1. Additive Manufacturing continues andvance
Additive Manufacturing (AM) is defined as “the process of joining materials to make objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies, such as traditional machining”. The first form of creating a three-dimensional object using computer-aided design (CAD) was rapid prototyping, developed in the 1980’s for creating models and prototype parts. Rapid prototyping is one of the earlier additive manufacturing processes. Compared to the traditional, subtractive manufacturing processes like milling, drilling and turning AM offers distinct possibilities. The subtractive techniques start with a block of the basic material, which is then subtracted from until the final desired product is reached, leaving much of the initial block as wasted scrap. In the AM process, the part is constructed by depositing material layer by layer in the Z-direction until the final product is produced, leaving little to no waste.
This offers a new freedom in the design of products. There are a variety of applications in the medical field that allows individual adaptation to the human body. Furthermore, there is an enormous interest in the automotive and aerospace sector, because it’s possible to create complex structures in a one-piece design and integrate functionalities or redesign products to save weight without sacrificing stability. In addition, in many cases this process is more economical than traditional methods, due to reduced tool and storage costs.
Now, with the desire to use this technology, the fist concerns arose. There are a lot of internal or regional standards and guidelines for casting and welding products. It is regulated which defects can occur and with which methods this is to be checked. But what about the AM products? The methods are diverse and differ greatly, seen in figure 1. Just as diverse are the types of defects that can occur such as pores, cracking, inclusions, delamination, lack of fusion, undercuts or trapped powder, just to name a few. With advances in the AM technology, there must be an increase in quality control of the AM parts, to ensure their structural integrity. The paper discusses the applicability of the non-destructive computed tomography technique to evaluate defects in AM parts and which analyses can be used especially for the selective laser melting (SLM) process for metals.
2. Types of defects in Metal AM parts
Common discontinuities observed in material fabricated through metals AM include lack of fusion (delamination) shrinkage porosity, gas entrapped porosity, cracking, thermal distortion, warpage and swelling. Lack of fusion discontinuities arise when a newly deposited layer of powder is not adequately heated and, in turn, melted. This prevents the fusion of the new layer with the underlying solid layer. Shrinkage porosity is a discontinuity that occurs when the liquid metal available during solidification is not able to compensate for shrinkage/density changes as the material undergoes the liquid to solid phase transformation. The identifiable characteristics of shrinkage porosity include an elongated void containing secondary dendrite arms within the void. In the powder-bed processes, entrapped gas porosity is spherical in shape. It is the result of trapped gas within the powder feedstock that cannot escape the melt pool because of the rapid solidification conditions that occur in metal AM Builds. In SLM process, part warpage occurs due to a buildup of residual stresses within the part. This causes distortion in the part geometry because the stress relaxes when the part is removed from the build plate. Cracking in metals fabricated through SLM processes may occur due to the sensitivity of the material to strain age cracking. This is attributed to the precipitation of secondary phases or solidification cracking at the high solidification rates observed in AM processes.
3. CT analysis of AM
With the rapid growth of AM technology, some evaluation criteria for processing techniques, processing parameter optimization, quality control and detection of possible discontinuities should be developed. Computed tomography, as non-destructive testing method, is one of the promising techniques for part inspection. The following sections show the most relevant analysis methods based on CT.
2.1. Prosity Analysis
With this tool the software detects irregularities because of a deviation of the grey values in one region compared with the local area. Pores with different sizes can be marked in different colours (Figure 3) and statistically analysed obtained to the diameter, volume, surface, frequency distribution or probability.
Figure 3: Left: Cross sectional plan without and with marked pores, right: A transparent 3D volume of the AM part with colored pores.
2.2. Dimensional Measuring
Different measurement tools allow a verification of distances and angles. This is very useful, if it’s necessary to check values on inboard structures.