Anja Buchwalder

EB Surface Alloying and Plasma Nitriding of Different Al Alloys

Within the last years, considerable progress was achieved in the research field of plasma nitriding of Al alloys. However, due to large property differences between the very hard AlN layer and the soft Al matrix material the load capacity of the nitride layer is limited. Electron beam (EB) surface alloying modifies the chemical composition of the area near the surface up to a certain depth. This, for instance, results in high hardness levels, and therefore this layer acts as support for the hard and wear-resistant thin AlN layer generated by plasma nitriding. In the present study, surface modifications produced by a combination of EB alloying with Fe based additives and plasma nitriding of wrought, cast and spray-formed Al alloys were investigated. After the EB treatment the layers were examined regarding their influence on the structure, the nitride layer growth mechanism, the effect of the EB layer for the support of the AlN layer and the resulting duplex layer properties, e.g. hardness and wear behaviour.

Neue Entwicklungen auf dem Gebiet der thermischen Elektronenstrahl-Randschichtbehandlung von Aluminium-Legierungen

Kurzfassung Der Elektronenstrahl (EB) ist ein ausgezeichnet geeignetes Werkzeug für die Flüssigphasen-Randschichtbehandlung von Al-Werkstoffen. Unter Nutzung einer hochfrequenten 3-D-Strahlablenkung lassen sich riss- und (nahezu) porenfreie Randschichten mit Umwandlungstiefen bis zu 8 mm mit neuartigen Struktur-/Gefüge-/Eigenschaftskomplexen einstellen. Am Beispiel der Al-Gusslegierung AlSi10Mg und den sprühkompaktierten Al-Werkstoffen AlSi35 und AlSi20CuFe wurden die Prozesse EB-Umschmelzen (EBU) und -umschmelzlegieren (EBUL) untersucht. Für das EBUL kamen vordeponierte Ni-, Cu- und Co-Basis-Zusatzstoffe zum Einsatz. Es wird gezeigt, dass sich bei Anwendung der EB-Flüssigphasen-Mehrspot-Technologien (Mehrbahn, Mäander) bzw. einer erstmals realisierten EB-Mehrprozess-Technik (Reinigen/Verdichten – Umschmelzlegieren – Homogenisieren/Glätten) maximale Härtesteigerungen bis auf das 6-fache, Erhöhungen der Ritzenergiedichte auf das 3-…5-fache und Verbesserungen der Reibungs- und Verschleißkennwerte, insbesondere der Verschleißrate k, um einen Faktor von max. 10…(25) erzielen lassen.

Plasmanitrieren von sprühkompaktierten Al-Legierungen*

Kurzfassung Auf dem Gebiet des Plasmanitrierens von Al-Knet-bzw. Gusslegierungen wurden in den letzten Jahren beachtliche Fortschritte erzielt. Das Nitrierverhalten wird in entscheidendem Maße von den Legierungselementen bestimmt. Für Al-Knet-bzw. Gusslegierungen ist bekannt, dass Mg-Anteile das Nitridschichtwachstum befördern, wogegen es durch höhere Si-Konzentrationen gehemmt wird. Sprühkompaktierte Al-Si-Legierungen weisen, bedingt durch den speziellen Herstellungsprozess, typischerweise hohe Si-Gehalte (15…35%) zur Verbesserung der Verschleißbeständigkeit sowie für eine gute Warmfestigkeit hohe Anteile an Fe, Ni, Co u. a. auf. Erstmalig durchgeführte Untersuchungen zum Plasmanitrieren derartiger Werkstoffe ergaben in Abhängigkeit von den Nitrierparametern rissfreie AlN-Schichten mit einer Dicke von bis zu 6 μm. Diese ausgezeichnete Nitrierbarkeit, trotz hoher Si-Gehalte, ist offensichtlich auf die werkstofftypischen ausschließlich primär erstarrten, fein verteilten, kleinen (2…5 μm) Si-Ausscheidungen zurückzuführen. Es wurden verschiedene Legierungszusammensetzungen hinsichtlich ihres Einflusses auf die Schichtausbildung und -eigenschaften (Härte, Verschleiß) untersucht. Durch den sprunghaften Wechsel der Eigenschaften von sehr hart (AlN-Schicht) zu weich (Al-Grundwerkstoff) ist die Tragfähigkeit der Nitridschicht im Belastungsfall eingeschränkt. Aus diesem Grund wurden die Möglichkeiten und Grenzen einer Nachbehandlung (Aushärtung) untersucht.

The influence of dilution on dissimilar weld joints with high-alloy TRIP/TWIP steels

In dissimilar welded joints without filler materials, the dilution is one of the most important factor for the weldability. Due to other factors, the high quality of the welding seam and a good reproducibility of welding parameters the electron beam can be advantageously used to join challenging materials. Several dissimilar welded joints were successfully produced as a result of an optimised variation of the beam offset and the beam current to achieve suitable levels of dilution. For microstructural evaluation, the welded joints were examined by light optical and scanning electron microscopy methods, such as Electron Backscatter Diffraction (EBSD) and Wavelength-dispersive X-ray spectroscopy (WDS). To characterise the mechanical properties and the deformation behaviour of the welded material, microhardness measurements and tensile tests were carried out. For the welds of a high-alloy TRIP steel and the steel DP590, an optimised dilution lead to flawless joints with a tensile strength superior to the TRIP steel. Furthermore, welds of a zirconia-reinforced TWIP steel with an as-cast TRIP steel free of pores and cracks could be produced.

Eine neue kombinierte Randschichttechnologie für hochbeanspruchte Gusseisenwerkstoffe

Kurzfassung Legierungs- und gießtechnische Neu- und Weiterentwicklungen von Gusseisen haben zur Erweiterung der Einsatzgebiete dieser Werkstoffe für Bauteile und Werkzeuge auch unter dem Aspekt des Leichtbaus geführt. Zur zusätzlichen lokalen Verbesserung der tribologischen und korrosiven Beanspruchbarkeit wurde eine Technologie zur kombinierten Randschichtbehandlung entwickelt. Dabei wird das Gusseisen lokal mittels Elektronenstrahl umgeschmolzen, sodass eine harte (650 HV 0,3), graphitfreie ledeburitische Schicht entsteht. Diese thermisch gut beständige Schicht (bis ca. 1100 °C) dient als Stützschicht für eine nachfolgend durch Plasmanitrieren erzeugte relativ dünne (< 10 μm), harte Verbindungsschicht. Es werden sowohl die werkstoffspezifischen Einflussfaktoren, wie Graphitmorphologie, chemische Zusammensetzung etc., als auch die Auswirkungen der Prozessparameter auf das Ergebnis der Einzel- und Kombinationsbehandlungen erörtert. Vergleichende Untersuchungen zum Verschleißverhalten (Kugel-Scheibe) haben gezeigt, dass insbesondere bei höheren Lasten (> 100 N) der Mehrwert der Kombinationsbehandlung gegenüber den Einzelbehandlungen voll zum Tragen kommt. Die deutliche Verbesserung des Korrosionsverhaltens nach der Kombinationsbehandlung resultiert aus der defektfreien Ausbildung der Nitrierschicht auf der graphitfreien Umschmelzschicht.

Electron Beam Surface Engineering of Spray-Formed Aluminium-Silicon Alloys

Aluminium alloys are commonly used lightweight construction materials. Spray-formed alloys, in particular, represent a group of materials with very high Si contents and a homogeneous distribution of primary Si and other alloying elements in solid solution and intermetallic compounds. The paper deals with current results of EB surface alloying and dispersing of such alloys using a high frequency beam deflection technique. The results concerning the interactions between the EB and the material and its effects on the layer microstructure, characteristic layer properties as well as detailed researches into friction and wear behaviour and future prospects for the technological transferability to industrial applications will be discussed.

PVD Hard Coatings for High Loaded Al Alloys – Potentials and Limitations

PVD hard coating is a well-known surface treatment technology for steels to improve wear resistance and, to some extent, corrosion resistance. In principle, hard coating can be carried out for Al alloys, but due to the natural oxide layer and the insufficient load-bearing capacity of the soft base material, the application of this technology for wear protection of components is not regarded as being particularly promising. The research activities described in this paper focused on electron beam (EB) surface alloying with a Co-based additive, and the influence of two different hardness levels (270HV0.1 and 390HV0.1) on the improvement of the local load-bearing capacity of Al alloys with thin PVD hard coatings. A further focus of this research was on the material-specific aspects of the coating deposition. Compared to steels, the hard coated surface of Al alloys is rougher and the measured adhesion of the coating is significantly lower. For this purpose, different technological PVD parameters (e.g. Ti interlayer, deposition temperature, and time) were adapted to optimize the coating properties – especially adhesion. The paper deals with comparative studies of single (PVD hard coating of Al base material) and duplex treatment (EB alloying of Al base material and subsequent PVD hard coating) by means of improvement of the coating and compound hardness, friction and wear behavior (pin-on-disc test), as well as the corrosion resistance (potentiodynamic measurements in 0.05M H2SO4). While the level of improvement in wear resistance as a result of the duplex treatments strongly depended on the adhesion of the thin coatings, the corrosion behavior was strongly influenced by the PVD deposition process and coating thickness.

Influence of Microstructure on the Corrosion Resistance of AZ91D after EB Surface Alloying

This paper focuses on liquid phase surface treatment of the Mg alloy AZ91D by electron beam alloying (EBA) using Al-based additives to improve, primarily, the materials corrosion resistance, as well as its hardness. By variation of EB energy input, layer depths between 0.7 and 2.5 mm were generated. As a result, layers with different Al contents were produced, and were categorized into three different types of microstructure. A correlation could be observed between these three types of microstructure and hardness, as well as between microstructure and corrosion behaviour. Hardness was improved by approximately 2 to 4 times from 67 HV0.1 (base material) up to 140-250 HV0.1 (EBA layer). For high Al contents, the corrosion potential could be increased by about 400 mV compared to the base material. Moreover, it is shown that, after a homogenization process, a corrosion potential of 450 mV above the corrosion potential of the base material was achieved even for low Al contents.

Combined Surface Treatment of Electron Beam Alloying and PVD Hard Coating for Al Alloys

Due to their typically high hardness, excellent resistance against wear, and their low coefficient of friction, Physical Vapor Deposition (PVD) hard coatings are used on steels for a wide range of tools and components. Currently, however, the potential for wear protection of Al alloy components cannot be exploited. The thin PVD layers tend to collapse and disintegrate due to plastic deformation of the soft base material. Present research is focused on electron beam (EB) surface alloying, using Co-based additives to increase the surface hardness of the Al base material, producing an improved supporting effect for PVD coatings. The influence of different beam deflection techniques and EB parameters on the microstructure and hardness of alloyed layers was investigated. The properties of the duplex composite layers produced are strongly dependent on the thermal stability of the EB alloyed layers (type and amount of intermetallic compounds, coarsening effects) which are affected by the temperature-time cycle of the PVD process. This will be discussed by means of SEM and EDX investigations in correlation with XRD analysis. Measurements using scratch test with increasing load result in critical load values for the combined treatment that are 3 to 5 times higher when compared to only PVD-coated base material.

Joining of TWIP-matrix composites by electron beam brazing

Zirconia reinforced, high alloy TWIP steels offer unique mechanical properties, due to the combination of stress- and deformation-induced phase transformations within the matrix and particles. However, these composites are considered as non-weldable, due to the strong evaporation of zirconia during fusion welding. In this work, an alternative approach for the joining of zirconia reinforced TWIP steels by electron beam brazing is presented. The electron beam was chosen as heat source due to its outstanding reproducibility of parameters, the independence of energy input from the materials surface, the inert vacuum atmosphere, and the flexibility of the energy transfer function. For the experiments, a stainless steel and a Fe-Cr-Mn-Ni-based TWIP steel reinforced with zirconia were brazed as butt joints with a Ni-based filler. The samples were characterized by light optical microscopy and EBSD measurements to evaluate the microstructure. Furthermore, Martens hardness and tensile tests were performed to characterize the mechanical properties of both, the fusion zone and the joints. It was shown that TWIP matrix composites with zirconia particles can be successfully brazed with Ni filler. With an optimized energy transfer function, the distortion of the butt joints can be minimized. After brazing with Ni-based filler, the brazing zone contains ≈ 35 to 50% brittle phases. During tensile tests, rupture took place within the brazing zone alongside the brittle phases. Nevertheless, the samples exhibited a maximum tensile strength of 358 MPa.