Corrosion Behaviors of Aluminum

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Corrosion Behaviors of Aluminum on Self Piercing Riveted Joints.

INTORDUCTION

Self piercing riveting (SPR) is a new joining technology in automotive industry, where used for joining the thin sheet materials. It doesn"„¢t require pre-drilled holes, because it differs from conventional riveting. In this technique, sheet clamping should be joined between the upset die and blank holder. Under the influence of upset die, the forcing of rivet to pierce from the upper sheet and flare in to the bottom sheet. Due to effect of the upset die a button is left on the underside of the bottom sheet. If a counter sunk rivet is used the joint flush can be set flush in one side. If non-flush on both sides oval head rivets can also be used and they are acceptable. Ideally, through the bottom sheet the rivet tail should not break. Apart from no pre-drilled hole requirements the technique has many advantages like wide range of materials can be joined, including combination of dissimilar or similar materials such as aluminium to steel. The process is environmentally friendly due to the low energy requirement, no fumes, low noise emissions and involves no heating when compared to spot welding. The self-piercing riveting of aluminium alloys gives joints of superior fatigue behaviour and comparable static strength to spot welding in this area of research. This technique offers solution to the automotive industry as the increasing use of light weight aluminium alloys for the body-in-weight application thatneeds to balance against the well known problems of spot welding of aluminium alloys. Self-piercing riveting is a joining of sheet material technique which is suitable for dissimilarmetals. Self piercing riveting is used widelynowadays in manufacturing of car industry where for body construction aluminium alloys are used. For design of mechanical structures, formed by the joining of component parts, a knowledge of vibration characteristics of different joint types ( adhesive bonding, spot welding, spr, etc). For setting the rivet equipment normally comprises a C-frame with a hydraulic force application for relatively short reach applications manual (or) gun systems are available. As it is one step joining techniqueit relies on a mechanicalinterlock rather than fusion. In steel joint thickness should be from 0.5 to 3 mm thickness and a total joint thickness is up to 6 mm. For up to 10 mm total joint thickness of aluminium alloys can be riveted. For heavily zinc-coated steel sheets, heating ventilation and building industries, for pre-painted steel for white goods, forroad sign manufacture of joining aluminium alloys.The SPR havingforces of 30 to 50 KN.

The assembly of steel as areinforcement in aluminium due to galvanic effect has raised some corrosion concerns. In order to prevent corrosion, coating is used and will improve the resistant to wear gives a good finish. Oneof the main coating and widelyused for automotive industry to enhance corrosion resistant is E-coat that is electrophoreticcoating. Under medium range voltage andrelatively high electrical current to the conductive parts a organic polymer coating is applied. The E-coatings has four other types of coatings commonly like cathodic acrylic, anode epoxy, anodic acrylic, cathodic epoxy. Cathodic epoxy is the most useful for automotive parts. There is another means for corrosion protection of steel parts that is by the zinc-coating (Zn-plate) where zinc-tin alloy is used commonly. When this is applied to the surface of the steel by electroplating and it protects the steel from the water, moisture and oxygen. Zinc will be more electronegative than mild steel when scratched and it oxidizes and thus repairing itself in the process. By using E-coat and zinc plate we can successfully apply on the parts of the HSLA ( high strength low alloy). Previously there is no report to the public domain that how thecoatings would affect the behaviour andquality of the SPR. The aim of this report that has studiedis to examine the effects ofcoatings on the mechanical behaviour and quality of self piercing riveted joints in aluminium alloy sheets joined with HSLA and thathas been coated with Zn-plate(or) E-coat.

BACKGROUND

Due to growing effects of emissions in vehicle and the natural resources depletion, the aluminium is mainly taken for high volume production vehicles, because the density of aluminium is lower than steel this allows to reduce the weight of the vehicle, emissions which come from exhaust and fuel consumption. To improve the performanceof the vehicle automotive manufacturers required touse the aluminium AA6016 and other advance materials. Because of corrosion resistance, aluminium is chosen for number of application. Aluminium is fairly a active metal, aluminium has got excellent corrosion resistance as a result aluminium oxide will form on its surface. The range of PH from 4 to 9 the film is stable and most of the corrosion observed in the aluminium is localized corrosion. If oxide film became unstable uniform corrosion can occur. Metallurgical and environmental are variable affecting the corrosion behaviour of the aluminium.

Jaguar, bmw, and Volvo these top automobile companies are looking for strength, safety, speed in the car. So from above of them we can notice that they are looking for lighter and stronger bodies. The usage of aluminium, high strength steel and magnesium are becoming prominent for car companies. So joining of these by using spot welding is very difficult because they are not so constant, and also not suited for mass production because of high costs and variable quality. During that time an HSPR offers a joining process which is strong enough and effective cost and environment friendly. A class leading automobile manufactures agree that SPR is 30% stronger than spot welding and has got low energy absorption, quality in repeated joints and fatigue resistant. Manufactures who used HSPR had got so much security because they are rated consistently high on European new car assessment programme (NCAP).

HISTORY

Henrob self piercing riveting was established in 1985 and from that time the organisation has developed their operations in different countries USA, UK, AUSTRALIA, and JAPAN. Henrob company holds number of his patents because of their designs and manufacture of self piercing riveting systems and rivets. 80% of henrob production will be exported and manufacturing will be carried out in UK. In white goods industries and automotive sector henrob has got now over 1500 self piercing rivet installation worldwide.

And network distribution across the world wide henrob is assuming that we still innovating, still expanding, still finding out new problems and new solution to old ones. In automotive industry German car manufacturers Audi has used self piercing riveting. Spot welding is failed to join consistently to Audi looked further more tests like fatigue, extensive crash and environmental testing and decided to choose the henrobs preclamping, patented high force. Audi used robotic assembly technique and manual assembly technique. In 1994 henrob followed their success with A8 Audi applying technology to North American truck builders. After this, Volvo FH12 truck cab has been used for thin, to reduce weight high strength steel is used. Jaguar XJ is the new model car in which spr is used for body construction. Jaguar chosen aluminium for HSPR because of difficulties in spot welding. It has been recognised as market leader in joining techniques.

Keith Jones was the founder and for the 150 employees he is the chief executive. Jones discovered spr in Australia and formed partnership with some inventors and they developed jointly together in UK and grow ever since from there and occupied 50 to 60 percent of European market share. By the year 2009 SPR market will be worth around $100 million this we can known by some forecasts. In competitive market SPR is confidence that his products make a benchmark for SPR technologies.

AIM: investigate the effects of processing faults during the self piercing rivet process on the corrosion behaviour of joints.

OBJECTIVES

Identification of the samples.

Clean samples.

Dry the samples.

Weigh samples.

Preparation of samples for corrosion treatment (salt spray corrosion rig)

Weigh every week, take photos of the samples.

Run the test to 1000 hrs.

Monitoring rate of corrosion.

Microscopic analysis of the samples.

Side investigation on the mechanical properties of samples.

LITERATURE REVIEW

SELF PIERCING RIVETING (SPR):

Self piercing riveting is a cold joining process used to join two or more overlapping process. For point joining of sheet material technique SPR is a high speed mechanical fastening technique. In a press tool between a punch and die a semi tubular rivet is driven in to materials to join. To form a mechanical interlock, with in the lower sheet the rivet pierces the top sheet and the die shape will cause the rivet to flare. When using the counter sunk rivet head and rivet may be set flush with the top sheet. On the underside of the lower sheet a button will be formed by using die shape. Button will not be pierced by the rivet tail ideally. The process was hydraulically operated actuator or electrically.

SELF PIERCING RIVETING PROCESS.

For setting a rivet the equipment will normally have c-frame and will have a hydraulic force application as well. For short reach applications, manual or robotic gun systems are available. Fixed c-frames will be normally uses for long reach applications. The diameter of rivet and lengthof rivet is critical.

In many industries SPR is used because it is simple and the technique will be in one step. SPR will be used on materials and also combined materials because rather than fusion it will depend on mechanical interlock. It is difficult or impossible for resistant spot welding for instance. Zinc coated, organic coated, combination of steel to aluminium, steel pre-painted, and plastics to metals are the some of the examples. The sheet should have at least 12% of elongation on die side. From 0.5 to 0.3 thickness of steelsheet joints can be made, up to 6mm of total joint thickness. A 10mm of total joint thickness can be riveted for the aluminium alloys. In automotive, ventilation, heating, and building industries, joining of heavily zinc coated steel sheets of SPR is used and it can be also used in pre painted steel for white goods, and for road sign manufacture the joining of aluminium is used and also in automotive industry typically from 30 to 5o KN, the SPR will be forced and which is high. For long reach application large stiff c-frame will be necessary, but for robotic application light weight equipment will be used because c-frames are too heavy for them. SPR will be less suitable for many industries which are driven by weight reductions and which are moving towards thinner and stronger materials. For those specific materials by further optimisation the SPR is feasible to make.

AD-VANTAGES

With no pre-drilled holes required it is a simple and one step process.

For single and combined materials SPR is suitable.

Operations will be fast and automated.

Repeatable quality.

For pre-coated materials there will be no damage or there may be little.

Joining more than 2 sheets is available.

There will be no fumes, no heat, and low noise emissions (environmental friendly)

The life of the tool will be so long and it can join more than 20,000 joints.

It doesn"„¢t burn zinc-coatings.

Energy consumption is less.

SPR is compatible with adhesives and lubricants.

There is availability of process monitoring equipment Often SPR is good enough fatigue performance than spot welds.

SPR can join different types of sheet material and also dis-similar sheet materials like aluminium to aluminium, steel to steel, aluminium to magnesium, and metals to composites. Spr can make good static strength and good fatigue resistant to the joints and which will be 30 % stronger than spot welding process.

APPLICATION OF PIERCE RIVETING:

It is only used for aluminium intensive vehicles like Audi A8, Audi A2, jaguar XJ and Plymouth prowler. In Audi A8, around 70% of single point fastening of spr is used because of superior strength, fatigue and crash worthiness of joints compared to spot welding ( 2600 pierce rivets in total)Audi A2, 1800 pierce rivets in total for the preference to the spot weld.

Jaguar XJ, to get a body stiffness of 60% greater than its predecessor and 40% lighter the 3180 self piercing rivets is used along with epoxy adhesive bonding.

The very luxury UK automobile companies are developing a new jaguar car the X350 and for the first time they are using aluminium body panels for the body shell. For metal joining aluminium is very difficult to weld so self piercing rivets are used instead of spot welding rivets are used but they used rivet setters attached to robots.

HENROB SELF PIERCING RIVETING:

Henrob is an innovative joining solution. HSPR will join two or more sheet materials by driving rivet through the top sheet and upsetting the rivet as well, without piercing in to the lower sheet under the influence of a die. The time of the riveting cycles ranges from 1.0 to 4 seconds

ADVANTAGES

HSPR can also join welding and blind riveting.

It joins dis-similar metals (steel, al, plastics).

Low energy consumption.

Very fast cycle times.

Multiple stack material can be joined.

No fumes, no heat, spark or waste and HSPR is environment friendly.

There will be continuous production because of automatic rivet feed.

Non-thermal process Joint can be checked visually.

The process is repeatable with good quality.

Lengths of rivets are available from 3.5mm to 14mm and diameter 3mm to 5mm diameter. Joints in aluminium can be joined up to 10mm and joints in steel can be joined up to 6mm. HSPR is available in wide ranges and styles like hand held riv lite tool, for low volume production, riveting at on-site, for repair work. Hydraulic power systems are available for higher volume production. When robot is mounted HSPR electric servo systems are highly effective.

Henrob has got four basic types of self piercing riveting. To achieve high quality cost effective and reliable production, the appropriate riveting system is fundamental for an application.

RIV LITE

The systems are used for very low volume production, work at the site, repairs in the vehicle, prototype build. Rivlite is a hand held actuator with batteries or from main.

HYDRAULLIC DOUBLE ACTING (DA)

It has got separate hydraulic power pack and a tool head. with variety of different accessories and allowing access to various shapes and sizes of components the tool head is configured. In manufacture of steel frame buildings, white goods industries, road signs and fabrication that has been used in construction.

HYDRAULLIC PC (PRE-CLAMPING)

A tool head and hydraulic power pack is comprised with this system. This system gives very good quality riveted joints and used in the automotive vehicle assembly. The system is also used for manual, automated or robotic productions as well. With henrob rivmon process monitoring unit this system can also be fitted.

ELECTRO SERVO SYSTEM

A separate control cabinet and tool head is comprised with this system. For automated and robotic application this system has been specially developed and also has got reduced cycle time. But this system is not recommended for manual operation. Henrob rivmon process monitoring unit can also be fitted with this system.

RIVET FEED SELECTION

There are four types of spr systems available for feeding of rivets in to the tool.

Manual tape feed

Mechanical tape feed

Sprocket tape feed

Rivtrans loose feed.

MANUAL TAPE FEED:

The only rivlite riveting system has got this type of rivet feeding method is used. Rivets that are used through the nose of the rivlite tool by hand are supplied on a short length of tape.

MECHANICAL TAPE FEED

For only hydraulic double acting and hydraulic pre clamping this type of rivet feeding method is used. Rivets that are fed in to the riveting tool automatically by a mechanical cam and spring mechanism built in to the nose of the tool are supplied in tape on spools or in cassettes.

SPROCKET TAPE FEED

For only hydraulic double acting and hydraulic pre-clamping and electro servo riveting system this type of rivet feeding system is used. Rivets are fed in to riveting tool automatically by a pneumatic sprocket mechanism are supplied in cassettes or in tape on spools.

RIVTRANS LOOSE FEED:

For only hydraulic preclamping and electro servo riveting system is used for this type of rivet feeding system. Rivets are feed in to riveting tool by automatically from a supply cabinet which is separate by a special tube blowing along them and rivets are supplied loose. Manage waste can be avoided by this type of feed and its very useful in high volume automated applications. To get the signal a sensor is built in the nose and it gives ready to rivet signal to both electric control cabinet and hydraulic power pack.

JOINT STRENGTH OF ALUMINIUM AND STEEL:

material combination

  • Peel strength Steel
  • Peel strength Aluminium
  • Shear strength Steel
  • Shear strength aluminium
  • 1.0 to 1.0mm
  • 2.5 KN
  • 1.2 KN
  • 3.5 KN
  • 2.6 KN
  • 2.0 to 2.0mm
  • 2.8 KN
  • 2.5 KN
  • 7.5 KN
  • 6.0KN
  • 3.0 to 3.0mm
  • 4.5 KN
  • 9.0 KN
  • 12.0 KN
  • 9.0 KN

DIAMETRE 5mm RIVETS IN STEEL AND ALUMINIUM.

SALT- SPRAY TEST:

This method is a standardized method checks the corrosion resistance on the samples. Metal which is made of steel, zamak, and Brass for these metals, coatings will provide the corrosion. Salt spray test is called an accelerated corrosion test, for coated samples the test provides corrosive attack for the prediction of suitability in use as a protective finish. After a period of time oxides will come out, this shows the appearance of the corrosion product. The duration or elapsed time of the test will be depending on the corrosion resistance of the coating. If there is more corrosion resistant of the sample then the test time will be more and there will be no signs of corrosion. Salt spray test is cheap, quick, repeatable test. Salt spray test are used in industrial sector for evaluating a corrosion resistant of surface finished parts. It consists of a closed testing chamber where by nozzles the salted solution of NACL is sprayed and this will provide us a atmosphere like corrosion on the samples in the chamber. The smallest volume of the chamber accepted by ASTM-B-117 IS 15 cubic feet (420 L) because of some historic reasons. With the solution of NACL when tests are done then it is called NSS (neutral salt spray). For the decorative coatings a solution of acetic acid ( ASS TEST) and acetic acid with copper chloride ( CCASS TEST) has been chosen for electroplated copper-nickel-chromium, electroplated copper-nickel or anodized aluminium. Because of ASS and CASS are very difficult to clean after the test some companies don"„¢t recommend to use ASS and CASS to use the test cabinet of NSS.

Salt spray corrosion rig.

From glass reinforced plastic (GRP) a salt corrosion cabinet is manufactured. It has been constructed in 2 parts.

The outer shell (cabinet outer) is of blue colour which has got control chassis, control panel, flow meter, and humidifier tower.

The inner shell (cabinet inner) is of grey colour sits with in the outer cabinet and it has been mounted on the heater panels.

With in the test chamber the 2132 will maintain the temperature controller accurately. To the side wall heaters a 220/240 is supplied from solid state relay which has been energised by low voltage control circuitry. The thermal over current trip is protected individually by each side wall heaters. To get the desired set point, the heat from the heater penetrates the side walls by raising the temperature in the inner side of cabinet chamber. During FOG cycle the heating will be disabled automatically and during DRY cycle the heating is enabled and cabinet temperature to the 35 degree centigrade.

STANDARDIZATION OF SALT SPRAY:

Under the national and international standards such as ASTMB-117 & ISO 9227 the cabinet construction, testing procedures and testing parameters are standardized and we can carry out the test by using this test under the describe standard, like temperature, sprayed solution air pressure, spraying solution preparation, concentration and Ph etc. The testing parameters should be checked daily to keep up to the standards and records should be maintained and used as reference standards. According to the specified requirements testing cabinets are manufactured. Requirements should be agreed between the customer and manufacturer because the testing standards will not provide the information about the evaluation of testing period for the coatings, and also the appearance of products in corrosion in form of salts. Under material specification automotive industry requirements are specified. In salt spray different coatings have different behaviour and also duration of the test time differs from one coatings o another coating. Test duration and coatings should have the minimum testing period according to the established requirement.

EXAMPLE:

Electroplated zinc-nickel steel parts can stay more than 720 hours without red rust in test and electroplated zinc and yellow passivated steel parts will be have test up to 96 hours. The surface coating of zinc on the rivet of the aluminium alloy runs up to 1000 hours.

Zinc flake coated bolt electroplated and yellow chromated bolt with red rust after testing with white corrosion.

zinc coating on the rivet of aluminium alloy.

USES:

Coatings that have been evaluated with this method are:

Zinc plating and zinc alloy plating.

Surfaces which are phospated.

According to ISO 10683, zinc flake coatings which are not applied electrically.

Coatings which are organic.

In salt spray test the hot dip galvanised surfaces are not tested generally. But painted surfaces of underlying hot dip galvanised coatings can be tested. This test which accepts the coated surfaces are in the applications of automotive, aerospace, construction industries. The time of the test differs from 8 to 24 hours for the phospated steel and for the 720 hours zinc-nickel coating and for 1000 hours the zinc-flake coating.

CYCLE PROHESION TEST:

In a series of 2 cycles, the cyclic prohesion test will run repeatedly throughout the test. There are 2 cycles which are.

FOG CYCLE

DRY CYCLE

FOG CYCLE:

For 1 hour with no heat or moist air the fog cycle should run to get required settings.DRYC CYCLE:

Introducing air to assists the samples in drying and temperature to set 35 degree centigrade the dry cycle should run for one hour to get to the required settings.

It is necessary to check when we run cyclic prohesion test:

To 35 degree centigrade the temperature of chamber should be set.

Prohesion switch should be selected and when this switch has been selected, during fog cycle the heating circuit is disabled and during dry cycle the heating circuit will be enabled.

The timer should be set for one hour for fog/dry (10 minutes for fog cycle and 50 minutes for dry cycle). Humidifier heat switch and moist air switch should be pre-selected.

For the cyclic prohesion test, the makeup solution should be

0.05% sodium chloride (NACL)

0.35% ammonium sulphate.

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