Galling Mechanisms Research Articles

Mechanisms of elevated temperature galling in hardfacings

The galling mechanism of Tristelle 5183, an Fe-based hardfacing alloy, was investigated at elevated temperature. The test was performed using a bespoke galling rig. Adhesive transfer and galling were found to occur, as a result of shear at the adhesion boundary and the activation of an internal shear plane within one of the tribosurfaces. During deformation, carbides were observed to have fractured, as a result of the shear train they were exposed to and their lack of ductility. In the case of niobium carbides, their fracture resulted in the formation of voids, which were found to coalesce and led to cracking and adhesive transfer. A tribologically affected zone (TAZ) was found to form, which contained nanocrystalline austenite, as a result of the shear exerted within 30μm of the adhesion boundaries. The galling of Tristelle 5183 initiated from the formation of an adhesive boundary, followed by sub-surface shear in only one tribosurface, Following further sub-surface shear, an internal shear plane is activated. internal shear and shear at the adhesion boundary continues until fracture occur, resulting in adhesive transfer.

Adhesive transfer operates during galling

In order to reduce cobalt within the primary circuit of pressurised water reactors (PWR’s), wear-resistant steels are being researched and developed. In particular interest is the understanding of galling mechanisms, an adhesive wear mechanism which is particularly prevalent in PWR valves. Here we show that large shear stresses and adhesive transfer occur during galling by exploiting the 2 wt.% manganese difference between 304L and 316L stainless steels, even at relatively low compressive stresses of 50MPa. Through these findings, the galling mechanisms of stainless steels can be better understood, which may help with the development of galling resistant stainless steels.

A Predictive Model for Galling Phenomenon and Its Applicability for Deep Drawing Processes

Abstract Galling is a recurring phenomenon in deep drawing processes which requires frequent maintenance of tools to improve the product surface quality. Adhesive transfer of softer material on the hard tool surface results in sharp features which causes surface roughening of the tools and deterioration of deep drawn products. In this article, an adhesive wear model based on a deterministic approach is developed to predict the galling behavior in a deep drawing process. The model uses the surface topography, material properties and contact conditions to predict the surface roughening of tool surfaces under perfectly plastic conditions. The adhesive transfer of material is considered by the growth of the asperities based on its geometry for the increase in height and radial direction by preserving the original shape and volume consistency. The results of the multi-asperity models show the growth of the transfer layer and its effects due to load, sliding cycle, sliding distance, and affinity of the materials. The results show the influence of the above-said parameters and its applicability for deep drawing process conditions. The simulated results show an 85% level of confidence in comparison with the experiments from literature for the prediction of the surface evolution due to the galling mechanism.

Tribological Behaviour of Ti or Ti Alloy vs. Zirconia in Presence of Artificial Saliva

Abutment is the transmucosal component in a dental implant system and its eventual appearance has a major impact on aesthetics: use of zirconia abutments can be greatly advantageous in avoiding this problem. Both in the case of one and two-piece zirconia abutments, a critical issue is severe wear between the zirconia and titanium components. High friction at this interface can induce loosening of the abutment connection, production of titanium wear debris, and finally, peri-implant gingivitis, gingival discoloration, or marginal bone adsorption can occur. As in vivo wear measurements are highly complex and time-consuming, wear analysis is usually performed in simulators in the presence of artificial saliva. Different commercial products and recipes for artificial saliva are available and the effects of the different mixtures on the tribological behaviour is not widely explored. The specific purpose of this research was to compare two types of artificial saliva as a lubricant in titanium–zirconia contact by using the ball on disc test as a standard tribological test for materials characterisation. Moreover, a new methodology is suggested by using electrokinetic zeta potential titration and contact angle measurements to investigate the chemical stability at the titanium–lubricant interface. This investigation is of relevance both in the case of using zirconia abutments and artificial saliva against chronic dry mouth. Results suggest that an artificial saliva containing organic corrosion inhibitors is able to be firmly mechanically and chemically adsorb on the surface of the Ti c.p. or Ti6Al4V alloy and form a protective film with high wettability. This type of artificial saliva can significantly reduce the friction coefficient and wear of both the titanium and zirconia surfaces. The use of this type of artificial saliva in standard wear tests has to be carefully considered because the wear resistance of the materials can be overestimated while it can be useful in some specific clinical applications. When saliva is free from organic corrosion inhibitors, wear occurs with a galling mechanism. The occurrence of a super-hydrophilic saliva film that is not firmly adsorbed on the surface is not efficient in order to reduce wear. The results give both suggestions about the experimental conditions for lab testing and in vivo performance of components of dental implants when artificial saliva is used.

Galling categories investigations in stainless steels

This paper focuses on the galling mechanisms occurring in stainless steels and aims to provide a better comprehension of the effects of microstructure on galling resistance. Five stainless steels are studied in this paper, namely Nitronic60, AISI660, 316L, 316LN (austenitic stainless steels) and Uranus45 N (duplex austenite-ferrite). Both surface topography and in-depth microstructure are characterized in order to determine the consequences of galling apparition. Experimental investigations at macroscopic and microscopic scales show that galling can occur following several mechanisms. Galling leads to either adhesive wear spots randomly distributed on the surface (tolerant galling), adhesive wear initiated on the periphery of the pin (moderate galling) or abrasive wear and smearing (severe galling). Depending on these categories, the galling threshold and severity are highly variable. Studying these specific mechanisms can help us predict and eventually increase galling resistance for a given material couple. Thus, several microstructural investigations have been performed in order to discuss about the possible origins of these galling categories.

Surface Galling Mechanism Analysis of Rotary Shouldered Thread Connection

In this paper, surface galling failure process and mechanism of drill pipe’s rotary shouldered thread connection (RSTC) was studied by means of full-scale make-up and break-out (M&B) testing. The results showed that surface galling damage mainly appearred at the positions of leading flank, bearing flank and make-up shoulder of RSTC. The galling damage of three different positions was a process of mutual independence and mutual promotion, and they might appear simultaneously, and also appear at different stage. The surface galling failure was mainly due to metal’s plastic deformation and peeling off under the effect of compressive stress caused by friction and compression effect among the contacting metal surface. During M&B testing, deformed and peeling metal accumulated continually and resulted in "cold welding" at local position. In the subsequent M&B process, "cold welding" metal was torn open and peeled off from metal matrix, and metal surface galling was getting serious gradually. M&B testing results showed that torque load and thread compound had decisive influence on surface galling damage, while the influence of RSTC’s surface wear, rearranging, torque fluctuations and tensile load was not obvious.

Material transfer during high temperature sliding of Al-Si coated 22MnB5 steel against PVD coatings with and without aluminium

Press hardening of Al-Si coated 22MnB5 steel is the dominant technology to enable light weight design in automotive applications. Transfer of the Al-Si coating onto the tool surface occurs during hot forming. This affects process economy and quality of produced components. The reported galling mechanisms are adhesion and compaction of wear debris. Surface engineering of forming tools has been proposed to minimise the transfer of Al-Si coating. Plasma nitriding of tool steel surfaces reduces adhesion but has poor abrasive wear resistance. PVD coatings have generally been found to promote galling due to higher chemical affinity but improve abrasive wear resistance. Most studied PVD coatings are transition metal nitrides containing aluminium. The aim of this study is to investigate the role of aluminium in PVD coatings and its effect on transfer of Al-Si coating material during sliding against coated tool steel at high temperatures. This work has focussed on PVD coatings (AlCrN and CrWN) deposited on plasma nitrided tool steel. Their tribological behaviour was studied using a hot strip-drawing tribometer capable of simulating the conditions prevalent in press hardening. The results showed that PVD coatings containing aluminium induce more material transfer. The material transfer is mainly related to chemical affinity since all coatings were polished to a low surface roughness (Sa =~120 nm) to minimise transfer initiated by surface defects. The hardness of the PVD coatings does not seem to influence the material transfer since the softer coating (CrWN, HV0.05 = ~1850) showed less transfer compared to AlCrN (HV0.05 = ~2100). The CrWN coating showed longer running-in compared to AlCrN due to reduced initial material transfer. Formation of thicker transfer layers governs the steady state friction mechanisms. Material transfer of Fe-Al intermetallic compounds occurs at the initial stages of sliding through direct adhesion to the PVD coating. The layers grow to > 5 µm thickness within a few decimetres of sliding.

Towards the Numerical Prediction of Galling Onset in Cold Forming

Aluminum alloys are materials that have a strong tendency to galling when they are cold formed. Caused by a breakdown of the lubricant film between the part and the tool, galling can have dramatic consequences on the forming operation: scratches and cracks in the surface of the piece, clogging and deterioration of tools, etc. The present work studies the galling mechanisms of the aluminum alloy 6082 during its cold forming. Trials involving the Upsetting-Sliding Test (UST) are performed first. The UST is a test bench able to simulate in laboratory conditions the contact encountered at the part/tool interface of industrial processes. Trials are achieved under varying contact pressure and lubrication. UST results show that galling is strongly influenced by tool roughness and is not accompanied by a significant increase of friction. Three sets of finite element computation of the UST are then run to predict galling onset. Lubricant and adhesion forces are not modelled in this simplified approach: only the mechanical aspects are taken into account, the chemical ones are implicitly taken into account by coefficients of friction. The Lemaitre’s and the Xue’s damage models are compared. Results show that the Lemaitre model needs the tool roughness to be modeled to detect the galling onset. The Xue model is able to detect the occurrence of galling without modelling roughness. This result is due to the used of the Lode angle with enable the calculation of damage under low stress triaxiality.

Galling mechanism in metal forming process with TRD coated die against advanced high strength steel sheet

Abstract Galling behavior in sheet metal forming of advanced high strength steel (AHSS) is of great concern in automobile industry. To examine the galling mechanism in sheet metal forming, Finite element analysis and U-bending test with VC coating die coated by thermo-reactive diffusion process (TRD) against DP780 sheet were carried out. The results show that scratches are firstly observed on the die radius surface, mainly distributed at 0-10°and 30-50°. As the forming number increases, the galling mechanism changes from adhesive wear to abrasive wear, accompanied by adhesive tumors and debris. The Ry increases from 0.481μm to 3.44μm for TRD coated die surface and from 0.718μm to 6.82μm for workpiece, respectively. The wear depth predicted by finite element analysis agrees with the experimental result.

Experimental and numerical investigation on galling behavior in sheet metal forming process

During the sheet metal forming, one of the major causes for die failure is transfer and accumulation of adhered material to the die surface, generally referred to as galling behavior. In the present work, the galling mechanism in sheet metal forming is investigated by a multi-scale approach. Firstly, the macro-galling behavior in square cup drawing of high-strength steel is examined by laboratory experiment and numerical simulation. Then, the first-principle calculations are used to present an insight to the atomic level galling at TiC/bcc-Fe interface. As a result, the macro-galling behavior in sheet metal forming is dominated by the critical contact pressure and the effective sliding distance. At atomic scale, the theoretical modeling using perfect crystals suggests that the separation most likely occurs at TiC/bcc-Fe interface or within the sub Fe slab, where the initiation galling would be arose and then formed as micro-adhesion.

Galling mechanisms during interaction of tool steel and Al–Si coated ultra-high strength steel at elevated temperature

Occurrence of galling in hot forming is detrimental to the quality of produced parts and process economy. Material transfer from Al–Si coated work-piece to the tool material has been studied in this work. PVD coatings (AlCrN, TiAlN and DLC) on tool steel substrate have been considered as well as plasma nitriding and their tribological behaviour was compared to the case of an untreated tool steel. Galling initiates through accumulation and compaction of wear debris when untreated tools are used whereas the PVD coatings resulted in increased galling due to adhesion. Plasma nitrided tool steel showed negligible galling due to formation of glaze layers and the formation of such layers depends on the occurrence of wear of the nitrided tool steel.

Investigations into wear and galling mechanism of aluminium alloy-tool steel tribopair at different temperatures

Aluminium alloys show poor formability at room temperature, and the production of complex components requires a series of high temperature forming processes, such as warm and hot forging, extrusion and hot sheet metal forming. Forming aluminium in these conditions subjects the tools to severe adhesive wear and galling, leading to increased energy needs, shorter tool life, lower part quality and increased cost. In this work, the wear mechanisms generated by aluminium alloys on forming tools have been studied by means of linear reciprocating sliding tests. Aluminium alloy AA2017 balls were slid against DIN 1.2344 (AISI H13) tool steel samples with various surface finishes at temperatures up to 450°C. The main results show that the observed wear mechanisms are extremely dependent on the system temperature, ranging from pure abrasive wear to formation of layers of compacted aluminium debris and gross aluminium transfer in the form of lumps. On the other hand, tool surface finish has a limited effect on gross material transfer, but does affect the material transfer micromechanisms.

Tribological Peculiarity of Galling Position in Square Cup Drawing

The high tensile steel sheet attracts attention in recent years. Galling occurs easily in the press forming processes of the high tensile steel sheet because of the high contact pressure. However, the galling behavior can’t be predicted in the forming process yet. The object of this study is to clarify the galling mechanism and develop a prediction method of galling behavior in press forming. Experiments were carried out to investigate the galling behavior in the square cup drawing of a high tensile strength steel sheet. On the die surface, galling occurs at the bottom of the boundary between the straight and corner. On the drawn cup galling starts at the top of the same boundary. The variations of contact pressure, sliding distance and temperature were investigated by using FEM. It was found that galling occurs on the sheet surface position having the history of maximum temperature and at the moment just before sliding out from the die.

Review of Research Progress on Galling in Sheet Metal Forming

Galling is a known failure mechanism in sheet metal forming. It results in increased cost of die maintenance and scrap rate of products. For high strength steels, problems with galling are of major interest, since forming pressure is high as well as shear stress. In the last ten years, significant progress has been made on development and understanding of galling in sheet metal forming. In this paper, the main test apparatus and the methods used to reduce galling tendency are reported. And the galling mechanisms of galling are also analyzed.

Wear at the die radius in sheet metal stamping

In sheet metal stamping, it is known that wear is unevenly distributed over the die radius and that multiple wear mechanisms may occur simultaneously. However, there has been little or no work that details the types of wear mechanisms, and quantifies the locations at which they occur. Furthermore, the link between recently identified time-dependent contact conditions and the wear response is currently unknown. An experimental study is presented in this paper to examine the location, type and severity of wear that occurs over the die radius during a typical sheet metal stamping process. It is found that the wear over the die radius consists of a combination of ploughing and galling mechanisms. The relative severity of the ploughing mechanism is divided into two distinct zones on the die radius, which correlate well with the contact pressure and sliding distance behavior predicted in our recently published numerical study. The galling mechanism results in failure of the stamping process and is, therefore, critical to the overall tool wear response. Our analysis indicates that the severe contact pressure/small sliding distance conditions, which occur during the initial stage of the process, cause the galling behavior observed over the radius. Therefore, it is concluded that the overall tool wear response and tool life is primarily dependent on the initial transient stage of the stamping process.

The effect of lubricant selection on galling in a model wear test

Galling is a known failure mechanism in sheet metal forming (SMF) processes. As a result of this wear process, the amount of waste increases, the production process becomes hard to control and eventually expensive maintenance is required in order to continue production. Delaying or avoiding galling mechanisms by optimising the contacting materials and lubricant is therefore of high industrial importance. The presented work focuses on the effect of lubricant selection on galling, using a model wear test: the TNO slider-on-sheet tribometer. With this tribometer, galling mechanisms are studied using tool steel in sliding contact with lubricated deep draw steel DC 06. The wear data obtained with the test method agrees well with a general framework of lump initiation, lump growth and scoring (galling). Experimental data clearly demonstrates that a low coefficient of friction at the start of the experiment is not a reliable indicator for galling prevention. Hence, lubricant evaluation should be done based upon long-term testing instead of on short-term testing. Comparative results with a lubricant set shows that the TNO slider-on-sheet model wear test is suited for ranking forming lubricants with respect to their ability to avoid material transfer.

The fretting corrosion resistance of PVD surface-modified orthopedic implant alloys.

The objective of this study was to evaluate the fretting corrosion resistance of both modified and unmodified Ti6Al4V flats fretted against CoCr-alloy spheres in a buffered Hank's solution at 37 degrees C using an original fretting apparatus. A physical vapor deposition (PVD) cathodic arc evaporation technique was used to deposit 3-4 microm thick titanium nitride (TiN), zirconium nitride (ZrN), or amorphous carbon (AC) coatings onto the Ti6Al4V substrates. The fretting behavior of the nitride films (TiN and ZrN) was characterized by the absence of surface damage and the deposition of a Cr-rich oxide transferred from the CoCr-alloy spheres to the modified surfaces. This oxide led to a slight increase in surface roughness. Three of the six multilayered AC coatings tested exhibited extensive fretting damage and generated large, deep, wear scars. Cohesive failure of the AC coating was observed in the low contact stress areas of the fretting scars. The remaining AC-coated specimens experienced only slight polishing wear. The reason for the different behavior within the AC-coated specimens is not clear at the present time. The unmodified Ti6Al4V surfaces experienced severe surface damage consistent with the adhesive galling mechanism to which these alloys are susceptible.

Galling mechanisms in sheet forming operations

The paper reviews some important observations concerning the initiation of galling for various combinations of tool and sheet materials. The experimental conditions have been chosen to resemble the contact conditions in sheet metal forming. Typically, a hard and smooth tool surface is repeatedly in contact with a soft and rough sheet surface. The plastic wave is introduced as a general mechanism for subsurface plastic working of the contact asperities on the sheet during friction contact. The same mechanism is used to describe the transfer of sheet material to the tool surface. Generally, different types of tool surface defects act as traps for sheet fragments which stick to the tool surface. Typical defects are grinding scratches and holes of various origin. A possible initiation from transfer aluminium oxide to the tool surface for hot-dip galvanized sheet steels has also been observed. The action of defects as initiation sites for sheet material transfer seems to be virtually independent of the material combination, as well as the presence of lubricants. However, the further build up of the transfer layer and formation of big lumps of sheet material on the tool surface is very sensitive to the material combination and the presence of lubricants. Special initiation mechanisms (e.g. aluminium oxide) can, however, be observed in action together with the defect mechanisms.

Galling mechanisms in lubricated systems: A study of sheet metal forming

In many sliding systems, transfer of material occurs between the contacting surfaces. Since the transfer modifies the contacting surfaces, the friction and wear behaviour of the system are also affected. A basic understanding of transfer mechanisms is therefore important, not only for specific applications, but also for a general understanding of friction and wear. The initiation of galling is proposed to be primarily due to tool surface defects. The defects are either present from the surface preparation of the tool (as grinding marks), or are produced during testing, owing to scratching/ cutting of hard sheet fragments (or occasionally other hard foreign particles) into the tool surface. The transfer proceeds with an accumulation of sheet debris on some of the initiation sites, where the contact stresses become more and more severe, owing to the hard tool surface irregularities produced during the accumulation. At later stages, cracking occurs in the transfer layer, and larger fragments move over the tool surface, causing even more severe tool surface irregularities. At this stage, back transfer of sheet fragments to the sheet surface is observed occasionally. The action of the lubricant seems to be mainly a delay of the transfer process compared with testing without lubrication. Similar mechanisms with production of hard sheet fragments that plough the tool surface and create surface defects and with a following build-up of larger lumps on these defects are observed for non-lubricated testing. However, the process is speeded up through a much larger number of fragments and a more rapid build-up of the transfer layer.

Development of tribology in metal forming.

This paper reviews recent progress in metal forming tribology. Tribo-testing methods are chronologically listed, and the correlation among various indices obtained by a compression-twist type tribometer is examined in detail. Results show that clear correlations exist between indices with similar physical meanings. Various factors influencing lubrication mechanisms are discussed, including plastic roughening and its effect on limit strain in the forming limit diagram, the application of the Reynolds equation to rolling, extrusion and drawing lubrication, elasto-hydrodynamic lubrication, micro-plasto-hydrodynamic lubrication, the galling mechanism and temperature analysis. Various lubricants and coating techniques for workpieces are briefly mentioned. Surface treatment techniques for tools such as TD process and Si3N4 ceramic provide excellent galling and wear resistance.