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01 May 2018 11:09
5 Common Problems Faced with Metal 3D printing – and How You can Fix Them
Metal 3D printing has made impressive strides in the last few years, with companies increasingly investing in the technology for highly complex, industrial applications. However, alongside the advantages of producing lightweight, sophisticated metal components, there are also a number of challenges that need to be overcome during the metal 3D printing process. Today’s tutorial will explore the main problems faced when 3D printing metals, and how you can solve them.
Metal 3D Printing – An Overview
When it comes to metal 3D printing, there are a range of printing processes. These can broadly be divided into three groups:
- Powder bed fusion processes (SLM, EBM)
- Direct energy deposition (DED)
- Metal binder jetting
Powder bed fusion is the most common method for the production of metal parts using AM, and involves the use of a laser beam (SLM) or electron beam (EBM) to selectively melt a layer of powder material, evenly distributed on the build platform.
Direct energy deposition covers a range of technologies, and typically involves a process by which the material is melted by a laser or electron beam before being deposited onto a build platform. The object is then formed layer by layer. While polymers and ceramics can be used with this process, DED is typically used with metals in either powder or wire form.
Metal binder jetting uses a print head to apply a liquid binding agent onto layers of powder, which fuses the powder particles together layer bylayer. The bound powder can be then optionally infiltrated with another metal (usually bronze) to achieve a higher density.
Each of the processes has its strengths and limitations, but there are common issues that occur in general when 3D printing metals — and these challenges must be in order to achieve best possible mechanical characteristics for your metal 3D printed parts.
5 Common Problems to Watch Out For
3D printed metal parts are often plagued with high porosity, which occurs during the printing process as small holes and cavities are formed within the part. These tiny, usually microscopic, pores can cause low density — the more pores there are, the lower the density of your part. They can also directly affect the mechanical properties of a part, making it prone to cracks or other damage, particularly when exposed to high loads.
There are typically two main reasons for highly porous 3D printed metal parts: either it is due to a problem with the powder production technique or due to the 3D printing process itself. For example, using gas atomisation can sometimes cause pores to form in the powder material. However, the more common source of such tiny holes is the printing process, when the energy of the is insufficient and therefore cannot melt the metal properly. The opposite can also apply: excessive laser energy can cause the droplets of melted material to splatter, resulting in pores.
How to reduce the porosity of your metal parts
Fortunately, there are a number of ways to eliminate porosity in your 3D printed metal parts and achieve stronger, more durable parts:
- Since material quality can at times be the source of high porosity, make sure to buy raw materials from a trusted supplier.
- Porosity caused during the printing process can be eliminated by tuning your printer’s parameters.
- The correct density can be achieved with post-processing methods, such as hot isostatic pressing. This eliminates any possible cavities while improving the mechanical properties of a 3D printed metal part.
- For powder bed fusion parts, infiltration is another post-processing option. This method is used to fill the remaining voids in the metal part.
Industrial applications of metal 3D printed parts frequently require high mechanical properties, which is why the density of a part is extremely important. When a part operates under cyclic stress conditions, its density will determine whether or not the part will fail under load. In other words, the lower the density of a part, the more likely it is to crack under pressure. Powder-bed technologies (SLM, EBM) can produce parts with densities of 98% and higher, which are crucial for stressful applications.
Improving the density of your parts
To ensure a part has consistent quality and density, it is necessary to optimise the material’s specific parameters, such as particle size, shape, distribution and flowability. Particles with spherical shape can lead to a higher density, for example, as they can achieve the maximum relative density compared to other shapes.
However, as there are a range of variables that can affect a part’s density, the general rule of thumb is to first consider the quality of your metal powder, and adjust the parameters of the process accordingly.
3. Residual stress
Heating and subsequent cooling are the common features of the metal AM processes. However, when a part is subjected to such extreme thermal changes, this can lead to residual stress. Residual stress has an unfavourable impact on the integrity of a manufactured part, resulting in different forms of deformation. The highest concentration of residual stress is found at the contact area between the bottom of a printed part and a print bed.
Reducing residual stress
As residual stress can make the difference between a successful metal print and structural failure, this issue should be properly addressed, and there are a number of ways to do so:
- Predictive modelling can be used to estimate the appropriate parameters such as heat input and layer thickness in order to build components with low residual stress.
- Implementing support structures and optimising part orientation can also minimise the occurrence of residual stress.
- Preheating the print bed and build material before the printing begins reduces temperature gradients, which are often the cause the residual stress. However, since EBM operates at a lower temperature, this technique is more successful with EBM than with SLM or DED.
- In powder bed fusion processes, the “island” scanning strategy can help to mitigate the build-up of residual stresses. This strategy works by dividing the exposure area into smaller sections, termed “islands”, and keeps the lengths of scan vectors shorter.
4. Cracking and Warping
Residual stress can be highly destructive, resulting in a number of structural issues in a part, with cracking and warping being the most frequent among them. Such problems typically occur when the melted metal cools down after printing. Cooling causes contraction, which makes edges of a part curl up and deform. In extreme cases, stress can exceed the strength of the part, leading to the part cracking (cracking may also occur if the powder material wasn’t properly melted).
Preventing cracking and warping
There are two main ways to prevent the cracking and warping of your metal part. One option is to preheat the print bed, while another is to improve the adhesion of a part to print bed and placing the necessary amount of support structures. Thermal post processing can also help to repair minor cracks, while establishing the correct number of support structures on your part is essentially to prevent warpage.
5. Post processing and surface roughness
Typically, metal parts are not ready for their final applications when they are first printed, and will need to undergo some form of post processing, such as powder and supports removal, thermal treatment and surface finish. But very often, you will come across some challenges during post processing steps.
For example, you may face challenges removing the support structures on your parts. This can occur, for instance, if your metal part has supports in small holes and tubes. These can be difficult to remove without damaging the part and subsequent machining will be needed.
Surface roughness is another issue. Additively manufactured components for high end applications require an average surface roughness — but 3D printed parts are often produced with rough surfaces and require additional post-processing such as machining, grinding or polishing to achieve better finish. As surface roughness is directly related to layer thickness, it can be mitigated by printing with thinner layers. However, this producing a part using finer layers can significantly increase build time.
Rough surfaces can also result from improper powder melting. This occurs when not enough energy has been applied to melt the metal completely. In this case, surface roughness can be reduced by increasing the power of your laser.
To sum up
While there are a range of potential challenges when using AM to produce metal parts, understanding these challenges is the first step in producing high quality, reliable components. And with the continued growth of metal 3D printing, we’ll certainly see a rise in the use of additively manufactured metal components used in industrial applications.