Notes On Welding

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Welding/Brazing/Bronze Welding

Introduction

Welding
Brazing
Bronze Welding
Electric Welding

Welding/Brazing/Bronze Welding

Introduction

I have been reading the various debates about the joining of metals with some interest for a considerable time. As with any engineering subject the choice of materials and the various process that are used in manufacturing are always based on a number of compromises and there is rarely a perfect solution for any specific application. The main factors that need to be considered usually involve cost and practicality.

When competition or other low volume production cars are considered, practicality usually wins the day. A more detailed consideration would include the following points:

Availability/Capital? cost of equipment available for manufacturing.
The level of skill available in the manufacturing environment
The development time available for the component/vehicle being considered
The "market price" of the component/vehicle relative to demand
The anticipated life of the component/vehicle.
The availability/cost of existing technology

I am sure that other factors can also be considered but I think that these are probably the main points that influence decisions.

We also need to be aware that there has been a significant increase in the available technology over the last 40 years and we should make certain allowances for this factor when we consider older vehicles. I think that it may be interesting to consider the techniques used in manufacturing Lotus space-frames and the developments that have occurred during the lifetime of the Seven.

We should also bear in mind that a "competitive situation" exists between different manufacturers and that it has become very common to justify "different" techniques as superior in order to gain financial benefit. These justifications are often quoted in the specialist press and become accepted as fact even when there is no good engineering justification, a factor which I tend to find rather irritating.

We need to define the basic processes that are being used and their advantages/disadvantages.

Welding

In a modern context we need to sub-divide welding into two distinct categories:

Fusion welding, which involves the melting of the parent components of a weldment. Gas Welding and Electric Welding are the most common forms.
Solid-phase welding techniques that have been developed in the last few years and include Spot Welding, Friction Welding, Ultrasonic Welding and Diffusion Bonding.

We will only concern ourselves with fusion welding for the purpose of this document as although the solid-phase techniques are commonly used in automobile manufacture they always involve a very high capital cost.

The most common form of fusion welding that was available in the late 1950's was Gas Welding. We all know that oxygen and acetylene burn with a flame that is sufficiently hot to melt the steel materials used for the manufacture of racing car space-frames.

As we know early Lotus space-frames were manufactured by Progress Engineering who seemed to use a very basic gas welding technique.

There are a few simple advantages that must have made gas welding relatively attractive:

Very small capital outlay for equipment
Very basic process requiring a minimum of skill and training
Very low cost welding consumables (Steel welding rods are cheap!)

The disadvantages of gas welding techniques were either not significant or were unknown at that time. These disadvantages are as follows:

Relatively high levels of distortion due to the high temperatures involved in any welding process.
Inconsistent weld quality due to the use of low grade welding rods and other effects, discussed later. (Gas welding rods are unsuitable for the majority of electric welding process due to high levels of dissolved gases)

As well designed space-frame chassis are fully triangulated all of the principle joints are usually subjected to either tension or compression loading. The amount of bending or torsion that is applied is generally limited. This factor makes the weld area relatively insensitive to the effects of metal fatigue.

I don't believe that Metal Fatigue was ever considered in the design of 1950s' chassis for three important reasons:

The mathematical tools to describe dynamic chassis behaviour and the alternating loads applied were very limited.
Little or no information about Fatigue was published in the literature or taught to mechanical engineers until after the Comet disaster.
The life of the vehicle was not estimated for the above reasons and chassis loading was relatively light due to the relatively poor tyres that were in use.

My conclusion is that cost and availability were the most important considerations that were made by Lotus. The financial problems caused by lack of cash-flow and the Elite programme have been well documented and "practicality" and timescale to market must have been a significant influence.

Manually gas welded joints suffer from significant strength variation for a number of reasons:

Poor quality welding rods with high dissolved gas contents can lead to brittle weldments.
Variation in penetration caused by lack of skill or operator fatigue (Hangover?). This becomes particularly relevant when higher production volumes are considered and the need for more skilled staff increases.
Inconsistent regulation of oxy-acetylene flames. The incorrect adjustment of the flame used for welding can create either strongly oxidising conditions which contaminates welds with iron oxide particles or unburned acetylene which is soluble in molten iron and can cause embrittlement.
The surface cleanliness or the parent material has a significant effect on the properties of gas welds as fluxes are not commonly used. Oil and other contaminants can cause either lack of fusion defects or alter material properties quite significantly.

I think that the main conclusion that can be made is that when gas welding is correctly employed by a skilled operator in reasonably controlled conditions satisfactory results can be obtained but there are other techniques available that are more tolerant of error.

Brazing

This process should be differentiated from Bronze Welding. It should be referred to more correctly as Capillary Brazing. Successful joints depend on the ability of filler metal to penetrate small gaps between metal surfaces by capillary attraction. Under the correct conditions the brazing alloy (usually copper based) wets and bonds by surface diffusion alloying to form a strong joint. There are three important conditions that must be met to achieve an effective joint:

The provision of a chemically clean metal surface, normally accomplished, when brazing in air, by a flux, which is active over the brazing temperature range.
The provision of the correct joint gap for the particular brazing filler metal
The correct heating pattern so that filler metal flows up the thermal gradient and into the joint.

welding3.gif The design of a joint for brazing is radically different from that of welding or bronze welding. The joint designs shown in Fig 3a may be considered as reasonable for welding, the full advantage of brazing will only be available from joints such as those shown in Fig 3b where the maximum capillary bond is achieved.

Brazing is still a very common process and is used for a great many production components particularly in the field of electrical contacts. It is commonly carried out using pre-formed inserts to control gap sizes and vacuum furnaces to avoid contamination.

The majority of brazing alloys have been developed to have a very low surface tension so that they can penetrate very small and well controlled gaps. The strength of a brazed joint is not dependent on the strength of the filler material but of the support provided by the surrounding structure. In general terms provided that the filler wets the entire surface of a joint, joint strength is inversely proportional to the thickness of the filler.

Even the most basic assessment of a typical racing car space-frame design should reveal that the basic design is rather unsuitable to allow brazing to be carried out in a cost effective manner. A significant re-design of all of the joints would be needed if the resulting structure were to have sufficient strength. Cost and practicality obviously prevent this from happening.

I believe that it is basically incorrect to describe any Lotus Seven chassis as being brazed.

Bronze Welding

Bronze Welding is an effective, low-cost joining technique. The term "bronze welding" is misleading since it implies fusion of the parent metal and the use of a bronze filler metal. This is clearly not the case.

Bronze welding, until the development of the low temperature capillary brazing process, was a classic process in which a bronze alloy was used as a filler metal to form a surface bond between unfused parent metals. Unlike capillary brazing the strength of the bronze welded joint is derived from the tensile strength of the filler metal deposited in a joint preparation that is virtually identical to that used in fusion welding as well as the actual bond strength developed between the filler and parent metals.

As for brazing, clean metal surfaces are required for the production of sound weldments and it is common to use a fluoride/borax flux. This can be applied to surfaces as a paste prior to bronze welding or be combined into the filler rod itself.

The process of bronze welding did have a poor reputation for joint strength, as early filler materials were a basic 60/40 commercial brass with relatively poor tensile strength. Bronze welding alloys have developed significantly in the last thirty years and the addition of elements such as Nickel or Silicon have significantly increased the tensile strength of the filler. Alloys with a tensile strength in the Â“as castÂ” condition, which is higher than mild steel are readily available and also are more resistant to corrosion than capillary brazing alloys.

These changes have lead to the introduction of the term Â“Nickel Bronze WeldingÂ?” to provide confidence that sound joints of adequate strength are being produced.

There are several other benefits associated with Nickel Bronze Welding:

It is a relatively low temperature process, which minimises both distortion and metallurgical damage to the parent structure.
It is more tolerant of operator error as the parent material always remains in the solid phase and is not prone to the contamination of the weld pool that can occur with gas welding.

I believe that if tensile test pieces from gas welded components were compared to nickel bronze welded components on a scientific basis, the bronze welded components would produce the most consistent results.

It is also interesting to note that modern nickel bronze filler alloys that are used to join mild steel have such high tensile strength that failure usually occurs in the parent material.

As we have previously discussed correctly designed space-frames are designed to have all of their joints in tension or compression and I believe that Nickel Bronze Welding is the ideal method for the manufacture of the Seven chassis and that Arch Motors have made a sound decision.

I think that there may be some relevance in the change from gas welding to nickel bronze welding at some stage in the life of the Series ll Seven. As the number of tubes was significantly reduced to make the Series ll cheaper to manufacture, chassis stiffness obviously reduced. This must have lead to increased and undesirable bending and torsion being applied to chassis joints.

I am firmly convinced that a Nickel Bronze Welded structure would be more tolerant of this type of loading than a chassis fabricated by gas welding and that the extra cost of the bronze filler material was outweighed by a reduction in warranty costs.

I also think that during the late 50Â’s bronze welding was not always completely reliable. Some of the more modern alloys, developed by companies such as Eutectic-Castolin, have alloy additions that have been designed to reduce the intergranular damage that can occur when too high a temperature is used for this technique.

Electric Welding

Just to finish off this topic, most Â“Lotus SevenÂ?” inspired replicas are now manufactured using a relatively modern form of electric welding, known as MIG welding.

Clearly electric welding offers several advantages over the more traditional gas welding used by Progress Engineering but to try to imply that it is the most suitable technique for space-frame manufacture is to grossly overstate the case and is misleading.

Electric welding was developed in the late 1920s in American shipyards and was used to disastrous effect in the manufacture of the T-42 Tankers and Liberty ships used during WWII. The vast majority of these ships broke in half during their voyages on Atlantic convoys and caused significant loss of life.

The technique used was Manual Metal Arc (MMA), which we all know quite well. A fluxed welding rod is used to strike an arc between the parent metals and the filler. A high current flow produces a significant amount of heat, and fusion of all three components takes place. The main advantage of this process is that very high production rates can be achieved due to the very high energy input that can be developed.

The disadvantage of MMA techniques is that the filler rod is coated with a flux, which is used to prevent oxidation and contamination of the weld pool. This flux must be removed after welding. In order to allow the flux to melt and flow sufficiently relatively high heat input is needed and this make the MMA process poor for joining relatively thin sections and therefore unsuitable for space-frame manufacture.

A modern development of MMA has been the use of Metal Inert Gas (MIG) welding.

In this technique a gas shield is used to prevent oxidation and contamination of the weld pool, thus eliminating the need to remove the fused flux layer from the finished weldment. The use of a gas shield also means that the high currents needed to melt the flux used in MMA are not required. MIG welders can therefore weld relatively thin sheet with minimum distortion. The use of different types of gas for shielding has also been quite important as different gases influence the mode of metal transfer between the workpiece and the electrode. These effects are very complex and are a science in themselves but the correct gas does help when welding thin materials.

These factors make MIG welding a very attractive proposition and as the technique is reasonably simple to learn and the equipment relatively cheap to buy, it has become the favoured by special builders.

Unfortunately this convenience has lead to the technique being justified on the basis of "Best Practice and this is certainly not quite correct.

The main disadvantage of MIG welding is that the arc required to produce heating is still struck from the filler metal. In the majority of welding machines this takes the form of a roll of copper plated wire. The feed speed required to produce "satisfactory" welds is a function of the thickness of material being joined. A continuous bead of metal is then deposited into the weld joint preparation. As the majority of kit-car space-frames are manufactured from 16swg (Lotus Seven was 18swg?) no weld preparation takes place and a large bead of metal is produced at every joint. In order to skin the car with alloy this is then ground back with an angle grinder !

I think that this procedure could weaken tubes unless carried out with great care. It is also important to be aware that when MIG welds achieve full penetration they slightly reduce the thickness of the parent plate just to the side of the weld bead and I don't consider this to be ideal when considering thin walled tubes. MIG welders were developed to enable commercial welding companies to lay down the large volumes of filler needed for the joining of thick sections with angled preparations and to avoid the time consuming and costly work of removing flux after welding. The fact that it can be used for low distortion thin section work does not make it ideal or "Best Practice". I am sure that the structures produced are fit for purpose but not the most elegant.

In the world of F1 the sophisticated suspension components that tend to be fabricated from Titanium or the Nickel alloy based exhaust systems are made using a process known as Tungsten Inert Gas Welding (TIG), in this process the arc is still shrouded by a protective gas, but the arc is not struck from the filler metal but from a Tungsten electrode. This design allows even higher energy input than MIG welding. The filler rod is usually added manually as in Gas Welding and the added metal can be kept to a minimum and the heat input controlled in a very accurate manner.

TIG welding is the process used for all thin sheet fabrication used in F1, Aerospace and military applications. The equipment used has an extremely high cost and very skilled operators are needed but is possibly the technique that should be considered as "Best Practice" but its use would significantly increase the cost of manufacture of the basic space-frame and the associated suspension components.

My conclusion is that Nickel Bronze Welding is a good compromise between cost and quality and I am entirely happy to drive a car that is manufactured using this technique.

Chris Flavell

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