Page - 68 - in 3D Printing of Metals

Metals 2017,7, 91 Figure 17. Relationbetweenheight andcompressive strength for SLMparts (HIP) andEBMparts (NoHIP) in0◦ and45 ◦orientation incomparisontoSLMparts (NoHIP) in0◦ and45◦orientation A linear relationship was found between equivalent diameter and compression strength. EBM parts showed a high correlation in both orientations (0◦—R2 = 0.976; 45◦—R2 = 0.956). SLMpartsalsoshowedahighcorrelation in the testedorientation(0◦—R2 =0.922;45◦—R2 =0.982). The correlation of the SLMpartswithout heat treatmentwaswithin the range of all other parts (0◦—R2 =0.957;45◦—R2 =0.987). ThedifferencesbetweenHIPedandNoHIPedparts that affect themechanicalproperties are well-known[40,46,51]. EspeciallyHIPingleadstodecreasedstrength. Thereasonis the transformation ofmartensite intoαandβphases. Themarginally lowerstrength ischaracteristic for theα+βphase. Thiscorrelationapplies to theSLMparts. ThedifferencebetweenHIPedandNoHIPedpartsdwindles awaywith increasingheightuntil it isgone. Although the EBM samples achieved higher values in the hardening measurement, the compressive strength values are below the values for SLMsamples. Here lies the reason for thesignificantlyhigherroughnessRZandRaof theEBMparts.Kasperovichetal. [46]describedthat therough“as-built” surfaceneeds tobeconsideredascrack initiatorandthat its lowerstrengthhas to be taken intoaccount. This leads to lowercompressionstrengthofEBMpartscomparedtoSLMparts withoutheat treatment. The influenceof thegeometricconditionscanbedescribedverywellby indicating thedegreeof slendernessλ (seeEquation(3)). Thecorrelationbetweenthecompressivestrengthandtheslenderness ratioalsorevealed lineardependencieswithhighcoefficientsofdeterminationofover90%asshown inFigures18and19. Thecoefficientswere loweronly in twocases (45◦orientedpartswithvariabled). Figure 18. Correlationbetween compressive strength and slenderness ratio of SLM-manufactured parts. Resultsareshownasmeanvalueswith thecorrespondingstandarddeviations. For the linear regressions, thecoefficientofdetermination isshown. 68