The Effects of Machining on Chemical Finishing of Titanium EBM Parts

Belgian scientsits have taken on a unique study in evaluating AM parts, releasing their findings in ‘Influence of Conventional Machining on Chemical Finishing of Ti6A14V Electron Beam Melting Parts.’ Acknowledging the positive impacts additive manufacturing has made in a wide range of industries—from medicine to aerospace, construction, and so much more—the authors point out that there is still much to learn as challenges continue to arise in the face of ongoing innovation.

In this study, titanium alloy printing (Ti6Al4V) with electron beam melting (EBM) is the focus, along with challenges in production—mainly dealing with roughness that reduces necessary resistance to fatigue. Such defects can be common and may discourage users from branching out into using new technology.

Finishing is often required to ensure optimum performance, with numerous processes available for refining the surface further:

• Shot peening
• Tribofinishing
• Vibratory finishing
• Conventional machining
• Laser polishing
• Shape adaptive grinding
• Electrochemical treatments such as chemical etching, electrochemical polishing, and plasma electrolytic polishing

(a) Sample design and coordinate axis; (b) Resulting part geometry

Samples were printed and then evaluated, as the researchers compared the roughness of Ti6Al4V EBM parts that were chemically finished before and after robotic machining. An Arcam A2 machine was used for printing, and part and core surfaces were evaluated, using cylindricity and roughness measurements.

Witnesses of the experimental steps

Arithmetic Roughness [μm]

• Preliminary analysis
• First chemical etching
• Robotic milling
• Second chemical etching

Arithmetic roughness was minimally affected by the first chemical etching, showing a decrease of 2.5 percent.

“Roughness decreases dramatically after robotic milling and reaches results < 0.5 μm for some parts (reduction  of  97%),” stated the authors. “This last roughness result allows to foresee demanding application of the treated part cylinder according recommendation of Ra <  1.6 μm for contact surface.”

Ultimately, total roughness measurement conclusions were the same as RA in the initial chemical etching, showing a decrease of 6.5 percent, and 95 percent for robotic milling:

“These measurements have been only made on two parts. So, it would be interesting to pursue analysis and make a chemical etching with more parts robotically milled beforehand to assess this influence of chemical etching on robotic milling in terms of Rt,” stated the researchers.

Total Roughness [μm]

Complementary measurements of Ra [μm] and Rt [μm]

“Even if the parts were coming from the same region of the building plate, cylindricity of raw parts were heterogeneous (σ = 0.027 mm for an average cylindricity of 0.062 mm). The first chemical  etching  did  not  change  these  results.  However,  robotic milling degraded cylindricity of the part by 300% due to  the  process  itself.  Chemical  etching  does  not  degrade  cylindricity while robotic milling does,” concluded the researchers.

“This process is very promising since it allows to treat the part regardless of its geometry and without inducing  stresses on the part surface. However, it was  not possible to reach an arithmetic roughness as good as after robotic milling.  Moreover, after having applied a chemical etching on the core material of the  part, Rt increased and the shiny surface became dull. More analysis will be  required to assess if this aspect modification is linked to a metallurgical  change of the part surface material.”

Titanium has been the source of much research to date, from evaluation of scaffolds in bioprinting to testing medical devices, improving 3D-printed implants, and more. What do you think of this news? Let us know your thoughts! Join the discussion of this and other 3D printing topics at 3DPrintBoard.com.