Hard Metals vs. Soft Metals for CNC Machining

Posted On June 16, 2021 By Amanda Tierney

Like all things, having multiple choices is usually a good thing. But having too many options without a clear objective for an upcoming CNC machining project can be overwhelming and costly—so we broke down the top six factors you should consider before settling between a hard or soft metal for machining. 

Mechanical Properties for Metals

Let’s start with mechanical properties, which are measured by the performance of the material when different forces are applied.

The top key mechanical properties for metal to consider are:

  • Strength (hard metals)
  • Ductility (soft metals)
  • Elasticity (hard metals tend to be more elastic than soft metals)
  • Hardness (hard metals)
  • Density (ranges vary going from soft to hard on the density scale)
  • Magnetic (steel)
  • Fracture toughness (all metals have the highest range of fracture toughness, but it ranges from soft to hard being the toughest)
  • Damping (hard metals tend to have small damping capacities)

If any of the above properties are important to your project, we recommend doing some research to get actual property ratings per material. Check out our materials page  for a comprehensive list of all our metals with links to detailed data sheets.

CNC machined part
When producing metal machined parts, carefully consider several factors before choosing between a hard or soft metal.

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.  Indeed, according to research on this topic, “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.   

Manufacturability of Metals

When it comes to manufacturability, each supplier or manufacturing partner probably has a different set of requirements based on their capabilities. If you discover your part cannot be machined and you have little to no flexibility in your part design, you might need to change manufacturing methods to 3D printing, which offers unique solutions in metal and plastics. The good news is if you have decided to machine your part and upload for quote, we at Protolabs offer almost instant design for manufacturability (DFM) feedback with every quote. 

Thermal Properties of Selected Metals

MATERIAL

Thermal
Conductivity
(Btu/ (hr-ft
F)

Density (lbs/in)

Specific Heat (Btu/lb/F)

Melting Point (F)

Thermal Expansion (in/in/F x 10-6)

Aluminum 136 0.098 0.24 1,220 13.1
Brass 69.33 0.306 0.096 1,724 11.2
Copper 231 0.322 0.095 1,976 9.8
Mild steel 26.0-37.5 0.284 0.122 2,570 6.7
Stainless steel 8.09 0.286 0.120 2,550  9.6 
Titanium 12.65 0.164 0.13 3,035  4.7 

Source: Tubingchina.com

Within the quote analysis, you’re able to access an interactive three-dimensional image of your part design, which allows you to review your part geometry and assess any potential design issues such as wall thickness, narrow pockets, tolerances, threading, and holemaking.

Cost of Metals

Last up on our list of important factors to check before choosing your material is cost. It probably comes as no surprise to you, but if you have more property requirements or needs, then it’s likely you will be paying more for that material. Aluminum is an affordable machining material if you can trade off some of the other features we talked about above. If trade-offs are not an option, we recommend referring to your DFM to optimize your part design, saving you time and money regardless of your material selection.

There is a lot to unpack here, but we hope you feel a little more knowledgeable about the trade-offs between hard and soft metals when it’s time for you to start your next machining project. If you still have questions, we suggest reaching out to our highly experienced engineering team for input.

Amanda Tierney is the product marketing manager for CNC machining and sheet metal at Protolabs. She has 10-plus years of experience marketing to manufacturing engineers and designers across multiple industries. She holds a B.A. in business and marketing from Saint Michael’s College in Vermont.