Nick J den Uijl

N, den Uijl: Thermal and electrical resistance in resistance spot welding. Proceedings of the 17th International Conference Computer Technology in Welding and Manufacturing; 18 – 19 June 2008; Cranfield University, Cranfield, UK. ISBN 13-978-1-903761-07-6. In Resistance Spot Welding (RSW) heat generated by a current passing through a series of thermal and electrical resistances of metal sheets is used to melt material and form a weld. This paper reports on research work conducted at Corus RD&T on mechanisms involving thermal and electrical resistance in RSW and how they interact. The goal of the work is twofold: • To get a better understanding of how various material properties influence the weldability of steels in RSW. These results can be used to support the development of new spot weldable steel grades . • To investigate the impact of various material properties on the simulation results. Material properties can be very cumbersome and expensive to determine. Therefore, better understanding of material properties from simulations saves money by cutting unnecessary work. First an overview is given of the physics underlying the thermal and electrical resistances in resistance spot welding. The actual investigation is done using a verified RSW model. Experimental results are used to support the simulation results. Additionally, these results are used to establish the level of accuracy needed for material data used in RSW simulations. The results can be used to increase the accuracy of weld modelling simulations. The results will also enable manufacturers to use simulation more efficiently in the set up of their production process. Finally the results can save costs by cutting unnecessary determinations of material properties

Tijdens de conferentie 'Trends in Welding Research' van juni jl. in Chicago was een speciale sessie gewijd aan de microstructuurontwikkeling tijdens het lassen. Een van de bijdragen was een presentatie van de onderzoeksresultaten van een M2i-onderzoek (Materials Innovation Institute), waarbij Tata Steel en TU Delft betrokken waren [1]. Het onderzoek had betrekking op de lasbaarheid van in ontwikkeling zijnde hogesterktestaalsoorten.

The goal of this article is to investigate resistance spot welding of a complicated welding configuration of three sheets of dissimilar steel sheet materials with shunt welds, using simulations. The configuration used resembles a case study of actual welds in automotive applications. One of the steel sheets is a newly developed high strength steel, DP800 HyperForm (DP800HpF). Hyperform steels offer the engineer high strength combined with improved formability and excellent weldability. First hyperform steels are introduced and positioned within the pallet of advanced high strength steels. Then the influence of various material characteristics is discussed. Next the impact of actual complicated welding operations, compared to laboratory coupon testing, is discussed. Having set the background, the experimental results and the set up of the model used for simulations are explained. Finally the results from simulations are used to investigate the welding operations, with a focus on the ...

The introduction of advanced high strengths steels enabled automotive manufacturers to simultaneously reduce weight and increase safety of vehicles. However issues were reported concerning the weldability of these steels. These issues concerned both the manufacturability as well as the performance of resistance spot welded joints. The manufacturability addresses the ability to produce joints in an industrial set up, especially the welding range and the electrode lifetime. The performance includes the post weld hardness and the failure mode of resistance spot welded joints. Both aspects are addressed in the dissertation using experimental data and software to model and simulate welding processes.

The introduction of advanced high strengths steels enabled automotive manufacturers to simultaneously reduce weight and increase safety of vehicles. However issues were reported concerning the weldability of these steels. These issues concerned both the manufacturability as well as the performance of resistance spot welded joints. The manufacturability addresses the ability to produce joints in an industrial set up, especially the welding range and the electrode lifetime. The performance includes the post weld hardness and the failure mode of resistance spot welded joints. Both aspects are addressed in the dissertation using experimental data and software to model and simulate welding processes.

Currently the majority of joints within the automotive industry is made using resistance spot welding (RSW). An accurate model of the RSW process predicts the resultant material properties. The model enables manufacturers to predict the final characteristics of a work piece. It also enables the steel producer to identify process windows which can be used for the development of materials. To support these activities research projects have been initiated and carried out at Corus RD&T to simulate RSW and to link chemistry of materials to weldability. Weldability of steels is often summarised as: weldability is reciprocal to hardness. Otherwise put, the maximum hardness after welding is inversely related to the weldability of a material. Because an RSW weld can be subdivided in zones such as the weld nugget, the heat affected zone and base material, all these zones can be considered in a broader look on weldability. Finite element simulations give first the maximum hardness in the nugge...

Uijl, NJ den, Smith, S, Moolevliet, T, Goos, C, Aa, EM van der & Veldt, T van der (2008). Failure modes of resistance spot welded advanced high strength steels. In W Zhang & N Scotchmer (Eds.), Proceedings 5th International Seminar on Advances in Resistance Welding, Toronto, Canada, 24-26 September 2008 (pp. 78-104). Denmark & Canada: Swantec Software and Engineering ApS & Huys Industries Ltd. Sheet steels are typically made using an oxygen (BOF) or electric arc furnace (EAF). Due to differences between raw materials and process, EAF steels have higher N, Si, Cu, Cr, Mo, Ni, and Sn contents compared to similar grades of BOF steels. As well, steel chemistries will slightly vary when produced by different mills. Automotive companies are concerned that these natural variations impact weldability through increased susceptibility to hot cracking, hardenability, or changing thermal and electrical properties. So there has been resistance to accepting EAF made steels and full source approvals are often needed if there are changes to the steelmaking process. This study explores these concerns. SORPAS was used to calculate the effect chemistry variation has on material properties and weldability. As well, the affect of chemistry variation due to steelmaking technique was tested by comparing the weld morphology and hardness of EAF and BOF made materials.

Toru Okada, Masato Uchihara, Kiyoyuki Fukui, Sullivan Smith, Nick den Uijl & Tony van der Veldt: The effect of paint baking cycles on the spot weld strength of advanced high strength steel Consequences for testing procedures; Preprints of the National Meeting of JWS, November 2008, Kyoto, Japan; pp.101-102.

In automotive acceptance tests, material performance is assessed for a range of key applications. Resistance spot welding performance is a key criterion in acceptance testing; welding current range, weld failure mode and weld mechanical properties are examined. Simply depending upon the spot welding results a material may be accepted or rejected. For conventional steel grades, testing in 'as received' oiled condition generates data which are representative for the end product. However in automotive manufacturing the entire body-in-white goes through a paint baking cycle after it has been assembled. In the case of AHSS an alteration in weld properties may occur as a result of heating in the paint baking cycle. For AHSS welds, significantly higher peel strengths and improved failure modes can be recorded after paint baking. This means that testing data gathered on 'unbaked' welds are not necessarily representative of the final weld properties of a completed automobile....

In automotive acceptance tests, material performance is assessed for a range of key applications. Resistance spot welding performance is a key criterion in acceptance testing; welding current range, weld failure mode and weld mechanical properties are examined. Simply depending upon the spot welding results a material may be accepted or rejected. For conventional steel grades, testing in 'as received' oiled condition generates data which are representative for the end product. However in automotive manufacturing the entire body-in-white goes through a paint baking cycle after it has been assembled. In the case of AHSS an alteration in weld properties may occur as a result of heating in the paint baking cycle. For AHSS welds, significantly higher peel strengths and improved failure modes can be recorded after paint baking. This means that testing data gathered on 'unbaked' welds are not necessarily representative of the final weld properties of a completed automobile....

Abstract: In this chapter an overview is given of the forming technologies available to produce body and chassis parts in automotive manufacturing. First, a review is given of the metallurgical background of forming technology. Next the different forming techniques are presented and the materials available are discussed. Then some aspects of the modelling technology that has helped to advance forming technology in recent decades are discussed. The chapter closes with some economic considerations on the application of forming and materials for the automotive industry. Obviously a short chapter like this can never give full details on a subject so wide and essential to automotive manufacturing as forming technology, it could easily be expanded to a full volume, detailing various aspects, but it should give some insight about the subject and enable the reader to find information for further study.

The mechanical properties of advanced high strength steels (AHSS) for automotive applications are sensitive to their multi-phase microstructures. Phosphorous is added in AHSS mainly as a solid solution strengthener and to suppress the formation of cementite thereby enhancing the room temperature stabilisation of austenite in TRIP and Quench and Partitioning steels. However, the weldability of phosphorous containing steels is poorer than conventional low carbon steels due to the grain boundary embrittlement resulting from the segregation of phosphorus to the grain boundaries during the solidification of the weld pool. In order to reduce the grain boundary embrittlement, the addition of boron has been suggested. However, the addition of boron also increases hardenability and the ratio of boron to other alloying elements present in the steels should be properly controlled to achieve the required mechanical properties of the welds. In this work, the partitioning behaviour of phosphorous...

In automotive applications welding is increasingly one of the first, as well as one of the last production steps. Where traditionally welding during assembly would prove to be important for the distortion it could cause in a finely designed structure, in modern car-manufacturing a weld produced in an early production stage (e.g. T.W.B.'s or tubes) is carried through a sequence of production steps. Therefore in these modern applications the changes induced in the material through welding could have a profound influence on subsequent production steps. Change in hardness may affect stamping tools. In additionally changes in microstructures may affect formability. A good example is hydroforming where tubes are formed from sheet material through roll bending after which the tubes are sealed with a weld. These tubes are subsequently hydroformed. They form important structural elements that have to be able to meet high demands on issues as fatigue and crash. Lacking information, the cu...

Structurele ontwikkelingen in de autotechniek zijn vaak gericht op lichtgewicht construeren of het vergroten van de veiligheid. Moderne hogesterktestalen bieden ontwerpers en constructeurs de mogelijkheid om beide aspecten tegelijkertijd aan te pakken. Ongewenst faalgedrag van gepuntlaste verbindingen kan de optimale benutting van deze nieuwe materialen echter in de weg staan.

Richardson, IM, Amirthalingam, M, Hermans, MJM, Uijl, NJ den & Wibowo, MK (2010). The influence of welding on advanced high strength TRIP and dual phase steels. In H Cerjak & N Enzinger (Eds.), Mathematical modelling of weld phenomena 9 - Proceedings 9th International Seminar on Numerical Analysis of Weldability, September 28-30, 2009, Schloss Seggau, Graz, Austria (pp. 185-214). Graz, Austria: Technische Universitat Graz.

Toru Okada, Sullivan Smith, Nick den Uijl, Tony van der Veldt, Masato Uchihara, and Kiyoyuki Fukui: The effect of paint baking cycles on the spot weld strength of AHSS and consequences for testing procedures.8th International Symposium of Japan Welding Society, At Kyoto, Japan, November 2008.

Advanced high strength steel sheet (AHSS) is widely used in the automotive body for weight reduction and the improvement of crash performance. Such a use requires higher joint strength of the resistance spot weld in AHSS. In laboratory tests, the joint strength of specimens is evaluated in as-welded condition. The effect of paint baking cycles [1][2] is usually not considered, so the actual joint strength in the automotive body is not clear. In this study, we focus on the effect of the thermal cycle corresponding to paint baking treatment on the joint strength of spot welds.

De moderne ontwerper of constructeur denkt al lang niet meer alleen in termen van metalen en lasverbindingen. Het toepassen van lijmverbindingen, eventueel in combinatie met meer conventionele verbindingstechnieken, is een manier om ten volle gebruik te maken van de mogelijkheden die moderne materialen bieden voor het verwezenlijken van constructies die optimaal zijn ontworpen.
In dit inleidende artikel worden enkele basisbegrippen en beginselen doorgenomen waarmee de bruikbaarheid van lijmen en lijmverbindingen in innovatief productontwerp beter beoordeeld kan worden.

The introduction of advanced high strengths steels enabled automotive manufacturers to simultaneously reduce weight and increase safety of vehicles. However issues were reported concerning the weldability of these steels. These issues concerned both the manufacturability as well as the performance of resistance spot welded joints. The manufacturability addresses the ability to produce joints in an industrial set up, especially the welding range and the electrode lifetime. The performance includes the post weld hardness and the failure mode of resistance spot welded joints.

Both aspects are addressed in the dissertation using experimental data and software to model and simulate welding processes.

S. Smith, N.J. den Uijl, T. van der Veldt, T. Okada, M. Uchihara, F. Fukui. The effect of ageing on the spot weld strength of AHSS and the consequences for testing procedures, IIW-1976-08, Welding in the World, p.p. R12 – R26, Vol. 54, no.1 / 2 – 2010.

Market trends within the automotive industry are leading to an ever-increasing use of high-strength and advanced high-strength steels (AHSS). The attraction of these materials is the advantage of excellent formability, combined with increasingly high tensile strength. It is a well-known fact that weld performance can be an issue with AHSS, where susceptible weld microstructures can lead to low strengths and undesirable failure modes. Much research has been conducted and published on this subject. A less well-documented effect in the weld performance of AHSS is ‘ageing,’ whereby a weld exhibits poor mechanical performance immediately after welding, but after a certain period of time, the weld properties improve significantly. In the Corus – SMI research cooperation, this ‘ageing’ effect was first observed in weld samples in 2004, since this time ageing has been a major topic of combined research. This presentation is a summary of the Corus – SMI weld ageing study, highlighting the major issues and characteristics of the effect: the influence of process parameters, the susceptible alloying systems and the possible mechanisms that can cause ageing of the weld. The ‘ageing’ effect has serious implications for standardized testing procedures, where the timescale between welding and testing is not specified, ageing can have a huge influence on the welding results obtained in ageing susceptible materials. The final aspect of this report is to consider the consequences of ageing for weld testing procedures.

Toru Okada, Masato Uchihara, Kiyoyuki Fukui, Sullivan Smith, Nick den Uijl & Tony van der Veldt: The effect of paint baking cycles on the spot weld strength of advanced high strength steel  Consequences for testing procedures; Preprints of the National Meeting of JWS, November 2008, Kyoto, Japan; pp.101-102.

In Japanese.

Okada, T, Nishibata, T, Uchihara, M, Fukui, K, Smith, S, Uijl, NJ den & Veldt, T van der (2009). Effect of time between welding and tensile testing on the spot weld peel strength of AHSS. Tokyo, Japan, National Meeting of JWS.

In Japanese.

N. den Uijl, T. Okada, T. Moolevliet, A. Mennes, E. van der Aa, M. Uchihara, S. Smith, H. Nishibata, K. Fukui: IIW-2162-11 (III-1573-10) Performance of resistance spot welded joints in advanced high strength steel in static and dynamic tensile tests. Welding in the World 7/8, 2012.

The performance of resistance spot-welded joints in advanced high-strength steel sheets is critical for the application of these materials in safety-critical areas. To be able to predict the performance of such joints from available material data would be of great benefit to the automotive industry. This report starts with a review of literature about various aspects of spot weld performance in advanced high-strength steels. It then describes experimental work whereby a set of resistance spot-welded joints in various advanced high-strength steels was tested in lap shear and peel-type tensile testing. Testing was done both statically and dynamically. The steel sheet materials varied in microstructural and chemical compositions, strength and thickness. The goal of the tests was to investigate possible relations between material characteristics and performance of the welded joints. Therefore, the experimental results are related to several material parameters, i.e., sheet thickness, base metal yield and tensile strength, carbon content and various Carbon Equivalent numbers.

N. den Uijl, F. Azakane, S. Kilic, V. Docter, B. Neelis, C. Goos & E. van der Aa: IIW-2327-11 Performance of tensile tested resistance spot and laser welded joints at various angles. Welding in the World 11/12 2012.

The performance of welded joints is usually evaluated using coupon tests under either normal or shear tensile loading. The actual loading of these joints in an automotive structure may be quite different, especially concerning the angle in which the load is applied. In this report an overview is given of published results on the performance of welded joints in automotive applications. Next the results from a series of resistance spot and laser welded joints in different steel sheet materials (HSLA and DP of varying thickness) subjected to tensile tests under varying loading angles are presented. The focus is on resistance spot welded joints, but the results are compared to similar tests performed with laser welded joints. The performance of the welded joints in terms of failure mode and strength are analysed, and the possible implications for automotive applications are discussed. Finally some work using finite element simulations is presented. Here the characteristics of the base material and welded joints of different grades of materials are evaluated to investigate the differences in performance in tensile testing. It is concluded that the thickness of the materials is the main parameter determining the failure characteristics of materials. The grade (HSLA or DP) is less of a factor determining failure mode. The fact that joints in DP steel perform as well as HSLA steels allows designers and engineers to use the advanced high strength steel without having to worry about unpredictable failure behaviour leading to decreased performance for safety critical applications.

Okada, T, Uchihara, M, Fukui, K, Smith, S & Uijl, NJ den (2010). The effect of paint baking cycles on the spot weld strength of advanced high strength steel. Preprints of the National Meeting of JWS, Tokyo, Japan Vol. 2010s. (pp. 50-51). Tokyo, Japan: JWS, Japan.

In Japanese

Uijl, NJ den, Smith, S, Nishibata, H, Okada, T, Uchihara, M & Fukui, K (2008). Prediction of post weld hardness of advanced high strength steels for automotive application using a dedicated carbon equivalent number. IIW-1873-07. Welding in the world, 52(11/12), 18-29.

Weldability of advanced high strength steels in automotive manufacturing is a key issue. There are two important aspects to weldability: producing the welds and the quality of the welds. Producing the welds concerns the process to be used, possible addition of filler materials, the electrodes to be used, et cetera. Weld quality concerns the performance of the welds in a construction (e.g. strength and crash). With advanced high strength steels issues arise with increasing strength levels concerning the weld-quality. Traditionally carbon equivalent numbers are used to predict weldability. These traditional carbon equivalent numbers are not sufficient to predict post weld hardness of advanced high strength steels. Sumitomo Metal Industries and Corus cooperate to research weldability of advanced high strength steels. This paper concentrates on the influence of the chemical composition on weldability, as a first step to assess weldability of advanced high strength steels. This is done in two steps. First the traditional use of carbon equivalent numbers to predict weldability is explored. Literature is reviewed and possible issues with welding of advanced high strength steels are identified. Next the application of carbon equivalent numbers to predict post weld hardness for various welding processes (e.g. laser beam welding and resistance spot welding) is discussed. A wide range of steels was evaluated experimentally to determine the relationship between chemical composition and post weld hardness. The influence of welding processes expressed in terms of the cooling rates. The results are combined into simple models to predict post weld hardness of advanced high strength steel joints.

Uijl, NJ den, Smith, S, Moolevliet, T, Goos, C, Aa, EM van der & Veldt, T van der (2008). Failure modes of resistance spot welded advanced high strength steels. In W Zhang & N Scotchmer (Eds.), Proceedings 5th International Seminar on Advances in Resistance Welding, Toronto, Canada, 24-26 September 2008 (pp. 78-104). Denmark & Canada: Swantec Software and Engineering ApS & Huys Industries Ltd.

Sheet steels are typically made using an oxygen (BOF) or electric arc furnace (EAF). Due to differences between raw materials and process, EAF steels have higher N, Si, Cu, Cr, Mo, Ni, and Sn contents compared to similar grades of BOF steels. As well, steel chemistries will slightly vary when produced by different mills. Automotive companies are concerned that these natural variations impact weldability through increased susceptibility to hot cracking, hardenability, or changing thermal and electrical properties. So there has been resistance to accepting EAF made steels and full source approvals are often needed if there are changes to the steelmaking process. This study explores these concerns. SORPAS was used to calculate the effect chemistry variation has on material properties and weldability. As well, the affect of chemistry variation due to steelmaking technique was tested by comparing the weld morphology and hardness of EAF and BOF made materials.

N. den Uijl & S. Smith: The Influence of Electrode Geometry on Resistance Spot Welding of Advanced High Strength Steels for Automotive Applications. Proceedings of International Conference on Total Welding Management in Industrial Applications; J. Martikainen (ed.); Acta Universitatis Lappeenrantaensis 274. 3rd JOIN Conference, August 21-24, 2007, Lappeentanta, Finland. ISBN 978-952-214-413-3; ISSN 1456-4491.

N, den Uijl: Thermal and electrical resistance in resistance spot welding. Proceedings of the 17th International Conference Computer Technology in Welding and Manufacturing; 18 – 19 June 2008; Cranfield University, Cranfield, UK. ISBN 13-978-1-903761-07-6.

In Resistance Spot Welding (RSW) heat generated by a current passing through a series of thermal
and electrical resistances of metal sheets is used to melt material and form a weld. This paper reports
on research work conducted at Corus RD&T on mechanisms involving thermal and electrical
resistance in RSW and how they interact.
The goal of the work is twofold:
• To get a better understanding of how various material properties influence the weldability of
steels in RSW. These results can be used to support the development of new spot weldable
steel grades .
• To investigate the impact of various material properties on the simulation results. Material
properties can be very cumbersome and expensive to determine. Therefore, better
understanding of material properties from simulations saves money by cutting unnecessary
work.
First an overview is given of the physics underlying the thermal and electrical resistances in resistance
spot welding. The actual investigation is done using a verified RSW model. Experimental results are
used to support the simulation results. Additionally, these results are used to establish the level of
accuracy needed for material data used in RSW simulations.
The results can be used to increase the accuracy of weld modelling simulations. The results will also
enable manufacturers to use simulation more efficiently in the set up of their production process.
Finally the results can save costs by cutting unnecessary determinations of material properties

Nick den Uijl: “Resistance spot welding of a complicated joint in new advanced high strength steel” in Proceedings of the 6th International Seminar on Advances in Resistance Welding (Hamburg, Germany); Wenqi Zhang, Kim Pedersen, Ralf Bothfeld & Jörg Eggers (ed.); pp. 40-54. 2010.

Smith, S, Okada, T, Uijl, NJ den, Veldt, T van der, Uchihara, M & Fukui, K (2009). Influence of the paint baking cycle on the mechanical properties of AHSS welded structures in automotive assemblies. Proceedings International Automotive Body Congress (IABC) 2009, Global Powertrain Congress (GPC) 2009, June 23-24, 2009, Vaals, The Netherlands (pp. 1-15).

Uijl, NJ den (2006). Modelling the influence of resistance spot welding on material properties. In W. Lucas & V.I. Makhnenko (Eds.), Proc. Joint Int. Conf. "16th Int. Conf. Computer Technology in Welding and Manufacturing" and "3rd Int. Conf. Mathematical Modelling and Information Technologies in Welding and Related Processes" (pp. 306-313). Kiev, Ukraine: E.O. Paton Electric Welding Institute of the NAS.

Uijl, NJ den & Anwar, MA (2006). Mathematical modelling of weld phenomena 7. In H. Crejak, H.K.D.H. Bhadeshia & E. Kozeschnik (Eds.), Laser weld induced microstructural changes in automotive bodysheet (pp. 309-326). Graz: Verlag der Technischen Universitat Graz.

Within the framework of ACE, the Dutch Automotive Centre of Excellence a consortium has been set up in which academic and industrial partners cooperate to research, develop, design, realise and test concepts of lightweight suspension for automotive applications. The scope of the program is broad with several pillars addressing issues of materials, manufacturing, design, application (e.g. joining) and testing. An important aspect of the program is the multidisciplinary set up with a wide range of industrial partners (e.g. materials, manufacturing, bearings, tyres, engineering et cetera). Crucial to the success of the program is the realisation phase, in which prototypes will be tested using vehicles that are commercially available. The program is set up so that activities are conducted within pillars that are directed by relevant industrial partners. Within these pillars projects are defined that are suited for students of the academic partners to work out in internships for their graduation theses. Primarily these are B Eng students that have specialised in light weight construction and design, following their minor-courses at the universities of applied sciences. Additionally there are students finalising their M Eng projects in the recently set up Master of Automotive Systems specialisation of Lightweight Design. The original impetus may have been from the Automotive Universities of Applied Sciences, but from the start there has been cooperation with the Dutch Technical Research Universities, so that the program need not be limited to the applied side of research and development. There is room for and there are activities that are more academic in nature. Thus the scope of the program is expanded to more fundamental research at BSc, MSc, and PhD level. The stated goal of the program is to research, develop, design, realise and test concepts of lightweight suspension for standard commercial automotive customers. As mentioned the realisation and testing is key as this is what should distinguish the program from efforts already undertaken in the automotive industry, which have been focused primarily on high performance vehicles or at least high end models of commercial vehicles. A secondary, but not less important outcome of the program is educational. The program will first of all give students a chance to work within a wide framework, giving opportunities to gain valuable experience in their working careers. Additionally the academic institutions will gain knowledge and experience in a field that is of great importance to the future of automotive, allowing them to improve their courses, offering students and industry a more valuable education. Though there is no stated aim that the program should be focused on the application of composite materials, the nature of the consortium, the trends in education and research and the interests of students , do tend to put a focus on the use of these materials in the program. Care is taken that this preference should not become a limitation. The philosophy is that solutions should originate from an approach without constrictions. A second possible constraint to progress within the program is the focus on student work, which may limits the scope and time-frame of individual project-activities, Therefore an important task of the consortium is to ensure that communication is fluent and information is shared freely ensuring progress and optimal efficiency of activities. The program offers the participants in the consortium an unique opportunity to benefit from each others specific strengths to rapidly increase their knowledge and experience working on an application that is both of interest to the industry and the academic partners. Additionally it offers young professionals a valuable learning opportunity and great experience to start their careers.