On average, aluminum is the easiest to machine material group. It gets as high as 350% in soft wrought aluminum alloys. But also, as low as 100% in cast aluminum alloys with high silicon content. We will break it down into its sub-groups and discuss the machinability of each one.
Table of Contents
What is Aluminum?
Aluminum in machining is referring to Aluminum alloys made from Aluminum (Al), with additional alloys that are usually silicon (Si), copper (Cu), magnesium (Mg), and manganese (Mn). Aluminum is a popular engineering material due to its low density (2.5 times lower than steel) and high machinability rate.
Aluminum alloy groups
- Cast aluminum alloys: This type of alloy has high silicon (si) content, usually between 7% and 15%. With lower silicon content, aluminum alloys are not castable, and therefore cast aluminum will always have high silicon content. The machinability of cast aluminum is about half compared to conventional wrought aluminum, and the wear on the cutting edge is developed much faster. A popular machined component made from cast aluminum is car wheels.
- Aluminum Lithium alloys: This group of aluminum alloys contains 0.7%-3% Lithium (Li) content. The presence of Lithium reduces the density of the alloy and provides an excellent strength-to-weight ratio. The downside is that the cost of aluminum-lithium’s is about double, and the machinability about 60% compared with common aluminum alloys. It is a popular alloy in the aerospace industry where weight and strength are crucial properties but rarely used in other components.
- Wrought (forged) aluminum: This is the most popular group of aluminum alloys present in machine shops. It has reasonably good mechanical properties combined with excellent machinability
Main types of wrought aluminum alloys and their machinability
Aluminum Alloy Series 2xxx
Copper is the primary alloying element in the 2xxx series. Thus, these alloys are also commonly referred to as “copper alloys”. When heat-treated, these alloys have mechanical properties equivalent to mild steel. Less corrosion resistance is present in these alloys when compared to other aluminum series. The popular alloy in this series is 2024, very common in the aerospace industry.
Strength: 465 Mpa
Hardness: 75 HB
Average Machinability Rate: 240%
Aluminum Alloy Series 6xxx
Aluminum 6xxx alloys have silicon and magnesium as the main alloying elements. These alloys have better machinability compared with the 2xxx series at the expense of lower strength. The most popular alloy in this series is 6061. It is a versatile alloy found in many engineering components and is the single most popular aluminum alloy in CNC machine shops.
Strength: 300 Mpa
Hardness: 95 HB
Average Machinability Rate: 270%
Aluminum Alloy Series 7xxx
The main alloying element in the 7xxx series is zinc, but it also contains copper, chromium, and magnesium. It has much higher strength and hardness than 2xxx and 6xxx but has a lower machinability rating and costs more. In this series, the popular alloy is aluminum 7075, commonly used for mold making, marine, automotive, and defense industries.
Strength: 570 Mpa
Hardness: 150 HB
Average Machinability Rate: 170%
Machinability Chart for wrought Aluminum Alloys
In many cases, the actual cutting speed will be much lower than what is possible from a machinability rate perspective. Maximum spindle speed and clamping stability may limit the actual speed.
Boosting Machinability with PCD tipped inserts
All our discussion so far was based on carbide inserts. However, to further boost aluminum machinability, it is best to upgrade to PCD (Diamond) tipped inserts. With this type of advanced cutting material, the cutting speed can be doubled compared with carbide, provided stability, machine, and diameter are suitable for the RPM needed.