Hard Metals vs. Soft Metals for CNC Machining

Wear and Fatigue Properties for Metals

Typically, if you are machining a part for prototype fit and function you won't need to worry about the wear of a material. In cases where you need guaranteed strength or for the part to stand the test of environmental properties like extreme temperatures, your choice of material will be very important. Let's break down the most important fatigue properties to consider.

• Fatigue strength and toughness: This is the stress to which the material can be subjected for a specific number of cycles. These variations have been studied extensively to aid in the appropriate selection of materials to meet your end-use requirements. According to Samuli Heikkinen, 'Fatigue is estimated to be responsible for approximately 90% of all metallic failures.' Failure occurs rapidly and without warning so we typically measure in fatigue strength by ratio averages. When choosing your material, we recommend evaluating the fatigue-strength rating if you know your part will be under numerous stress cycles.
• Environmental cycling: There are many resources for environmental cycling testing. Most of the time materials are placed in a controlled environment and tested against high and low temperatures, high and low humidity, thermal cycling, and thermal shock to name a few.
  --Metals that can withstand high temperatures: titanium and stainless steel.
  --Metals that can withstand extremely cold temperatures and remain ductile in low temperatures: copper and aluminum.

• Creep resistance is defined as a materials ability to resist 'creep' which refers to the tendency of a solid material to deform over a long period of time-due to exposure to high levels of stress. It's important to note that creep resistance can happen beyond the standard stress limits of a material because it happens over a longer period of time. Creep becomes especially important to use cases that have the potential exposure to elevated temperatures-think aerospace applications or spacecraft. Creep resistance for metals is controlled by their alloy composition as well as their melting temperature. Nickel, titanium, and stainless steels have the highest creep resistance for metals. Aluminum tends to have very low melting temperatures and not recommended for aerospace use cases.

Corrosion (Oxidation) Resistance in Metals

Corrosion on metals is deterioration or oxidation, as a result of chemical reactions between it and the surrounding environment. There are many reasons for metal corrosion and it's worth noting all metals can corrode. Pure iron typically corrodes very quickly, but stainless steel, which combines iron and other alloys, is very slow to corrode. Stainless steel is a great option for a metal if you are worried about corrosion.

Another alternative to stainless steel would be anodizing aluminum. This method helps reduce corrosion and is a very durable finish. Since anodizing is a secondary service, it will likely add lead time to your project, so it might not make sense for your project needs.

Thermal Properties in Metals

We touched on it a little already, but metals react very differently when put under thermal pressure. Metal can expand, melt, and conduct to name a few changes we will explore. Let's break down the metals and their thermal properties in the table below.

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