What problems will metal 3D printed parts have if post-processing is not done?

1. Surface flaws: a chain reaction from "roughness" to "functional failure."
Metal 3D printed objects usually have bad surface quality at first, with big problems including layering, burrs, and porosity. For example, the SLM (Selective Laser Melting) process stacks layers on top of each other to make parts. This creates a big "step effect" on the surface of the item, making it rougher than standard machining, which has a roughness (Ra value) of 10-20 microns, which is much higher than the 0.8-3.2 microns of traditional machining. This rough surface not only changes how things look, but it also causes a number of functional problems:

Stress concentration and fracture initiation: Surface flaws can create locations of stress concentration, which speeds up the spread of cracks when the material is loaded or heated. For example, before sandblasting, the fatigue life of a given aviation engine turbine blade is only 30% of what it should be. After sandblasting, the fatigue life goes up to over 90%.
Less resistance to corrosion: The rough surface makes it easier for corrosive materials to get through. For example, unpolished 316L stainless steel parts showed signs of corrosion within 24 hours of salt spray testing. However, after electrolytic polishing, the parts were able to withstand corrosion for more than 500 hours.
Higher friction coefficient: When two surfaces are in touch and sliding against each other, surface roughness has a direct effect on how well friction works. When the gearbox shaft parts of a certain car weren't machined with ultra precision, the friction coefficient went up to 0.15, which meant that energy use went up by 12%. After ultra precision machining, the friction coefficient went down to 0.03, and energy use went down to the design value.
2, 2, Internal Defects: From "Hidden Killer" to "Catastrophic Failure," a Hidden Crisis
During the metal 3D printing process, thermal stress and the lack of fusion between the powder and the metal can cause problems like internal porosity and cracks. If these flaws aren't fixed during post-processing, they will make the parts very unreliable.

Too much porosity: A study found that Ti-6Al-4V parts that haven't been hot isostatic pressed (HIP) can have a porosity of 0.5% to 1%. After HIP treatment, the porosity can be cut down to less than 0.01%. High porosity can make a part less dense, which makes it more likely to break when it is under dynamic loading.
Residual stress is out of control: During printing, the part can build up residual stress if it heats up and cools down quickly. In one situation of making moulds, parts that weren't stress-relieved before usage warped and changed shape, which meant the mould had to be thrown away. After annealing, the parts' dimensional stability improved by 90%.
Organisation that isn't even: The powder bed melting process might cause the size of the grains to vary a lot in different parts of the part. The local grain size reached 100 microns when a certain part of an airplane's structure was not treated with a solution. But following solution treatment, the grain size became even, between 20 and 30 microns, and the fatigue resistance went up by three times.
3. Performance degradation: the difference in performance between "design compliance" and "actual failure"
Even if the parts' geometric dimensions fulfil the design criteria, their mechanical performance may nevertheless be well below what was expected if post-processing is not done:

A study looked at untreated and heat-treated 316L stainless steel parts. It found that the tensile strength of untreated parts was at the design value, but the elongation was only 60% of the design value. After heat treatment, the elongation went back to the design value.
Hardness is not evenly spread out: Parts made with the direct energy deposition (DED) technique commonly have hardness gradients. The surface hardness of a certain mould insert was only 35HRC before nitriding treatment, but it went up to 58HRC after the treatment and the wear resistance went up by 5 times.
Not enough thermal stability: A high-temperature alloy part that wasn't aged lost 20% of its hardness after working at 650 °C for 100 hours. But following the ageing treatment, the hardness retention percentage went up to above 95%.
4. Economic cost: costs that are out of control, from "partial rework" to "complete shutdown."
Not doing post-processing not only makes parts work worse, but it also has a chain reaction on the economy:

The cost of rework has gone up a lot. One car parts maker threw away a whole batch of parts because they didn't do any post-processing, and rework expenses made up 35% of the overall order value. But if non-destructive testing and repair are done right after printing, the cost can be kept to within 5%.
Longer production cycle: Because a certain aviation structural part didn't get the stress reduction treatment it needed, it got bent during assembly. This caused the whole manufacturing line to shut down for two weeks, costing the company $2 million.
The brand's reputation has been hurt: A medical implant maker's failure to polish the surface of their products led to a 15% rise in the number of recalls, a 25% loss of customers, and an immeasurable loss of brand value.