The effects of machine parameters on the integrity of WC-Ni coatings deposited onto titanium parts with laser additive manufacturing

Van Coller, Marius Cornelius (2019-04)

Thesis (MEng)--Stellenbosch University, 2019.

Thesis

ENGLISH ABSTRACT: Part quality specifications are of the utmost importance within industries that include specialized applications, for instance the medical, aerospace, nautical and mining industries. Attributable to the high costs of specialized parts and the risks involved in operation, such as the loss of human lives and company integrity. Parts are expected to showcase consistent properties throughout their life cycle as catastrophic consequences may ensue if they do not align with industry specifications. The goal of today’s part manufacturers is to extend part lifecycles and mitigate unwanted occurrences like crack formation, deformation or creep in the material. To maintain the highest achievable production rate and most efficient use of resources, consistent material homogeneity of components within stark operating conditions is a crucial pursuit. Titanium and its alloys have become popular contributors within these industries due to their advantageous material properties, e.g. biocompatibility, corrosion resistance and high strength. This said, titanium has poor tribological properties, which restricts its range of applications in environments where friction and wear is prevalent. Titanium’s high temperature wear and oxidation resistance is poor and to solve these inefficiencies, surface treatments have been applied to titanium and its alloys. These modifications largely incorporate the use of hardened coatings, like carbide composites, and employ laser beam technologies for application because of its high energy density, coherence and good directionality. Little research exists on the application of laser additive manufacturing technologies for production of carbide coated parts. In the pursuit to improve the current supply chain capabilities for carbide strengthening and refurbishment, this study investigated the feasibility of depositing tungsten carbide onto titanium parts using laser melting technology. Nickel was employed for the binder material in the cermet as an alternative to cobalt, which is conventionally used for additive manufacturing of hardened carbides. The research study was aimed at designing an experiment to determine which machine parameters result in the optimal adhesion characteristics between titanium alloy Ti-6Al-4V and tungsten carbide in a 10% nickel binder (WC-Ni). The machine parameters evaluated were the laser power, scanning speed and hatch spacing. The experimentation consisted of both a single-track screening experiment and single layer depositions. From the screening, suitable hatch spacings and a process window for scanning speed and laser power was deduced. These results were incorporated into further experimentation that evaluated layered depositions by varying the three parameters under study. The parameter combinations were tailored to maintain a consistent volumetric energy density range between 28.85 J/mm3 and 88.33 J/mm3 to test the individual effects of input parameters at a constant energy input. The evaluated responses were: 1) the surface quality of coatings, 2) penetration and diffusion of the deposition into the substrate, and 3) the consistency of coating material at the substrate interface. Optimal quality coatings yielded between 20wt.% and 40wt.% tungsten carbide at layer surfaces, while maintaining a layer thickness between 30μm and 60μm; diffusion depths of up to 50μm were also achieved. Higher energy densities resulted in deeper penetration and good diffusion of the coating into the substrate but lacked sufficient presence of the coating material at the substrate surface. It was concluded that higher laser powers and high hatch spacings combined with intermediate scan speeds produced the most desirable coatings. This corresponded to depositions in the energy density range of 28 J/mm3 and 58 J/mm3. Furthermore, the validity of the volumetric energy density equation as a predictive metric in laser additive manufacturing was addressed, as significant differences in layer quality were achievable by varying parameter values at identical energy densities.

AFRIKAANSE OPSOMMING: Die kwaliteit spesifikasies van onderdele is van uiterste belang in nywerhede wat hoogs gespesialiseerde toepassings insluit, byvoorbeeld die mediese, lugvaart-, seevaart- en mynbedrywe. Laasgenoemde word toegeskryf aan die hoë koste van die gespesialiseerde onderdele en die risiko's van operasie, soos die verlies van menselewens en maatskappy integriteit, sou die onderdele nie die mas op kom nie. Daar word van die onderdele verwag om konsekwente eienskappe gedurende hul lewensiklus te vertoon, aangesien dit katastrofiese gevolge kan meebring indien hulle nie in lyn is met bedryf spesifikasies nie. Die doelwitte van vandag se onderdeel vervaardigers is, onder andere, om ongewenste gebeurtenisse soos kraak vorming, vervorming of die kruip in die materiaal te versag, om die hoogste haalbare produksie snelheid en die mees doeltreffendste gebruik van hulpbronne te handhaaf. Die konsekwente materiële homogeniteit van die vervaardigde komponente/onderdele in moeilike bedryfstoestande is 'n belangrike strewe in die hedendaagse tyd. Titaan en sy legerings het gewilde bydraers geword in hierdie nywerhede as gevolg van titaan se voordelige eienskappe, bv. Bio-verenigbaarheid, korrosie weerstand en hoë sterkte. Dit gesê, moet mens titaan het swak tri-bologiese eienskappe ook in ag neem, welke eienskappe sy toepasbaarheid beperk in omgewings waar wrywing en slytasie algemeen voorkom. Titaan se slytasie in hoë temperature en oksidasie weerstand is baie swak en om hierdie tekortkoming op te los, word oppervlak behandeling op titaan en sy legerings toegepas. Hierdie oppervlak behandelinge van titaan en sy legerings bestaan grotendeels uit die gebruik van verharde bedekkings, soos metaal komposiete, en gebruik word dikwels gemaak van laserstraal tegnologie vir die toepassing daarvan as gevolg van die hoë energie digtheid en kleefbaarheid. Daar is min navorsing oor die toepassing van laser toevoegings produksie tegnologie by die vervaardiging van metaal-bedekte onderdele. In die strewe om die huidige verskaffings ketting vermoëns vir die versterking en opknapping van metaal te verbeter, het hierdie studie ondersoek ingestel na die uitvoerbaarheid van die toevoeging van wolfram karbied op titaan-dele met behulp van lasersmelt tegnologie. Nikkel was gebruik as die bindings materiaal in die struktuur as 'n alternatief vir kobalt, wat normaalweg gebruik word vir toevoegings vervaardiging van verharde karbied. Die studie is hoofsaaklik daarop gemik om 'n eksperiment te ontwerp wat bepaal watter masjien parameters die optimale kleefbaarheid eienskappe tussen titaan legering Ti-6Al-4V en wolfram karbied sou bied in ‘n 10% nikkel binder (WC-Ni). Die masjien parameters wat geëvalueer was is die laser krag, skandeer spoed en luik spasiëring. Die eksperimente het bestaan uit beide 'n enkelspoor siftings-eksperiment en enkellaag stortings. Uit die sifting is geskikte luik spasies en 'n proses vensterd vir skandeer spoed en laser krag afgelei. Hierdie resultate is geïnkorporeer in verdere eksperimente wat lae steekproef afsettings geëvalueer het deur die drie parameters onder die studie te verander. Die parameter kombinasies is aangepas om 'n konstante volumetriese energie digtheid tussen 28,85 J / mm3 en 88,33J / mm3 te handhaaf om die individuele effekte van inset parameters te toets teen 'n konstante energie-inset. Die geëvalueerde antwoorde was: 1) die oppervlak kwaliteit van lae, 2) penetrasie en diffusie van die afsetting in die substraat, en 3) die konsekwentheid van die laagmateriaal by die substraat-koppelvlak. Optimale kwaliteit van lae het tussen 20wt. En 40wt.% Wolfram karbied by laag oppervlaktes gelewer, terwyl 'n laagdikte van tussen 30μm en 60μm behou word; diffusie dieptes van tot 50μm is ook behaal. Hoër energie digthede het dunner penetrasie en goeie diffusie van die laag in die basis tot gevolg gehad, maar het nie genoegsame teenwoordigheid van die laagmateriaal op die basis materiaal gehad nie. Daar is bevind dat hoër laserkragte en hoë luik spasiëring gekombineer met intermediêre skandeer snelhede die mees wenslike oppervlakbedekking geproduseer het. Dit stem ooreen met afsettings in die energie digtheid berekening. Verder is die geldigheid van die volumetriese energie-digtheid vergelyking as 'n voorspelling metriek in die vervaardiging van laseradditiewe aangespreek, aangesien beduidende verskille in laagkwaliteit bereikbaar is deur verskillende parameterwaardes by identiese energiedigtheid.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/105941
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