel efficiency of their fleets, so naturally they are looking for stronger and lighter materials. In the commercial trucking industry, lighter trailers can result in greater weight of cargo carried per trip in addition to fuel savings. Aluminum is also used to reduce the weight of marine vessels, which helps their speed, maneuverability, stability and fuel economy. Lightweight hulls also allow for shallow water operation.
The challenges of working with aluminum
Aluminum alloys also have lower hardness, higher ductility, and lower melting points than steel alloys (932 degrees F to 1112 degrees F for aluminum versus about 2732 degrees F for steel). These differences can mean that metalworking tools and techniques used for machining steel are not always optimized for machining aluminum.
A common problem when it comes to hand sanding aluminum workpieces is the tendency for aluminum chips to stick to the sanding wheel itself. When the wheel becomes loaded (clogged) with metal shavings, it cannot remove more metal from the part. Figure 1 shows a standard sanding disc after just a few minutes of use on aluminum. Since this grinding wheel was designed for use on steel, not aluminum, a load occurred and the wheel stopped grinding effectively.
One practice that delays the onset of metal loading is applying wax to the grinding wheel. By applying a slippery substance to the surface of the wheel, it temporarily makes it difficult for the aluminum chips to stick. However, as the wheel is used, the wax wears off and needs to be reapplied. This option is not ideal because applying wax takes time away from sanding and creates additional dirt on the part that needs to be cleaned when sanding is complete. If the wax is not completely cleaned from the part, it can lead to defects in the weld.
A high-magnification camera focuses on the stressed areas on the surface of a used grinding wheel (see Fig. Figure 2) reveals an abrasive surface that cannot do the job it was designed to do. The bright areas are the aluminum metal glued to the face of the wheel. The white, blocky elements are the abrasive grain. The yellow area is the exposed, worn areas of the bond and the brown is the main bond and pores.
The image on the right of Figure 2 shows the cutting point of a single grain whose surface is coated with metallic aluminum. Behind the cutting point are many tough aluminum shavings that have been collected. Because these chips were not removed from the grinding zone, they were fused together by the friction and heat generated as the grain struck the workpiece. Streaks along the center of this table indicate friction marks between the aluminum workpiece and the aluminum stuck to the grinding wheel. As the aluminum collected on the face of the grinding wheel, it blocked the cutting tip from removing more chips – stalling the metal removal process.
Cross-section of this grinding wheel (see Fig Figure 3), viewed under a microscope, reveals the metal loading from a side view.
Close examination with a scanning electron microscope of the aluminum chips removed from the surface of this wheel reveals even more (see Figure 3, right). A close-up focus on the top side of the chips shows friction/plowing marks, suggesting a semi-solid-like behavior. The underside of the chip shows how the aluminum was able to deform and attach itself to the entire surface of the grinding wheel, conforming to both the grain and the bond. These deformation characteristics indicate that the metal was softened near its melting point when it attached to the surface of the wheel and that the mass grew cohesively when other pieces of aluminum were stuck.
Figure 4 shows a framework for how the abrasive grain, the bond holding the grain, and the workpiece being ground can interact in cutting (material removal), plowing (material displacement), and sliding (surface modification) processes. The features observed on the wheel surface are mainly indicative of sliding interactions from the moment the abrasive grains are in contact with the aluminum workpiece. Sliding interactions do not contribute to the metal removal process (chip formation) and only act to make the grinding process less efficient.
During grinding of aluminum (see Figure 4), the grain passes through the plastic part, which covers the tips of the grain with metal. Once the grain tip is coated, the frictional interaction between the chip (adhered to the grain) and the workpiece allows the stuck metal chip to begin to grow cohesively. As the stuck metal patch grows, further interactions between the bond and the part build up more heat, resulting in a larger area affected by metal loading.
During use, as the abrasive wheel becomes clogged with metal, grinding becomes less efficient, resulting in the operator’s natural reaction to push harder with the grinder to try to break down the wheel even more and open up the surface, to expose new cutting grains. However, this common approach does not work because the increased grinding pressure causes more heat to build up, which continues the process of aluminum chips softening and sticking to the disc surface. This creates a feedback loop that acts as a vicious cycle to put additional stress on the wheel until it can no longer grind and needs to be replaced.
New abrasive technology for aluminum
To break the loading mechanism feedback, the abrasive grain must become more resistant to metal loading. This is because the loading mechanism starts at the tips of the grains and grows cohesively to cover large areas of the grinding wheel.
During grinding, the individual abrasive grains experience thermal and mechanical stresses as they continuously impinge on the workpiece. These stresses can cause the grain to crack or break in various ways (see Figure 5). The type of grain fracture, as well as the overall rate of grain fracture, depends on grain microstructure and is related to several grain properties, including hardness and resistance to heat, shock, and impact. A grain that breaks and crumbles easily is known as friable, and one that wears slowly is known as durable.
The grain break is self-sharpening as it exposes new cutting surfaces. In the case of aluminum grinding, when the grain breaks, the ejected pieces can pick up bits of stuck aluminum metal, leaving behind a fresh, clean cutting point.
To demonstrate the effect of friability on grinding rate (metal removal rate) and loading rate, wheels containing grain types with different levels of friability were prepared and tested for grinding. All other experimental parameters were kept equal.
After the grinding test was completed, each wheel was imaged to determine the degree of metal loading by calculating the total chamfer area covered by trapped metal (see Fig. Figure 6).
As a result, a strong correlation was found between grinding wheels that contained highly friable grain types having less metal loading and higher grinding speed.
This led to the development of aluminum grinding wheels with a special, extremely friable abrasive grain that is able to break and shatter just before too much pressure and heat is generated, preventing metal build-up (see Figure 7). These abrasive discs are aggressive, allowing the hand grinder to work with less effort compared to using abrasive discs that are not specifically designed for aluminum removal.