Additive manufacturing (AM) offers numerous advantages when producing high-performance parts, enabling complex geometries and low-volume manufacturing. However, accurately assessing the mechanical properties of these parts poses a distinct challenge. Traditional methods rely on uniaxial testing of separate witness coupons, assuming they are representative of the whole component while overlooking the fact that AM parts often have inconsistencies in the material properties throughout the part.
Challenges with Witness Coupons
Witness coupons provide a limited view of the mechanical performance of an AM part, as they only represent the gauge section and specific locations within the build. Moreover, these coupons are often produced and tested separately, introducing geometric and thickness discrepancies that may not align with the final part. Consequently, the mechanical performance of a part can differ from the expectations based on coupon testing which in high performance industries such as aerospace and motorsports can result in the catastrophic failure of these parts.
The Role of PIP Testing
The objective of our latest case study, done in collaboration with Alloyed, was to compare the mechanical properties of a metal 3D-printed part with those obtained from a witness coupon using PIP testing. Unlike traditional methods, PIP testing allows for the assessment of un-machined samples, eliminating the need for separate coupons and offering insights directly from the part itself. By characterising the property changes seen throughout an AM part, PIP testing provides users with a comprehensive understanding of the relationship between process, geometry, and materials.
Methodology and Results
The case study focused on three distinct regions of an AlSi10Mg metal 3D-printed part, each with varying printing conditions. The mechanical properties were measured using our Indentation Plastometer, which employs an accelerated inverse finite element method to infer accurate stress-strain curves from indentation test data. The results of these tests were then compared with those obtained from traditional uniaxial tensile testing.
The study revealed significant variations in mechanical properties across different locations of the part, with differences of almost 20% observed. Testing in Region 2, which experienced aggressive downskinning, showed a substantial reduction in ultimate tensile strength (UTS) compared to Regions 1 and 3. The PIP testing results demonstrated that different regions of the part exhibit distinct mechanical behaviours, highlighting the need for multi-directional testing.
Comparison with Witness Coupons
By plotting the results from the testing of the part alongside the testing of the witness coupon, it became evident that the mechanical behaviour in different regions of the part significantly differed from that of the coupon. While the witness coupon closely matched the testing in Region 3, discrepancies were observed in other regions. This emphasised the importance of testing directly on printed parts to gain a holistic understanding of their mechanical properties. Without these insights, companies are often left in the dark.
Conclusion
The reliance on witness coupons for assessing the mechanical properties of additively manufactured parts can lead to misleading results and potential failures in critical industries such as automotive, defence, and aerospace. The PIP testing method offers several advantages, including speed, cost-effectiveness, and accuracy in representing the mechanical properties of the printed part. This enables more informed design strategies and increases efficiency and cost savings across the additive manufacturing process. Ultimately, it provides users with parts they can rely on to perform as intended.
To read the full case study, click here.
